Multianvil High-Pressure High-Temperature Synthesis: From Deep Earth Minerals to Advanced Functional Materials

Abstract

This article provides a comprehensive overview of multianvil high-pressure high-temperature (HP-HT) synthesis, a pivotal technique in geoscience and materials research. It explores the foundational principles of large-volume presses, detailing their unique capability to generate stable, homogeneous conditions exceeding 25 GPa and 2000°C for prolonged periods. The scope extends to advanced methodological applications, including the synthesis of deep Earth mineral analogs, novel superhard materials like nano-polycrystalline diamond and cubic boron nitride, and functional compounds. We also address critical troubleshooting and optimization strategies for pressure calibration and assembly design, and conclude with a comparative analysis of material validation techniques. This resource is tailored for researchers and scientists seeking to leverage HP-HT synthesis for discovery and innovation in planetary science and advanced materials development.

Principles and Evolution of Multianvil High-Pressure Technology

The Kawai-type multi-anvil apparatus (KMA) represents a cornerstone technology in high-pressure, high-temperature (HPHT) research, enabling the synthesis and study of materials under extreme conditions. Developed by the late Professor Naoto Kawai and his colleagues, this large-volume press (LVP) utilizes a double-stage multi-anvil system to generate significantly higher pressures than single-stage presses by amplifying force through a unique anvil arrangement [1]. The KMA's design provides substantial advantages over diamond anvil cells (DACs), including a larger sample volume and superior capacity for stable, homogeneous high-temperature generation, making it indispensable for both geophysical studies and materials synthesis [1] [2]. This technology has been pivotal in advancing our understanding of deep Earth mineralogy and in creating novel functional materials such as nano-polycrystalline diamond and transparent nano-ceramics [1].

The Double-Stage Multi-Anvil Design

Fundamental Operating Principle

The core operating principle of any high-pressure device, including the KMA, is defined by the fundamental relationship P = F/A, where pressure (P) is generated by applying a force (F) over a confined area (A) [3] [4]. The KMA ingeniously amplifies pressure by transmitting force from large first-stage anvils to smaller second-stage anvils, dramatically reducing the area over which the final force is applied to the sample [1] [5]. This force amplification strategy allows the apparatus to achieve extreme pressures that would be impossible with a single-stage compression system.

Component Architecture and Force Transmission

The KMA's double-stage design functions through a precise sequence of force transmission and compression, depicted in the workflow below.

The KMA system comprises several key components that work in concert:

• Hydraulic Press: Provides the primary uniaxial force required to drive the compression process [3].
• First-Stage Anvils: Typically six anvils (often made of steel) housed within guide blocks. These anvils convert the uniaxial force from the press into a multi-directional convergence [1] [5].
• Second-Stage Anvils: Eight cubic anvils (commonly tungsten carbide or sintered diamond) with truncated corners. These anvils form a cubic cavity and are directly responsible for compressing the sample assembly [1] [5].
• Kawai Cell Assembly: An octahedral pressure medium containing the sample, furnace, and other internal components. This assembly is compressed by the eight second-stage anvils [1] [4].

Current Technical Capabilities and Performance

Pressure and Temperature Generation

The performance of a KMA is primarily determined by the material and design of its second-stage anvils. Technological advancements have progressively expanded the pressure-temperature (P-T) conditions achievable with this apparatus.

Table 1: Pressure Generation Capabilities of KMA with Different Anvil Materials

Anvil Material	Maximum Pressure (Room Temperature)	Maximum Pressure (High Temperature)	Key Characteristics
Tungsten Carbide (WC)	65 GPa [6] [7]	~50 GPa at 1500 K [6] [7]	High toughness, cost-effective, widely used for moderate P-T experiments [1].
Sintered Diamond (SD)	120 GPa [7]	106 GPa at 1200 K [5]	Exceptional hardness, higher cost, enables access to lower mantle conditions [1] [7].

Table 2: Typical Sample Assembly Components and Their Functions in KMA Experiments

Component	Material Examples	Primary Function
Pressure Medium	Pyrophyllite, Boron Epoxy, MgO [3] [4]	Transmits pressure hydrostatically to the sample, provides electrical insulation.
Furnace	Graphite, Lanthanum Chromite (LaCrO₃), Metal Foils [3]	Heats the sample via electrical resistance heating.
Sample Jacket	Platinum (Pt), Gold (Au), Nickel (Ni) [3]	Contains the sample and prevents chemical reactions with the assembly.
Thermocouple	W-Re Alloys [3]	Measures temperature inside the cell assembly.
Pressure Standard	NaCl, MgO [3]	Allows in-situ pressure determination via X-ray diffraction.

In-Situ Measurement and Observation Capabilities

A significant strength of the KMA platform is its compatibility with various in-situ probes for real-time observation and measurement:

• Synchrotron X-ray Diffraction: Allows for direct observation of phase transitions, determination of crystal structure, and measurement of sample pressure using a known pressure standard [1] [3].
• Neutron Diffraction: Used to study magnetic structures and light elements [1].
• Electrical Conductivity Measurements: Performed by incorporating electrodes into the cell assembly [4].
• Ultrasonic Interferometry: Enables the determination of acoustic velocities and elastic properties of materials under pressure [4].

Experimental Protocols for High-Pressure Synthesis

Assembly Preparation and Loading

A successful HPHT experiment requires meticulous preparation of the cell assembly. The following protocol outlines the key steps for a typical solid-state synthesis experiment in a KMA:

• Component Fabrication: Machine the octahedral pressure medium (e.g., MgO or pyrophyllite) to the desired size. Drill a cylindrical chamber for the sample and furnace [3] [4].
• Furnace and Sample Loading:
  • Place a cylindrical furnace (e.g., graphite or LaCrO₃) within the chamber.
  • Insert the sample material, typically as a packed powder or a solid piece, inside the furnace. The sample is often encapsulated within a metal jacket (e.g., Pt or Au) to prevent contamination [3].
  • Position the thermocouple wires (e.g., W-Re) so their junction is adjacent to the sample for accurate temperature monitoring [3].
• Gasket Placement: Attach soft gaskets made of materials like pyrophyllite near the cell assembly and between the second-stage anvils. These gaskets are crucial for efficient pressure generation and provide lateral support to the anvils during compression [5].
• Anvil Assembly: Carefully arrange the eight second-stage anvils around the prepared cell assembly to form the Kawai cell. This entire cube is then precisely positioned within the first-stage anvils and guide blocks of the press [1] [5].

Pressurization and Heating Sequence

The core of the experiment involves a carefully controlled sequence of compression and heating:

• Initial Compression: Begin compressing the assembly at room temperature using the hydraulic press. The load is increased gradually to the desired value for the target pressure, based on a pre-established load-pressure calibration curve [3].
• In-Situ Pressure Measurement (if available): When using synchrotron X-rays, monitor the diffraction pattern of the pressure standard (e.g., NaCl) in real-time to determine the exact pressure within the sample chamber [3].
• Heating Phase: Once the target pressure is reached, pass an electrical current through the internal furnace to heat the sample. The power is adjusted to achieve the target temperature, which is monitored via the thermocouple [3] [4].
• Dwell Time: Maintain the target pressure and temperature for a predetermined duration (from minutes to several hours) to allow for phase transitions, chemical reactions, or sintering to occur.
• Quenching and Decompression:
  • First, rapidly cut power to the furnace to quench the high-temperature phase.
  • Then, slowly and carefully release the hydraulic load to decompress the assembly. Rapid decompression can cause sample damage or fracture of the anvils.
• Post-Experiment Analysis: Recover the sample assembly and extract the synthesized material for ex-situ analysis using techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), or Raman spectroscopy.

Applications in Geophysical and Materials Research

The KMA has been instrumental in advancing several fields of research by enabling experiments under planetary interior conditions and facilitating the synthesis of novel materials.

Geophysical Research

The apparatus has been extensively used to simulate the conditions of Earth's deep interior, providing critical data on:

• Phase Stability and Transitions: Determining the stability fields and transformation boundaries of mantle minerals, such as the olivine-wadsleyite-ringwoodite transition in the upper mantle and the perovskite to post-perovskite transition in the lowermost mantle [7].
• Physical Properties: Measuring key properties like density, viscosity, elastic wave velocities, and electrical conductivity of minerals and rocks under relevant P-T conditions, which are essential for interpreting geophysical observations [1] [3].
• Melting Relations: Investigating the melting behavior of rocks and minerals to constrain the origin and distribution of melts in the mantle [1].

Materials Synthesis

Beyond geophysics, KMA technology has opened a new frontier in materials science:

• Superhard Materials: Synthesis of nano-polycrystalline diamond (NPD) from graphite, which is actually harder than single-crystal diamond and boasts superior mechanical properties [1] [2].
• Transparent Nano-Ceramics: Fabrication of advanced transparent ceramics like nano-polycrystalline MgAl₂O₄ spinel for optical and engineering applications [1].
• High-Pressure Phases: Discovery and synthesis of novel high-pressure phases with unique functional properties, potentially useful for electronics, catalysis, and other advanced technologies [4].

The development of high-pressure, high-temperature (HPHT) technology represents a cornerstone of modern materials science, enabling the synthesis of novel functional materials and the study of matter under extreme conditions. This field, pioneered by Percy Bridgman, has evolved from basic anvil designs to sophisticated large-volume presses that are indispensable tools for research and industrial production. These advancements have made possible the synthesis of superhard materials, superconductors, and functional ceramics, fundamentally expanding our ability to engineer materials with tailored properties. This application note details the historical progression, technical specifications, and standardized protocols for multianvil HPHT systems, providing researchers with a comprehensive framework for advanced materials synthesis.

Historical Development and Technical Progression

From Bridgman Anvils to Modern Press Designs

The foundation of modern high-pressure technology was established by Percy Bridgman, who developed the first practical apparatus for generating sustained high pressures in the laboratory. His pioneering work with anvil-type configurations enabled systematic studies of materials under pressure and laid the groundwork for subsequent technological innovations [8] [9]. The fundamental principle of these early devices—the amplification of force through confined volumes—remains central to modern press designs.

The evolution of static high-pressure apparatus over the past fifty years has transformed our capacity to study materials under conditions mimicking the Earth's deep interior. Modern large-volume presses now routinely achieve pressures exceeding 25 GPa with sample volumes sufficient for detailed characterization and quantitative analysis [9]. This technological progression has unfolded through several key developments:

• Bridgman Anvil Systems: Early systems utilizing opposed-anvil geometry to achieve modest pressures with limited sample volume [8]
• Piston-Cylinder Apparatus: Development of one-dimensional compression systems capable of reaching ~3 GPa with relatively large sample volumes (1-1000 cm³) [8]
• Multianvil Systems (MAS): Implementation of cubic, tetrahedral, and octahedral anvil arrays providing quasi-hydrostatic compression from multiple directions
• Modern Large-Volume Presses: Integration of advanced anvil materials (tungsten carbide, diamond composites), sophisticated pressure calibration, and in-situ monitoring capabilities

The large-volume press and diamond anvil cell now represent the two primary approaches in high-pressure research. While diamond anvil cells achieve extreme pressures (>300 GPa), large-volume presses provide substantially larger sample volumes and very stable, uniform high-temperature conditions through internal resistance heaters, making them particularly suitable for materials synthesis [9].

The Belt Press and Cubic Press Revolution

A transformative advancement in HPHT technology came with Hall's 1954 development of the belt apparatus at General Electric, which solved fundamental engineering challenges preventing earlier success in diamond synthesis [10]. This design distributed pressure more uniformly than previous systems while providing superior temperature control throughout growth chambers, enabling reproducible diamond synthesis [10]. The successful synthesis of diamond on December 16, 1954, marked the transition of diamond synthesis from laboratory curiosity to industrial reality [10].

Following this breakthrough, the technology diversified into two principal designs for commercial HPHT production: the belt press and the cubic press. The belt apparatus employs a cylindrical pressure vessel that applies uniform force via hydraulic systems, while cubic press designs employ six anvils in a cubic geometry [10]. The technical specifications and applications of these systems differ significantly:

Table 1: Comparison of Belt Press and Cubic Press Technologies

Equipment Specification	Belt Press Technology	Cubic Press Technology
Production Capacity (carats/month)	2,500 - 4,000	800 - 1,200
Pressure Uniformity	±2% across chamber	±1% across chamber
Growth Chamber Size	25-40mm diameter	15-20mm diameter
Maximum Pressure Capacity	6.5 GPa (943,000 psi)	7.2 GPa (1,044,000 psi)
Temperature Range	1200-1700°C	1300-1650°C
Cycle Time (hours)	120-200	80-150
Equipment Cost (USD)	$2.8M - $4.2M	$1.8M - $2.5M
Maintenance Frequency	Every 500 cycles	Every 300 cycles
Annual Maintenance Cost	$180,000 - $250,000	$220,000 - $300,000
Energy Consumption (kWh/cycle)	15,000 - 22,000	12,000 - 18,000
Operator Training Required	6-8 weeks	4-6 weeks
Best Application	High-volume commercial production	Precision small-batch synthesis

The United States HPHT diamond cubic press market reflects this technological progression, with current growth driven by increasing investments in research and development and the rising adoption of synthetic diamonds for commercial and industrial purposes. Market analysts project a compounded annual growth rate (CAGR) of approximately 7-9% over the next five years, reflecting strong industry confidence and continuing technological advancements [11].

Modern HPHT Apparatus and Capabilities

Contemporary Large-Volume Press Designs

Modern large-volume presses have evolved through continuous refinement of multianvil geometries. The Kawai-type (DIA) apparatus and related multianvil systems represent the current state-of-the-art for large-volume HPHT research [9]. These systems utilize a complex arrangement of anvils to generate truly multi-axial compression, significantly improving pressure distribution and maximum achievable pressures compared to uniaxial systems.

Recent innovations include the development of the 6-8 type multianvil system, which combines six inner anvils with eight outer anvils to extend the pressure range while maintaining relatively large sample volumes. These systems have demonstrated the ability to reach pressures up to 65 GPa using tungsten carbide anvils, bridging the gap between conventional large-volume presses and diamond anvil cells [9]. The toroidal diamond anvil cell represents another innovative design, allowing for detailed measurements under extreme static pressures with improved access for in-situ characterization [9].

The integration of these advanced press designs with synchrotron radiation sources has been particularly transformative, enabling real-time investigation of materials behavior under extreme conditions. This combination has dramatically increased our knowledge of materials synthesis and transformation processes at high pressures and temperatures [9].

Ultra-High-Pressure Spark Plasma Sintering (UHP-SPS)

A recent significant advancement in HPHT technology is the development of Ultra-High-Pressure Spark Plasma Sintering (UHP-SPS), which combines a belt-type high-pressure apparatus with a conventional pulse electric current generator [12]. This innovative equipment allows processing up to 6 GPa, significantly higher than conventional HP-SPS systems based on modified SPS equipment.

UHP-SPS has demonstrated remarkable capabilities for sintering binderless polycrystalline diamond at unexpected P-T conditions—typically ~10 GPa lower pressure and 500-1000°C lower temperature than required in typical HPHT setups [12]. This non-equilibrium processing approach makes UHP-SPS a promising tool for sintering other high-pressure materials and represents a potential pathway for industrial transfer with reduced environmental impact.

Table 2: Technical Specifications of HPHT Synthesis Methods

Specification	HPHT Lab-Grown	CVD Lab-Grown	Natural Diamond
Pressure (GPa)	5.0-6.0	0.01-0.1	4.5-6.0 (natural formation)
Temperature (°C)	1300-1600	700-1200	900-1300 (natural formation)
Growth Rate (mm/day)	0.1-10	0.01-0.1	0.000001 (geological time)
Equipment Cost (USD)	$500K-$2M	$200K-$800K	Mining infrastructure $50M+
Production Time	7-14 days	14-28 days	1-3 billion years
Crystal Quality	VVS-SI, occasional inclusions	IF-VVS, fewer inclusions	Variable, IF-I3
Maximum Size (ct)	20+ carats	15+ carats	3000+ carats (rare finds)
Energy Consumption	High (pressure systems)	Medium (plasma generation)	Very High (mining operations)
Typical Colors	Colorless, yellow, blue	Colorless, brown, pink	Full spectrum
Catalyst Required	Metal (Fe, Ni, Co)	None	Natural minerals

Research Reagent Solutions for HPHT Synthesis

Successful HPHT synthesis requires carefully selected materials and reagents optimized for extreme conditions. The following table details essential research reagents for HPHT experiments:

Table 3: Essential Research Reagents for HPHT Synthesis

Reagent/Material	Function	Specifications & Considerations
Metallic Flux (Fe, Ni, Co alloys)	Solvent-catalyst for diamond synthesis	High purity (>99.97%) reduces inclusions; Specific alloys control growth rate and crystal morphology [10] [13]
High-Purity Graphite	Carbon source for diamond synthesis	Crystalline structure affects dissolution kinetics; Porosity influences carbon transport to growth zone [13]
Diamond Seed Crystals	Nucleation sites for epitaxial growth	0.5-2 mm specimens; Crystallographic orientation determines final crystal structure and properties [10]
h-BN Insulation	Thermal and electrical insulation within cell	Maintains temperature gradients; Minimizes contamination from pressure medium
ZrO₂ Pressure Medium	Transmits quasi-hydrostatic pressure to sample	Low thermal conductivity helps maintain thermal gradients; Phase stability at target P-T conditions
NaCl Pressure Medium	Hydrostatic pressure transmission	Ductile behavior provides excellent hydrostaticity; Electrical insulator for SPS applications
WC/Co Anvils	Pressure generation and containment	High compressive strength (>5 GPa); Fracture toughness critical for safety and maximum pressure
Pyrophyllite/Gaskets	Pressure containment and seal formation	Thermally stable; Controlled deformation properties essential for pressure sealing

Experimental Protocols for HPHT Diamond Synthesis

Standard Temperature Gradient Growth Method

The temperature gradient growth method is the predominant technique for producing high-quality single crystal diamonds via HPHT. The following protocol outlines the critical steps for successful implementation:

Apparatus Setup:

• Assembly Preparation: Configure the HPHT press (belt or cubic type) according to manufacturer specifications. Ensure all hydraulic systems and electrical connections are properly maintained.
• Growth Cell Construction: Layer the required materials in a precise sequence within the pressure capsule:
  • Outer insulation sleeve (pyrophyllite or ceramic)
  • Graphite heater element with precisely controlled resistance
  • ZrO₂ or similar thermal insulation layers
  • Metal solvent cartridge (Fe-Ni-Co alloy) surrounded by high-purity graphite source material
  • Diamond seed crystals (0.5-2 mm) positioned in cooler region of cell
• Cell Loading: Carefully position the assembled growth cell in the central cavity of the HPHT apparatus, ensuring proper alignment with anvil surfaces.

Synthesis Parameters:

• Pressure Application: Gradually increase pressure to 5.0-5.5 GPa (50,000-55,000 atmospheres) at a controlled rate of approximately 0.5 GPa/hour to prevent fracture of components.
• Temperature Ramping: Once target pressure is stabilized, increase temperature to 1350-1450°C at a rate of ~200°C/minute.
• Gradient Establishment: Maintain a precise axial temperature difference of 20-50°C across the growth cell, with the carbon source region at higher temperature and seed crystals at lower temperature.
• Growth Phase: Maintain stable P-T conditions for 7-14 days, depending on target crystal size. Monitor system stability through in-situ power measurements and occasional pressure adjustments.
• Termination: Gradually reduce temperature (100°C/minute) followed by slow pressure release (0.2 GPa/hour) to prevent crystal fracture due to thermal or pressure shock.
• Recovery: Carefully extract the growth cell and recover synthesized diamonds using chemical etching (acid treatment) to remove the metal solvent matrix [10] [13].

Advanced Process Monitoring and Optimization

Modern HPHT synthesis increasingly incorporates sophisticated monitoring and control strategies to enhance yield and quality:

Carbon Transport Management:

• Implement real-time monitoring of temperature gradients to maintain optimal supersaturation at growth interfaces
• Control dissolution rates of carbon source through precise thermal profiling
• Utilize convective flow patterns within the solvent to enhance uniform carbon distribution [13]

Defect Mitigation:

• Maintain pressure stability within ±0.5% to prevent growth striations and inclusion formation
• Implement slow ramp rates during phase transitions to minimize thermal stress
• Optimize catalyst composition to reduce metallic inclusion incorporation [10]

Quality Validation:

• Characterize crystal quality through X-ray topography and photoluminescence spectroscopy
• Assess nitrogen content and distribution through UV-Vis and FTIR spectroscopy
• Evaluate dislocation densities through etch pit counting and X-ray diffraction analysis [13] [14]

Visualization of Press Evolution and Experimental Workflow

Historical Evolution of High-Pressure Apparatus

The following diagram illustrates the technological progression from early Bridgman anvils to modern large-volume press systems:

HPHT Diamond Synthesis Workflow

The experimental workflow for HPHT diamond synthesis involves multiple critical stages from cell preparation to crystal recovery:

The historical progression from Bridgman's pioneering anvil designs to modern large-volume presses represents a remarkable technological journey that has fundamentally expanded capabilities in materials synthesis and research. Contemporary HPHT apparatus now enable precise control over extreme pressure and temperature conditions, facilitating the production of advanced materials ranging from single-crystal diamonds for quantum applications to novel superconducting ceramics.

Future developments in HPHT technology will likely focus on several key areas: enhanced process monitoring through advanced in-situ diagnostics, improved energy efficiency through innovative cell designs, integration with computational modeling for process optimization, and the development of hybrid techniques combining HPHT with other synthesis approaches. The recent emergence of UHP-SPS technology demonstrates the continued potential for innovation in this field, suggesting that non-equilibrium processing at lower temperatures may open new pathways for materials synthesis.

For researchers engaged in multianvil HPHT syntheses, successful implementation requires meticulous attention to assembly protocols, precise control of pressure-temperature trajectories, and comprehensive material characterization. The protocols and specifications outlined in this application note provide a foundation for advancing research in this demanding yet richly rewarding field.

In multianvil high-pressure high-temperature (HPHT) synthesis research, the precise generation and control of extreme conditions are paramount. The multi-anvil apparatus (MAA), a type of Large Volume Press (LVP), is engineered to expose cubic-millimeter-sized samples to pressures exceeding 25 GPa and temperatures surpassing 2300 °C, enabling the simulation of planetary interiors and the synthesis of novel materials [15] [16]. The apparatus's efficacy hinges on the synergistic interaction of three fundamental components: the outer wedges, the inner anvils, and the pressure media. This application note delineates the roles, material properties, and functional interdependencies of these core components, providing researchers with a detailed framework for experimental design and execution.

Core Components and Their Functional Interplay

The multi-anvil apparatus operates on the fundamental principle that pressure (P) is force (F) applied over a unit area (A), expressed as P = F/A [15] [3]. A hydraulic press generates a substantial force, which is meticulously transmitted and focused through the component assembly to achieve high pressure on the sample.

Figure 1: Force transmission pathway in a Kawai-type multi-anvil apparatus.

Outer Wedges: The Force Transmission Framework

Primary Function: The outer wedges constitute the first stage of force transduction. They are engineered to convert the uniaxial force from the hydraulic ram into precisely directed inward forces that synchronously compress the inner anvil array [15] [3].

Material Composition and Properties:

• Material: Typically manufactured from hardened steel for exceptional compressive strength and structural integrity [17].
• Geometry: Arranged in a Kawai-type (6-8) or DIA geometry, these wedges feature angled faces that slide against each other. This specific geometry is the key to transforming the simple forward motion of the hydraulic press into the multi-directional inward movement required [15].

Research Considerations:

• Lubrication: To minimize frictional losses and prevent electrical conductance between the wedges and the pressure module wall, they are often lined with lubricated Mylar sheets or similar low-friction materials [17].
• Electrical Path: In heated experiments, specific copper foil connections can be established between the inner anvils and the outer wedges to create an electrical path for furnace current from the module's top to bottom plates [17].

Inner Anvils: The Final Pressure Intensifiers

Primary Function: The inner anvils serve as the final pressure intensifiers. They receive the force from the outer wedges and focus it onto a small-volume sample assembly, thereby generating extreme pressures [15].

Material Composition and Properties: The choice of anvil material is critical for achieving the target pressure and determines the practical upper pressure limit of the assembly.

Table 1: Common Inner Anvil Materials and Their Performance Characteristics

Material	Typical Truncation Edge Length (TEL)	Maximum Pressure (Approx.)	Key Characteristics
Tungsten Carbide (WC) [16] [17]	11 mm [17]	~25 GPa [16]	High compressive strength, good toughness, widely used for upper mantle conditions.
	5 mm / 3 mm [16]	Up to ~120 GPa (with advanced designs) [16]	Smaller TELs enable higher pressures.
Sintered Diamond [16]	Not Specified	>120 GPa [16]	Superior hardness and wear resistance, essential for lower mantle conditions.

Research Considerations:

• Anvil Alignment: Precise alignment of the inner anvils is crucial to prevent catastrophic failure and ensure a uniform pressure distribution on the sample assembly.
• Insulation: The cubic assembly of inner anvils is often covered with epoxy resin fiberglass (G10) sheets to electrically insulate them from the outer wedges [17].

Pressure Media: The Hydrostatic Environment Manager

Primary Function: The pressure medium, typically shaped as an octahedron, encapsulates the sample assembly. Its primary roles are to transmit pressure quasi-hydrostatically to the sample, contain the sample and furnace components, and flow controllably into the gaps between the anvils, where friction helps to retain the high pressure internally [15] [17].

Material Composition and Properties: The ideal pressure medium possesses a balance of softness, thermal stability, and electrical insulation.

Table 2: Common Pressure Media Materials and Their Applications

Material	Typical Edge Length (OEL)	Key Characteristics and Applications
MgO (+Cr2O3) [17]	18 mm (with 11 mm TEL anvils) [17]	30% porosity aids in compression; common in large-volume assemblies for planetary science [17].
Boron Epoxy [15] [3]	Varies	Soft, X-ray transparent, ideal for synchrotron studies [15].
Pyrophyllite [15] [17]	Varies	Functions as both a pressure medium and a gasket material between anvils to prevent blow-outs [17].

The Scientist's Toolkit: Essential Research Reagent Solutions

A successful HPHT experiment relies on a suite of specialized materials within the sample assembly.

Table 3: Key Materials for Multi-Anvil Sample Assemblies

Item	Function	Common Examples & Notes
Furnace	Heats the sample via electrical resistance.	Graphite (X-ray transparent), LaCrO3 (high-T stability) [15].
Confining Media	Protects sample, distributes pressure evenly.	MgO, BN (inert, high-melting); NaCl (soft, X-ray standard) [15].
Sample Jacket	Contains sample, prevents chemical reactions.	Pt, Au, Ni tubes; choice depends on X-ray transparency or volatile retention [15].
Thermocouple	Measures temperature in situ.	W-Re alloys (high strength); can be fragile [15].
Pressure Standard	Calibrates pressure via in-situ X-ray diffraction.	Materials with well-known Equations of State (EoS) like NaCl or MgO [15].

Experimental Protocol: Assembly, Compression, and Calibration

The following protocol details the setup and calibration of a Kawai-type multi-anvil apparatus, synthesizing methodologies from the search results.

Figure 2: Workflow for multi-anvil experiment setup and calibration.

Procedure

• Assembly Preparation:

  • Machine or procure the octahedral pressure medium (e.g., 18 mm OEL MgO octahedron with 5% Cr2O3) with a central hole [17].
  • Construct the internal sample assembly: Insert a zirconia (ZrO2) sleeve into the octahedron for thermal insulation. For a typical synthesis experiment, place a graphite furnace sleeve inside the zirconia, followed by a sample capsule (e.g., a metal jacket) surrounded by soft confining media like MgO [15] [17].

• Anvil and Gasket Setup:

  • Assemble the eight tungsten carbide (WC) inner anvils (e.g., 11 mm TEL) into a cubic configuration. Cover the cube with G10 sheets for electrical insulation [17].
  • Place pre-formed pyrophyllite gaskets between the anvils near their truncated edges. These gaskets are critical for preventing anvil-to-anvil contact and controlling the extrusion of the pressure medium [17].
  • Load the outer wedges and inner anvil cube into the Walker-type pressure module [17].

• Sample Loading and Module Closure:

  • Load the prepared sample assembly into the cavity formed by the eight inner anvils.
  • For electrical heating experiments, ensure proper connection of copper foils to establish the circuit [17].
  • Close the high-pressure module with its steel lid [17].

• Compression and Heating:

  • Place the sealed module into the hydraulic press.
  • Increase the oil pressure in the hydraulic vessel to drive the module upward, compressing the anvils and sample assembly. The pressure is increased to a predetermined load corresponding to the target pressure based on prior calibration [17].
  • Once the target pressure is reached, apply a electrical current to the graphite furnace to heat the sample to the desired temperature (e.g., >1500°C). Temperature can be monitored in situ with a W-Re thermocouple or controlled via a pre-determined power-temperature calibration curve [15].

Pressure Calibration Protocol

Accurate pressure determination is non-trivial due to significant friction losses. The following calibration method is standard [15] [17]:

• Principle: Use known phase transitions in standard materials at specific pressures and temperatures to bracket the pressure inside the cell.

• Materials:

  • Room Temperature Calibration: Bismuth (Bi). Detect phase transitions (Bi I-II at 2.55 GPa, Bi II-III at 2.69 GPa, Bi III-V at 7.66 GPa) via characteristic changes in electrical resistance during compression [17].
  • High-Temperature Quench Calibration: Silica (SiO2) or Calcium Germanate (CaGeO3). Identify phase transitions (e.g., α-quartz to coesite at ~3.2 GPa and 1200°C; coesite to stishovite at ~9.4 GPa and 1300°C; CaGeO3 garnet to perovskite at ~6.2 GPa and 1000°C) by analyzing the quenched products [17].

• Method:

  • Conduct a series of experiments at varying hydraulic oil pressures and fixed temperature.
  • After each experiment, recover the sample and use techniques like X-ray diffraction to identify the phases present.
  • Determine the oil pressure at which the phase transition occurs. This creates a calibration curve linking the known oil pressure to the actual sample pressure for that specific assembly design [17].
  • For the highest accuracy at synchrotron facilities, use an in-situ pressure standard (e.g., NaCl, MgO) with a well-known Equation of State (EoS) and measure its unit cell volume via X-ray diffraction to calculate pressure directly [15].

The multi-anvil apparatus is a complex system whose performance is dictated by the integrated function of its key components. The outer wedges provide the necessary force conversion, the inner anvils act as the final pressure multipliers, and the pressure media ensures a stable, contained high-pressure environment. A deep understanding of the materials, geometries, and interactions of these three elements—supported by rigorous calibration protocols—is fundamental for researchers aiming to design and execute successful HPHT syntheses and reliably push the boundaries of high-pressure science.

High-pressure, high-temperature (HPHT) synthesis represents a cornerstone technique in solid-state chemistry and materials science, enabling the discovery and growth of novel materials with unique properties. Framed within the broader context of multianvil research, this methodology allows scientists to access thermodynamic conditions unattainable through conventional synthesis, stabilizing phases and structures that are otherwise metastable or impossible to form at ambient pressure. The capability to operate across a spectrum from 3 GPa to over 90 GPa places a vast landscape of material properties within experimental reach, from novel superconductors to advanced multifunctional materials. This application note details the practical capabilities, protocols, and reagent tools essential for researchers embarking on HPHT synthesis, providing a foundational guide for exploring this pressure-temperature frontier.

High-Pressure Capabilities and Quantitative Specifications

Multianvil presses, a workhorse of HPHT synthesis, provide a versatile platform for a range of experimental conditions. These large-volume presses are capable of treating samples at pressures greater than 20 GPa and temperatures exceeding 2,000°C [18]. The resulting sample sizes are typically in the millimeter range, making them suitable for subsequent characterization via techniques like X-ray diffraction and electron probe microanalysis [18].

The following table summarizes the capabilities of different high-pressure apparatuses, illustrating the extended range of the pressure-temperature frontier.

Table 1: Capabilities of High-Pressure Synthesis Apparatuses

Apparatus Type	Typical Pressure Range	Typical Temperature Range	Sample Volume	Key Applications
Multianvil Press	>20 GPa to over 90 GPa [18]	>2,000°C [18]	Millimeter scale [18]	Synthesis of novel dense phases, diamond production [18]
Multianvil (Example Experiment)	6.5 GPa [19]	1,070 K (~797°C) [19]	Not Specified	Synthesis of specific HP-phases (e.g., HP-Co3TeO6) [19]
Optical Floating Zone (OFZ)	Ambient pressure to ~10 bar O2 partial pressure [20]	Up to crystal melting point	Large single crystals (e.g., 6 × 5 × 3 mm3) [20]	Single crystal growth of oxides, nickelates [20]

Detailed Experimental Protocol: Multianvil Synthesis of HP-Co₃TeO₆

The synthesis of a high-pressure polymorph of cobalt tellurate (HP-Co₃TeO₆) serves as an exemplary protocol for multianvil HPHT synthesis [19]. The detailed methodology, from precursor preparation to final characterization, is outlined below.

Precursor Synthesis

• Starting Materials: Cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O) and telluric acid (H₆TeO₆).
• Mixing and Calcination: A stoichiometric mixture of the precursors is homogenized and calcined at 770 K for 7 hours.
• Annealing: The resulting intermediate product is powdered and subsequently annealed multiple times (for 7 hours per step) in temperature intervals of 100 K, up to a final temperature of 970 K, to obtain phase-pure normal-pressure Co₃TeO₆ [19].

High-Pressure/High-Temperature Treatment

• Cell Assembly: The polycrystalline precursor is wrapped in platinum foil and placed into a crucible made of hexagonal boron nitride (h-BN). This assembly is then inserted into an octahedral pressure medium within a Walker-type module.
• Pressurization: The module is compressed to a target pressure of 6.5 GPa using a controlled ramp of 72 bar/h [19].
• Heating and Sintering: Once the target pressure is stabilized, the sample is heated rapidly (within 5 minutes) to a synthesis temperature of 1,070 K. This temperature is maintained for a dwell time of 20 minutes to facilitate the reaction and crystallization [19].
• Controlled Cooling: The sample is then cooled steadily to 670 K over 120 minutes. This controlled cooling rate is critical for preserving crystal quality and minimizing internal stresses.
• Decompression: Finally, the pressure is released slowly with a ramp of 24 bar/h to prevent damage to the recovered sample [19].

Post-Synthesis Analysis

• The powdered product of HP-Co₃TeO₆ is reported to be air-stable and appears dark greyish to purple [19].
• Initial characterization is performed using powder X-ray diffraction (PXRD) to confirm phase purity and identity the crystal structure via Rietveld refinement [19].
• Energy-dispersive X-ray spectroscopy (EDX) is used for semiquantitative elemental analysis to verify the sample composition against the expected stoichiometry [19].

Experimental Workflow and Signaling Pathways

The following diagram visualizes the logical sequence and decision points in a generic HPHT synthesis experiment, from precursor preparation to final analysis.

HPHT Synthesis Workflow

The Scientist's Toolkit: Essential Research Reagents and Materials

Successful HPHT synthesis relies on specialized materials that function under extreme conditions. The table below lists key reagents and their critical functions in a typical multianvil experiment.

Table 2: Essential Materials for Multianvil High-Pressure Synthesis

Material/Reagent	Function in Experiment	Specific Example
Hexagonal Boron Nitride (h-BN)	Crucible material; provides chemical inertness and thermal stability within the pressure assembly.	Used as an 18/11-assembly crucible [19].
Platinum (Pt) Foil	Sample capsule; encapsulates the precursor material, preventing contamination and reaction with the surrounding medium.	Used to surround the polycrystalline precursor [19].
Pressure Medium (e.g., MgO, ZrO₂)	Transmits hydraulic pressure uniformly from the anvils to the sample assembly.	Part of the standard octahedral assembly in a Walker module [19].
Precursor Chemicals	Source of elemental components for the target material.	Co(NO3)2·6H2O and H6TeO6 for HP-Co3TeO6 synthesis [19].
Sintering Aid (e.g., B2O3)	Optional additive to promote densification and crystal growth at lower temperatures.	Not used in the cited example, but common in other HP syntheses.

In the field of high-pressure research, the diamond anvil cell (DAC) has long been a cornerstone tool, enabling extreme static pressures exceeding 100 GPa for detailed material characterization [21]. However, for high-pressure, high-temperature (HP-HT) synthesis, two significant limitations persist: the inherently small sample volumes and challenges in maintaining a stable, homogeneous temperature field, particularly for prolonged experiments [22]. The multi-anvil press (MAP) addresses these challenges directly, offering larger sample volumes and a more stable thermal environment, which are critical for the synthesis of novel materials, including pharmaceuticals and advanced alloys [22] [23]. This application note details the specific advantages of the MAP system, providing quantitative comparisons and detailed protocols for researchers engaged in multianvil HP-HT syntheses.

Comparative Analysis: Multi-Anvil Press vs. Diamond Anvil Cell

The selection of a high-pressure apparatus is a fundamental decision that dictates the scope and quality of an experiment. The table below summarizes the key performance characteristics of Multi-Anvil Presses and Diamond Anvil Cells.

Table 1: Quantitative Comparison of High-Pressure Apparatus Characteristics

Characteristic	Multi-Anvil Press (MAP)	Diamond Anvil Cell (DAC)
Typical Sample Volume	Large-volume samples; capability for "macroscopic" samples of a few micrometers for neutron diffraction [21] [24].	"Thinner samples" on a micrometer length scale; sample chambers of a few μm in dimensions [21].
Temperature Stability & Homogeneity	Provides a "stable and homogeneous temperature field"; with advanced controllers, temperature fluctuations can be within ±2 °C [22].	Temperature field stability not specifically quantified in results; laser heating is common for achieving high temperatures [25].
Maximum Pressure	Up to 120 GPa reported [22].	Conventional DAC: ~400 GPa limit; Toroidal-DAC: up to 603 GPa demonstrated [21].
Maximum Temperature	Up to 4000 K [22].	Limited primarily by laser heating system capabilities; temperatures over 2000 K are achievable [25] [23].
Primary Applications in Synthesis	Simulation of planetary interiors, synthesis of novel materials, and study of physical properties under extreme conditions [22].	Detailed exploration and material synthesis up to the TPa range; studies of phase behavior and melting [21] [25].

The Scientist's Toolkit: Essential Research Reagent Solutions

Successful HP-HT synthesis relies on a carefully selected assembly of components. The following table lists key materials and their critical functions within a multi-anvil experiment.

Table 2: Essential Materials and Their Functions in Multi-Anvil Experiments

Item	Function / Explanation
LaCrO₃ Heater	A heater material capable of reaching temperatures exceeding 3000 K at pressures above 3 GPa; provides a more uniform temperature field compared to other heaters [22].
Pressure Medium (e.g., MgO doped with Cr₂O₃)	The solid medium that encapsulates the sample; it transmits pressure quasi-hydrostatically to the sample and serves as thermal and electrical insulation [22].
Rhenium (Re) Gasket	A metal foil pre-indented to create a sample chamber; it contains the sample and pressure medium, preventing extrusion under extreme load [21] [23].
W/Re Thermocouple (e.g., D-type)	Placed at the geometric center of the assembly for direct and precise sample temperature measurement [22].
Nanopolycrystalline Diamond Micro-Anvils	Used in double-stage DACs to extend the pressure range; noted for their superior mechanical properties, though reproducibility can be challenging [21].

Experimental Protocol for High-Pressure Synthesis Using a Multi-Anvil Press

The following workflow and detailed protocol outline the key steps for conducting a high-pressure, high-temperature synthesis experiment in a multi-anvil press, with a focus on achieving temperature stability.

Figure 1: HP-HT synthesis workflow in a multi-anvil press.

Detailed Procedure

• Assembly Preparation

  • Pressure Medium: Machine an octahedron from MgO doped with 5 wt. % Cr₂O₃ to serve as the pressure-transmitting medium [22].
  • Sample Loading: Place the powdered sample reactants (e.g., Nd and Re metal for alloy synthesis) inside the prepared chamber within the pressure medium [23].
  • Heater and Thermocouple: Position a cylindrical LaCrO₃ heater (e.g., S6 type) around the sample chamber. Insert a W97Re3-W75Re25 (D-type) thermocouple with its junction at the geometric center of the assembly to ensure accurate temperature measurement. Fill the heater's interior with machinable MgO to provide mechanical support [22].

• Cell Assembly & Gasketing

  • Assemble the components within the multi-anvil stack.
  • Use a rhenium gasket that has been pre-indented to the desired initial thickness. The gasket material confines the sample assembly and prevents extrusion under high load [21] [23].

• Application of Pressure and Temperature

  • Compression: Increase the pressure smoothly and slowly to the target value (e.g., 23 GPa), allowing the gasket and anvil tips to relax under high load. Monitor pressure using known standards, such as the fluorescence of a ruby ball or the X-ray diffraction pattern of a metal like gold [21] [23].
  • Heating and Stabilization: Engage the specialized temperature control system (e.g., the MASTer controller). Use its "gentle approach control strategy" to ramp the temperature to the target (e.g., 2,200 K for NdRe₂ synthesis). The system should stabilize power fluctuations within ±0.1 W and temperature fluctuations within ±2 °C by using fast, precise measurements and gradual voltage adjustments [22] [23].

• Reaction, Quenching, and Decompression

  • Synthesis: Maintain the stable HP-HT conditions for a duration sufficient for the reaction to complete, as confirmed by in-situ X-ray diffraction [23].
  • Quenching: Rapidly terminate the reaction by cutting power to the heater, "quenching" the sample to preserve its high-temperature structure.
  • Decompression: Slowly decompress the entire assembly to ambient pressure to recover the synthesized product for ex-situ analysis [23].

The multi-anvil press stands as a powerful apparatus for high-pressure, high-temperature synthesis, effectively overcoming the limitations of small sample volume and temperature instability associated with diamond anvil cells. Its capacity for large sample volumes enables more robust material synthesis and detailed characterization using techniques like neutron diffraction. Furthermore, the development of advanced temperature controllers ensures the precise and stable thermal environment required for reproducible synthesis of novel materials, from pharmaceuticals to advanced alloys. For research focused on high-yield synthesis and detailed property measurement under extreme conditions, the multi-anvil press offers a superior and often indispensable platform.

HP-HT Synthesis in Practice: From Earth's Mantle to Industrial Materials

Multianvil high-pressure, high-temperature (HP-HT) systems are foundational apparatuses in solid-state physics, geoscience, and materials synthesis research. These systems enable the investigation of material behaviors and the synthesis of novel phases under extreme conditions that mimic planetary interiors or create previously inaccessible compounds. The core function of this technology is to compress a sample volume uniformly from multiple directions, thereby achieving static pressures ranging from a few gigapascals (GPa) to over 100 GPa, combined with temperatures exceeding 2500°C [1]. The Kawai-type multianvil apparatus (KMA), utilizing a double-stage multi-anvil system, represents a significant evolution in this field, first developed by Professor Naoto Kawai [1]. This design philosophy of using multiple anvils to converge pressure uniformly onto a sample volume has become a cornerstone for large-volume press (LVP) technology, providing larger sample volumes and more homogeneous temperature distributions compared to diamond anvil cells [1].

System Components and Their Functions

The efficient generation of ultrahigh pressures relies on the precise integration of three primary subsystems: the guide blocks that maintain alignment, the anvil assemblies that transmit force, and the hydraulic press that supplies the primary load.

Guide Blocks

Guide blocks are the first-stage anvils and alignment system that house and constrain the second-stage anvils. Their primary function is to ensure that the compressive force from the hydraulic ram is distributed evenly and precisely onto the anvil assembly, preventing misalignment that could lead to catastrophic failure at ultrahigh pressures.

• Split-Sphere Design (Kawai-Type): The original design used eight first-stage anvils split from a tungsten carbide (WC) sphere, which compressed an octahedral pressure medium [1]. This was later refined by setting the first-stage anvils within paired guide blocks or in an oil reservoir to improve handling and alignment of the central cubic anvil assembly, known as the Kawai cell [1].
• Walker-Type Module: A widely used design employs a cylindrically constrained module where guide blocks align the six first-stage anvils that compress the eight second-stage WC anvils [26]. This robust design is used in presses with force capacities ranging from 600 to 2000 tons [26].
• Split-Cylinder Design: This variation uses a guide block system comprised of a split cylinder to house the anvils, offering an alternative method for applying uniform pressure from the hydraulic press onto the Kawai cell [1].

Anvil Assemblies

The anvil assembly, or the second-stage anvil system, is the core component where pressure multiplication occurs. It typically consists of eight cubic anvils, each with a corner truncation, arranged to form a cubic cavity. An octahedral pressure medium containing the sample is placed within this cavity.

• Anvil Materials: The choice of material dictates the maximum achievable pressure.
  • Tungsten Carbide (WC): The standard anvil material for pressures up to approximately 25-28 GPa. Recent developments with ultra-fine grains and minimal binders (e.g., Fujilloy TJS01) have pushed these limits, enabling pressures up to 50 GPa and even 65 GPa with specialized tapered cubes [1].
  • Sintered Diamond (SD): For pressures exceeding the capabilities of WC, sintered diamond anvils are used. SD is a composite of diamond powder with a metallic binder, offering superior hardness and strength. Using SD anvils, KMAs can routinely achieve pressures of 40-50 GPa and have been confirmed to reach over 120 GPa in conjunction with synchrotron X-ray diffraction [16] [1].
• Truncation Edge Length (TEL): The size of the truncated corner on the cubic anvil is a critical design parameter. A smaller TEL creates a smaller sample chamber but enables higher pressure generation for a given applied force.

Table 1: Comparison of Second-Stage Anvil Materials in KMA

Material	Maximum Pressure (GPa)	Relative Hardness	Key Characteristics	Typical Applications
Tungsten Carbide (Standard)	~25-28 GPa	High	High toughness, cost-effective, widely available	Phase equilibria studies, material synthesis up to the lower mantle transition zone [1]
Tungsten Carbide (Advanced, e.g., TJS01)	Up to 65 GPa	Very High	Ultra-fine grain, minimal binders, more brittle	Extended pressure range for mineral physics and synthesis [1]
Sintered Diamond (SD)	>120 GPa	Ultra-High	Exceptional hardness, lower toughness, higher cost	Ultrahigh-pressure synthesis, lower mantle mineralogy studies [16] [1]

Hydraulic Press Systems

The hydraulic press provides the primary uniaxial force required to drive the multi-anvil compression. The press force is measured in tons and is a key determinant of the system's capability.

• Force Capacity: Laboratory-scale multianvil systems typically range from 500-ton to 2000-ton presses. For example, systems at ETH Zurich include a 1000-ton conventional press and a 600-ton press mounted on a rotatable cradle [26]. The highest-pressure generation with SD anvils often requires press capacities of 1000-1500 tons or more [1].
• Pressure Control: Modern systems use worm-gear-type automatic pressure control systems to manage the press load with high precision, which is crucial for stable experiments over long durations [26].

Figure 1: Force transmission logic in a Kawai-type multianvil apparatus.

Quantitative Performance Data

The performance of a multianvil apparatus is a function of the anvil material, the TEL, the composition of the pressure medium, and the available press force.

Table 2: Pressure Generation Capabilities of Kawai-Type Multianvil Apparatus

Second-Stage Anvil Material	Truncation Edge Length (TEL)	Approx. Press Load	Achievable Pressure	Source/Example
Tungsten Carbide (Standard)	3-5 mm	Not Specified	4 - 25 GPa	[26]
Tungsten Carbide (Fujilloy TF05)	Not Specified	Not Specified	~41 GPa	[1]
Tungsten Carbide (Fujilloy TJS01)	Not Specified	Not Specified	~50 GPa	[1]
Tungsten Carbide (Tapered TJS01)	Not Specified	Not Specified	~65 GPa	[1]
Sintered Diamond (SD)	5 mm	Not Specified	~41 GPa (Room Temp)	[1]
Sintered Diamond (SD)	Not Specified	Not Specified	>120 GPa	[16]
Sintered Diamond (SD) - Split-Sphere	Not Specified	Not Specified	Up to 40 GPa	[26]

The relationship between the anvil truncation and pressure generation is inverse; smaller TELs yield higher pressures but reduce the sample volume. This trade-off is critical for experimental design, especially when the synthesized material quantity is important for subsequent analysis.

Experimental Protocol for High-Pressure Synthesis

The following protocol outlines a standard procedure for synthesizing high-pressure silicate polymorphs, such as those found in the Earth's mantle, using a Kawai-type multianvil apparatus.

Preparation of the High-Pressure Cell Assembly

• Pressure Medium Fabrication: Machine an octahedron from a low-compressibility, thermally insulating material such as magnesium oxide (MgO) or a composite of MgO and dopants (e.g., Cr2O3). The size of the octahedron corresponds to the TEL of the anvils.
• Furnace Assembly: Inside the octahedron, assemble a cylindrical resistive heater, typically made from graphite, lanthanum chromite (LaCrO3), or a noble metal like rhenium, depending on the target temperature and chemical environment.
• Sample Loading: Place the starting material (e.g., a silicate glass or oxide mixture with the target composition) inside the heater. The sample may be a powder, a sintered pellet, or a single crystal seed for crystal growth.
• Sensor Integration: Incorporate a WRe thermocouple (Type C, D, or W) to monitor and control temperature during the experiment. For precise pressure determination, include an internal pressure standard, such as a small piece of MgO, whose unit cell volume under pressure is well-calibrated.
• Gasketing and Assembly: Place the fully assembled octahedral pressure medium into the cubic cavity formed by the eight truncated second-stage anvils. Use gaskets made from pyrophyllite or other soft materials at the anvil interfaces to facilitate pressure retention and control extrusion during compression.

Pressurization and Heating

• Assembly Loading: Position the entire anvil assembly (Kawai cell) precisely within the guide blocks of the hydraulic press.
• Compression: Increase the press load gradually at a controlled rate (e.g., 1-2 tons per minute) to compress the assembly. Monitor the press load, which is correlated with the sample pressure via pre-calibrated curves.
• Heating: Once the target press load is reached, apply power to the internal heater to increase the sample temperature. Use the thermocouple reading for feedback control to maintain a stable temperature profile.
• Synthesis Phase: Maintain the target pressure and temperature (P-T) conditions for a duration sufficient for the reaction or crystal growth to complete. This can range from minutes to tens of hours. For in situ studies, synchrotron X-ray diffraction can be used to monitor phase transitions in real time [27].

Decompression and Recovery

• Cooling: Turn off the power to the heater and allow the sample to quench to room temperature while maintaining pressure. This rapid cooling helps preserve high-pressure phases.
• Decompression: Slowly release the press load at a controlled rate to prevent shock-induced cracking of the anvils or the synthesized sample.
• Sample Recovery: Dismantle the anvil assembly and carefully extract the pressure medium to recover the synthesized sample for ex situ analysis (e.g., electron microscopy, X-ray diffraction).

Figure 2: High-pressure synthesis experimental workflow.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Essential Materials for Multianvil High-Pressure Experiments

Item	Function	Common Examples & Specifications
Second-Stage Anvils	Directly compress the sample volume; material defines pressure limit.	Tungsten Carbide (WC) cubes (e.g., 32 mm edge length); Sintered Diamond (SD) cubes for >30 GPa [26] [1].
Pressure Medium	Transmits quasi-hydrostatic pressure from anvils to the sample.	Prefabricated or castable MgO octahedra; Cr2O3-doped MgO for higher strength [26].
Furnace/Heater	Heats the sample to the target temperature.	Cylindrical graphite (reducing), LaCrO3 (oxidizing), or Rhenium (high T, inert) heaters [16].
Thermocouple	Measures and controls temperature inside the cell.	W3%Re/W25%Re (Type C) or W5%Re/W26%Re (Type D); maximum T ~ 2300-2500°C [16].
Internal Pressure Standard	Provides in situ pressure measurement via X-ray diffraction.	MgO, Au, Pt, or NaCl; their pressure-volume equations of state are well-characterized [27].
Gasket Material	Seals the anvil interfaces, controls pressure gradient and extrusion.	Pyrophyllite, amorphous boron, or composite materials [1].
Starting Materials	Precursors for synthesizing the target high-pressure phase.	Oxide powders (e.g., MgO, SiO2), glasses, or pre-sintered pellets of target composition [16].

In multianvil high-pressure high-temperature (HP-HT) synthesis, the sample assembly—comprising the octahedral pressure medium, furnace, and insulating components—is a critical determinant of experimental success. This integrated system defines the pressure and temperature uniformity, chemical environment, and operational limits within the large-volume press [6] [28]. The design dictates the stability of the pressure-temperature (P-T) conditions essential for synthesizing novel materials, such as superhard boron carbides and multiferroic compounds, which are inaccessible through conventional chemistry [28]. This application note details the design principles and practical protocols for constructing robust sample assemblies, providing a foundation for advanced materials research under extreme conditions.

Technical Specifications and Material Selection

Octahedral Pressure Media

The octahedron serves as the primary pressure-transmitting medium, containing the sample and internal components within the multianvil apparatus. Its material selection is paramount for achieving high-pressure generation.

Table 1: Common Materials for Octahedral Pressure Media

Material	Maximum Practical Pressure (GPa)	Key Characteristics	Typical Applications
MgO-Doped Ceramics (e.g., Cr₂O₃-doped MgO)	~25	Good machinability, moderate strength	Standard HP-HT synthesis, moderate P-T conditions [28]
Sintered Magnesium Oxide (MgO)	>25	High hardness, superior pressure generation	High-pressure synthesis exceeding 25 GPa [6]
Partially Stabilized Zirconia (PSZ)	>25	High fracture toughness, low thermal conductivity	Experiments requiring enhanced thermal insulation [28]

Recent breakthroughs have demonstrated that sintered MgO octahedra, coupled with advanced anvil designs, can extend the pressure limit of Kawai-type multi-anvil presses with tungsten carbide anvils to 65 GPa at room temperature and about 50 GPa at high temperatures [6].

Furnace and Insulation Systems

The internal heater and insulation create a controlled high-temperature environment while minimizing heat loss to the anvils.

Table 2: Furnace and Insulation Configurations for Different Temperature Ranges

Furnace Type / Heater Material	Maximum Temperature Range	Thermal Insulation Materials	Suitability
Graphite Furnace	Up to ~1800°C (in inert environment)	MgO, ZrO₂, Castable Ceramic Insulation	General-purpose HP-HT synthesis, reducing conditions [28]
Metal Furnaces (e.g., LaCrO₃, Molybdenum)	Up to ~2200°C	MgO, Al₂O₃	Oxidizing conditions (LaCrO₃), very high temperatures [29]
Resistive Wire Furnaces (One-Atmosphere)	Up to 2200°C (special design)	-	Gas-mixing experiments at controlled oxygen fugacity [29]

For thermal management, low-thermal-conductivity materials like MgO or ZrO₂ are commonly used as filler and liner materials. In some assemblies, NaCl can serve a dual purpose as both a pressure-transmitting medium and thermal insulator [30]. The integration of high thermal insulation is a key factor for successful high-pressure generation at elevated temperatures [6].

Experimental Protocols

Protocol 1: Assembly for Room-Temperature High-Pressure Synthesis

This protocol outlines the assembly for experiments targeting ultra-high pressures (e.g., >50 GPa) prior to heating.

• Octahedron Preparation: Machine a sintered MgO octahedron to the required dimensions (e.g., 8mm, 10mm, or 14mm edge length) [6].
• Gasket and Anvil Setup: Employ a precisely aligned guide-block system. Use second-stage anvils with tapered faces and a high degree of hardness [6].
• High-Pressure Cell Assembly:
  • Within the pre-drilled axial hole of the MgO octahedron, insert a high-pressure cell crafted from a material with a high bulk modulus [6].
  • The cell should contain the sample and a solid, quasi-hydrostatic pressure-transmitting medium (e.g., NaCl or MgO).
• Compression: Assemble the octahedron within the anvils and compress using a Kawai-type multi-anvil press to the target pressure.

Protocol 2: Assembly for High-Pressure/High-Temperature (HP-HT) Synthesis

This protocol is for simultaneous application of high pressure and high temperature, typical for synthesizing new materials like transition metal phosphides or boron carbides [30] [31].

• Octahedron and Furnace Assembly:
  • Place a cylindrical resistive furnace (e.g., graphite or metal) within the central cavity of a Cr₂O₃-doped MgO octahedron.
  • Ensure the furnace is surrounded by thermal insulation layers (e.g., MgO or ZrO₂ sleeves) to minimize radial heat loss.
• Sample Capsule Loading:
  • Seal the starting materials (e.g., mixed metal and phosphorus powders) within an air-tight capsule (e.g., gold, platinum, or h-BN) to prevent contamination and volatile loss [31].
  • Position the capsule axially within the furnace, packed with additional insulating MgO powder.
• Pressure and Temperature Application:
  • Assemble the octahedron into the multi-anvil press and compress to the target pressure (e.g., 4-8 GPa for phosphide synthesis) [31].
  • Once the target pressure is stabilized, pass an electrical current through the furnace to ramp temperature to the target (e.g., 900-1000°C) [31].
• Quenching and Recovery:
  • After the desired dwell time, terminate the power to the furnace for rapid temperature quenching.
  • Slowly decompress the assembly to ambient pressure and recover the synthesized sample.

The workflow for this assembly process is as follows:

The Scientist's Toolkit: Key Research Reagents and Materials

Table 3: Essential Materials for HP-HT Sample Assemblies

Item	Function/Purpose	Key Examples & Notes
Octahedron	Primary pressure-transmitting medium	MgO, Cr₂O₃-doped MgO, PSZ. Choice dictates maximum achievable pressure [6] [28].
Furnace/Heater	Generates high temperature within the cell	Graphite (reducing), LaCrO₃ (oxidizing). Must be stable at target P-T [29] [28].
Thermal Insulation	Minimizes heat loss, improves temperature uniformity	MgO, ZrO₂, Al₂O₃ (powder or sleeves). Low thermal conductivity is key [6].
Pressure Transmitting\nMedium (PTM)	Provides quasi-hydrostatic conditions around sample	NaCl, MgO, amorphous boron. Reduces pressure gradients [30].
Sample Capsule	Contains and isolates sample from assembly	Pt, Au, h-BN. Prevents reaction and contains volatile components [31].
Gasket Material	Seals the volume between anvils, supports pressure	Pyrophyllite, Cr₂O₃-doped MgO. Deforms plastically to contain the assembly.
Electrical Feedthroughs	Deliver power to the internal furnace	Molybdenum or tungsten rods. Must maintain electrical contact under compression.

The precision design of the sample assembly is the cornerstone of successful multianvil HP-HT research, enabling the synthesis of novel materials with exceptional properties. Future advancements will be driven by the integration of in situ characterization techniques, particularly synchrotron X-ray diffraction, which allows for real-time observation of phase transitions and reaction pathways [28]. The ongoing development of more robust octahedral media and high-temperature furnaces, coupled with advanced thermal insulation designs, continues to push the boundaries of accessible pressure-temperature space, opening new frontiers in the exploration of planetary interiors and the synthesis of next-generation functional materials.

The multianvil high-pressure high-temperature (HP-HT) apparatus is a cornerstone technology for simulating the physical and chemical conditions of Earth's deep interior, enabling the synthesis of major mineral phases found throughout the mantle. This methodology allows researchers to recreate pressure-temperature (P-T) conditions spanning from the upper mantle to the core-mantle boundary, facilitating direct experimental investigation of mineral stability, phase relations, and physical properties. The multianvil technique has been widely used for high-pressure and high-temperature experimental studies in Earth science, yielding many important results for understanding the Earth's components, structure, and evolution [32]. Unlike diamond anvil cells which offer higher pressures but smaller sample volumes, multianvil apparatuses expose cubic millimeter-sized samples to near-hydrostatic pressures exceeding 90 GPa with temperatures exceeding 2100°C when properly configured [32] [17]. This capability is essential for synthesizing adequate sample quantities for subsequent characterization of mineral properties and behavior under conditions relevant to planetary interiors.

The Multianvil Apparatus: Core Principles and Capabilities

Pressure Generation and Capabilities

The fundamental design of multianvil apparatuses employs a Walker-type module or Kawai-type geometry with multiple anvils (typically six outer wedges and eight inner cubic anvils) that converge to apply quasi-hydrostatic pressure to a sample assembly [17]. The attainable pressure in multianvil systems is principally limited by the strength of the anvil material. Standard tungsten carbide (WC) anvils generally limit attainable pressures to approximately 30 GPa, corresponding to depths of about 900 km in the Earth's mantle [32]. However, the introduction of sintered diamond cubes as second-stage anvils has dramatically enhanced the pressure generation capacity, with recent advancements achieving pressures exceeding 90 GPa (equivalent to approximately 2700 km depth) at room temperature using 14-mm sintered diamond anvils [32]. This technological advancement now enables experimental access to conditions spanning the entire mantle regime, including the core-mantle boundary.

Table 1: Pressure Capabilities of Multianvil Apparatus Configurations

Anvil Material	Maximum Pressure (GPa)	Equivalent Depth in Earth	Sample Volume	Primary Applications
Tungsten Carbide (WC)	~30 GPa	~900 km	Cubic millimeter	Upper mantle, transition zone minerals
Sintered Diamond	>90 GPa	~2700 km	Reduced volume	Lower mantle, core-mantle boundary phases

Temperature Generation and Control

Heating within the multianvil assembly is typically achieved through resistive heating using a graphite or refractory metal furnace housed within the pressure-transmitting medium. The 800-ton multianvil apparatus described in calibration studies can achieve temperatures exceeding 2100°C at high pressures, sufficient to simulate even the thermal conditions of the lowermost mantle [17]. Temperature measurement is accomplished using type C (W95Re5/W74Re26) thermocouples guided through alumina tubes to the sample vicinity, with the generated voltage interpreted through standard Seebeck effect relationships according to NIST standards [17]. The multianvil system offers superior temperature stability and uniformity compared to laser-heated diamond anvil cells, with temperature fluctuations typically within 5 K and pressure uncertainties of 0.05–1 GPa, making it particularly suitable for phase equilibrium studies [33].

Experimental Protocols for Deep Earth Mineral Synthesis

Apparatus Assembly and Calibration

Protocol 1: Initial Apparatus Setup and Pressure Calibration

• Assembly Preparation: Install six hardened steel outer wedges and eight cubic-inch-sized tungsten carbide (WC) inner anvils in Walker-type pressure module. Use 11 mm truncated edge length (TEL) anvils to enclose an octahedral sample assembly with 18 mm edge lengths (OEL) [17].

• Pressure Medium Assembly: Employ a Cr2O3-doped MgO octahedron (95% MgO + 5% Cr2O3, 30% porosity) as the primary pressure medium. Insert a zirconia (ZrO2) sleeve (4 mm inner diameter) into the central hole of the octahedron to serve as a thermal insulator [17].

• Gasket System: Position pyrophyllite gaskets (3.3/3.0 mm height/width) between inner anvils near truncated edges to prevent anvil contact and contain the pressure medium during compression. Apply PTFE tape behind gaskets to minimize extrusion during compression [17].

• Electrical Isolation: Cover cubically assembled inner anvils with G10 epoxy resin fiberglass sheets (0.65 mm thickness) to prevent electrical conduction between inner anvils and outer wedges [17].

• Pressure Calibration: Utilize known phase transitions in standard materials for pressure calibration:

  • Bismuth phase transitions (Bi I-II at 2.55 GPa, Bi II-III at 2.69 GPa, Bi III-V at 7.66 GPa) detected through electrical resistance measurements at room temperature [17].
  • SiO2 phase transitions (α-quartz to coesite at 3.2 GPa, coesite to stishovite at 9.4 GPa) determined through high-temperature quench experiments [17].
  • CaGeO3 transition (garnet to perovskite at 6.2 GPa) at 1000°C [17].

The following diagram illustrates the workflow for multianvil apparatus assembly and calibration:

Synthesis of Wadsleyite (β-(Mg,Fe)₂SiO₄)

Protocol 2: Wadsleyite Synthesis at Mantle Transition Zone Conditions

• Starting Material Preparation: Prepare homogeneous powder mixture of olivine composition (Mg0.9,Fe0.1)2SiO4 from high-purity oxide and carbonate precursors through repeated grinding and pre-sintering [34].

• Sample Loading: Encapsulate sample powder in a 2 mm long graphite sample bucket with 1 mm thick lid, placed within the zirconia sleeve and surrounded by crushable polycrystalline MgO parts [17].

• Pressure Application: Compress assembly to target pressure of 15-18 GPa, corresponding to the stability field of wadsleyite in the mantle transition zone (410-520 km depth) [34].

• Heating Protocol: Heat sample to temperatures of 1400-1800°C using graphite resistance heater, monitored by type C thermocouple with junction positioned immediately above sample container [17] [34].

• Reaction Duration: Maintain P-T conditions for 30-120 minutes to ensure complete phase transformation from olivine to wadsleyite, confirmed by in-situ X-ray diffraction where available [34].

• Quenching and Recovery: Rapidly terminate heating while maintaining pressure, then decompress system slowly to preserve high-pressure phase for ex-situ analysis [33].

Synthesis of Ringwoodite (γ-(Mg,Fe)₂SiO₄)

Protocol 3: Ringwoodite Synthesis at Higher Transition Zone Conditions

• Starting Material Options: Begin either with wadsleyite synthesized in Protocol 2 or with olivine of composition (Mg0.9,Fe0.1)2SiO4 identical to that used for wadsleyite synthesis [34].

• Pressure-Temperature Conditions: Compress to 18-23 GPa pressure range with temperatures of 1600-2000°C, corresponding to the ringwoodite stability field in the lower mantle transition zone (520-660 km depth) [33].

• Reaction Monitoring: For in-situ studies, monitor the wadsleyite to ringwoodite transformation through changes in X-ray diffraction patterns. In quench experiments, maintain conditions for sufficient duration to ensure complete transformation (typically 60-180 minutes) [33].

• Characterization: Recovered samples should be characterized by microfocused X-ray diffraction (MF-XRD), scanning electron microscopy (SEM) with backscattered electron imaging (BEI), and electron probe microanalysis (EPMA) to verify ringwoodite formation and assess chemical homogeneity [33].

Synthesis of Silicate Perovskite (Bridgmanite, (Mg,Fe)SiO₃)

Protocol 4: Bridgmanite Synthesis for Lower Mantle Conditions

• Compositional Considerations: Prepare starting materials with appropriate Mg-Si-Fe-Al-Ca ratios to match lower mantle chemistry. Typical bridgmanite contains significant Fe and Al substitutions, with ideal composition MgSiO3 but incorporating Fe2+, Fe3+, Al3+ through various substitution mechanisms [33].

• High-Pressure Assembly: For pressures exceeding 23 GPa (lower mantle conditions), utilize sintered diamond anvils to achieve required pressure conditions. Smaller sample assemblies may be necessary compared to upper mantle syntheses [32].

• Ultrahigh-Pressure Synthesis: Compress to target pressures of 23-135 GPa, covering the entire lower mantle regime, with temperatures of 1800-2500°C simulated using specialized resistive heating elements [32] [33].

• Stabilization and Quenching: Due to the metastability of some bridgmanite compositions at ambient conditions, careful pressure-temperature quenching paths may be necessary to recover samples for ex-situ analysis. In-situ characterization using synchrotron radiation is preferred for precise determination of phase relations [32].

• Cation Distribution Analysis: Employ transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS) and Mössbauer spectroscopy to determine Fe3+/ΣFe ratios and cation site occupancies in recovered samples, as these significantly influence bridgmanite properties [33].

Table 2: Synthesis Conditions for Major Deep Earth Minerals

Mineral	Crystal Structure	Pressure Range (GPa)	Temperature Range (°C)	Depth Range (km)	Key Chemical Considerations
Wadsleyite	Orthorhombic spinelloid	15-18 GPa	1400-1800°C	410-520 km	(Mg0.9,Fe0.1)2SiO4 composition; can incorporate ~1 wt% water [34]
Ringwoodite	Cubic spinel	18-23 GPa	1600-2000°C	520-660 km	(Mg0.9,Fe0.1)2SiO4 composition; higher water solubility than wadsleyite [33]
Bridgmanite	Orthorhombic perovskite	23-135 GPa	1800-2500°C	660-2700 km	MgSiO3 with Fe2+, Fe3+, Al3+ substitutions; multiple incorporation mechanisms [33]

The Scientist's Toolkit: Essential Research Reagents and Materials

Successful multianvil synthesis requires carefully selected materials for each component of the experimental assembly. The following table details essential research reagents and their functions in high-pressure mineral synthesis:

Table 3: Essential Research Reagents and Materials for Multianvil Synthesis

Material/Reagent	Function	Specifications	Alternative Options
Cr2O3-doped MgO octahedron	Pressure-transmitting medium	95% MgO + 5% Cr2O3, 30% porosity, 18 mm edge length	Pure MgO, ZrO2-based media [17]
Zirconia (ZrO2) sleeve	Thermal insulation	4 mm inner diameter, high-purity	Alumina (Al2O3) sleeves for lower temperature applications [17]
Graphite heater	Resistance heating element	Machined to fit assembly geometry	Lanthanum chromite, refractory metals for oxidizing conditions [17]
Pyrophyllite gaskets	Pressure containment and gasketing	3.3/3.0 mm height/width, machined to precise dimensions	Other phyllosilicates, composite gaskets [17]
Tungsten Carbide (WC) anvils	Primary pressure generation	Grade THM-U, cubic-inch size with 11 mm TEL	Sintered diamond anvils for ultrahigh pressure (>30 GPa) [32]
Type C thermocouple	Temperature measurement	W95Re5/W74Re26, alumina insulated	Type D (W97Re3/W75Re25) thermocouples [17]

Analytical Characterization of Synthesized Phases

Structural and Chemical Analysis

Comprehensive characterization of synthesized high-pressure minerals is essential for verifying successful synthesis and obtaining meaningful data. For mineralogical analysis, the following techniques are routinely employed:

• Microfocused X-ray Diffraction (MF-XRD): For phase identification and unit cell parameter determination on recovered samples [33].
• Scanning Electron Microscopy (SEM) with Backscattered Electron Imaging (BEI): To examine sample textures, grain sizes, and chemical zoning [33].
• Electron Probe Microanalysis (EPMA): For quantitative chemical analysis with precision better than 0.1 wt% for major elements, applicable to grains larger than 2-3 μm [33].
• Transmission Electron Microscopy (TEM) with Energy-Dispersive X-ray Spectroscopy (EDS): For nanoscale chemical analysis of finer-grained samples, particularly important for ultrahigh-pressure run products [33].
• Mössbauer Spectroscopy: For determining Fe3+/ΣFe ratios with precision of 0.02-0.05, crucial for understanding oxidation states in bridgmanite and other Fe-bearing phases [33].

In-situ Measurements at High P-T Conditions

For advanced research applications, in-situ measurements under high-pressure and high-temperature conditions provide invaluable data on material behavior:

• Synchrotron X-ray Diffraction: Real-time monitoring of phase transitions and equation of state determinations at simultaneous high P-T conditions [32].
• Ultrasonic Interferometry: Sound velocity measurements for deriving elastic properties, often combined with X-ray diffraction for simultaneous density determination [34].
• Brillouin Spectroscopy: Determination of single-crystal elastic properties at high pressures and temperatures, providing data on pressure derivatives of elastic moduli [35].

The following diagram illustrates the complete workflow from synthesis to characterization of deep Earth minerals:

Application to Earth Science Research

The synthesis of deep Earth minerals under controlled P-T conditions enables critical advances in understanding planetary interiors. Specifically, these experimental capabilities allow researchers to:

• Constrain Mantle Composition: By determining the phase relations of constitutive minerals like bridgmanite under varying P-T conditions, researchers can better understand the structure, dynamics and evolution of Earth's interior [33].
• Interpret Seismic Observations: Experimentally determined elastic properties of high-pressure phases at relevant conditions provide essential data for interpreting seismic velocity variations within the mantle [35] [34].
• Understand Mantle Dynamics: Phase transition boundaries, such as the olivine-wadsleyite transition at the 410 km discontinuity, directly influence mantle convection patterns and slab dynamics [34].
• Assess Water Storage Capacity: The ability of transition zone minerals (wadsleyite and ringwoodite) to incorporate significant water as hydroxyl groups affects our understanding of Earth's deep water cycle [35].

Through continued technical advancements in multianvil technology, particularly the implementation of sintered diamond anvils, the pressure frontier for large-volume press experiments continues to expand, enabling direct experimental investigation of previously inaccessible regions of Earth's deep interior [32]. These capabilities are fundamental to advancing our understanding of planetary formation, evolution, and dynamics.

The synthesis of advanced functional materials under high-pressure and high-temperature (HPHT) conditions represents a frontier in material science, enabling the creation of substances with exceptional properties unattainable through conventional methods. Within this domain, nano-polycrystalline diamond and cubic boron nitride (c-BN) stand out as superhard materials critical for industrial applications, from precision machining to advanced electronics [36] [27]. Their significance is further elevated when combined into composite structures, harnessing the superior hardness of diamond and the exceptional thermal stability and chemical inertness of c-BN [36] [37]. Framed within broader multianvil HPHT synthesis research, this application note details the protocols and quantitative data essential for synthesizing these advanced materials, providing a structured guide for researchers and scientists engaged in developing next-generation functional materials.

Key Material Properties and Performance Metrics

The pursuit of synthetic superhard materials is driven by the need for superior mechanical performance in extreme conditions. Nano-polycrystalline c-BN exhibits a Vickers hardness nearly twice that of its single-crystal counterpart, while sophisticated crystalline-amorphous composites can surpass the fracture toughness of traditional materials [36].

Table 1: Mechanical Properties of Selected Superhard Materials and Composites

Material Type	Vickers Hardness (GPa)	Fracture Toughness (MPa·m¹/²)	Key Characteristics
Single Crystal c-BN	~Half of natural diamond	N/A	Excellent thermal stability & chemical inertness [36]
Nano-Polycrystalline c-BN	~2x Single Crystal c-BN	N/A	High density of grain boundaries [36]
c-BN + Amorphous Diamond Composite	86.2	10.2	Homogeneous embedding; excellent crack deflection [36]
Polycrystalline Diamond	N/A	N/A	Hardest known material [27]

The integration of an amorphous diamond-like carbon phase within a dense c-BN matrix creates a composite material where the distinct strengthening mechanisms of crystalline and amorphous structures synergize. In this system, the amorphous diamond particles effectively impede crack propagation, forcing crack tips to deflect or be confined at the crystalline-amorphous boundaries. This mechanism simultaneously achieves ultra-high hardness and enhanced fracture toughness, a combination often mutually exclusive in single-phase materials [36].

Experimental Protocols for HPHT Synthesis

HPHT Synthesis of c-BN and Amorphous Diamond Composites

Principle: This protocol utilizes a Kawai-type multi-anvil press to achieve simultaneous phase transformation of hexagonal boron nitride (h-BN) to dense boron nitride phases (w-BN/c-BN) and the conversion of C₆₀ fullerene into amorphous diamond-like carbon, resulting in a superhard composite [36].

Procedure:

• Precursor Preparation:
  • Mix multi-walled boron nitride nanotubes (BNNTs) and C₆₀ fullerene in a 4:1 mass ratio.
  • Grind the mixture in a mortar for 30 minutes to ensure homogeneity.
  • Pre-press the resulting powder into a cylindrical shape (e.g., 1.6 mm diameter, 2 mm length) [36].

• High-Pressure Assembly:

  • Assemble the precursor cylinder within a standard octahedral cell assembly suitable for a Kawai-type multi-anvil press.
  • Use tungsten carbide anvils with a truncation edge length (TEL) of 3 mm [36] [5].

• HPHT Compression and Sintering:

  • Compress the assembly to a target pressure of 25 GPa.
  • Heat the sample to a temperature between 1,400 °C and 1,600 °C for 5-10 minutes. Note: The transformation of h-BN to its dense phases is typically complete by 1,400 °C [36].
  • Monitor pressure and temperature in real-time using in-situ synchrotron X-ray diffraction if available [27].

• Recovery and Processing:

  • Slowly quench the temperature to ambient.
  • Decompress the pressure gradually to recover the sintered bulk sample.
  • The final product is a compound of crystalline c-BN and amorphous diamond-like carbon [36].

Simultaneous Crystallization of Diamond and c-BN from BC₂N

Principle: This method employs a graphitic boron-carbon-nitrogen (BC₂N) precursor with a metal catalyst (Cobalt) under HPHT conditions. The process induces a disproportionating crystallization, yielding separate diamond and c-BN grains from the single precursor [37].

Procedure:

• Precursor Preparation:
  • Use graphitic BC₂N as the primary precursor.
  • Combine it with a pure Cobalt (Co) metal solvent catalyst [37].

• HPHT Synthesis:

  • Load the precursor mixture into a suitable pressure cell (e.g., a multi-anvil assembly).
  • Compress to a pressure of 5.5 GPa.
  • Heat to a temperature range of 1,400-1,600 °C and maintain for a specified dwell time [37].

• Product Analysis:

  • Recover the product and analyze using powder X-ray diffraction.
  • Confirm the simultaneous presence of two distinct cubic phases: diamond and c-BN, in approximately equal amounts [37].
  • Verify the composition and sp³ bonding of individual grains using electron energy-loss spectroscopy [37].

The Scientist's Toolkit: Essential Research Reagents and Materials

Successful HPHT synthesis relies on precise selection and control of precursors, catalysts, and equipment.

Table 2: Essential Research Reagents and Materials for HPHT Synthesis

Item	Function/Description	Example Use Case
BN Nanotubes (BNNTs)	Precursor for dense boron nitride phases; high-purity (99.9%) BNNTs with diameters of 20-50 nm are typical [36].	Synthesis of c-BN and amorphous diamond composites [36].
C₆₀ Fullerene	Carbon precursor for the formation of amorphous diamond-like carbon under HPHT [36].	Synthesis of c-BN and amorphous diamond composites [36].
Graphitic BC₂N	Single-source precursor containing B, C, and N for the simultaneous crystallization of diamond and c-BN [37].	Disproportionation synthesis of diamond/c-BN mixtures [37].
Cobalt (Co) Metal	Solvent catalyst that facilitates the hexagonal-to-cubic phase transformation in carbon and BN systems [37].	Catalyzed synthesis from BC₂N precursor [37].
Tungsten Carbide (WC) Anvils	First and second-stage anvils in multi-anvil presses; TEL of 3-5 mm is common for pressure generation up to ~25 GPa [16] [5].	General HPHT compression in multi-anvil apparatuses.
Sintered Diamond Anvils	Second-stage anvils for the highest pressure experiments (>50 GPa) in Kawai-type apparatuses [5].	Ultra-high-pressure synthesis.
Pyrophyllite Gasket	Gasket material placed between anvils to generate sample pressure efficiently and provide lateral support [5].	Containing the pressure medium in the cell assembly.

Workflow and Material Transformation Pathways

The synthesis of nano-polycrystalline diamond-cBN composites involves a precise sequence of preparation, compression, and transformation. The following diagram illustrates the core experimental workflow and the parallel material transformation pathways occurring during the HPHT process.

Apparatus Configuration for HPHT Synthesis

The multianvil apparatus, particularly the Kawai-type design, is the workhorse for generating the stable, high-pressure conditions required for synthesizing these materials. Its design enables precise control and in-situ monitoring.

The HPHT synthesis protocols for creating nano-polycrystalline diamond and c-BN composites demonstrate the power of extreme condition material science. The detailed methodologies—from precursor preparation and catalyst selection to precise control of pressure-temperature profiles in multi-anvil apparatuses—provide a reproducible framework for synthesizing these advanced functional materials. The resulting materials, characterized by their exceptional hardness and toughness, hold significant promise for advancing industrial applications and provide a foundation for future research. Future breakthroughs will likely emerge from optimizing crystalline-amorphous architectures, exploring novel precursor chemistries, and leveraging in-situ characterization techniques to further elucidate and control transformation mechanisms at the atomic scale.

In-situ synchrotron X-ray diffraction (XRD) represents a transformative advancement in analytical techniques for materials research, enabling real-time observation of phase transformations under extreme conditions. This technique is particularly invaluable within the context of multianvil high-pressure high-temperature (HPHT) synthesis, a method used to discover and synthesize novel materials mimicking deep-Earth conditions or possessing unique functional properties. Unlike conventional ex-situ methods that only capture static snapshots of pre- and post-transformation states, in-situ synchrotron XRD allows researchers to monitor the dynamic structural evolution of a material as it responds to varying pressure and temperature. The exceptional brightness and high photon flux of synchrotron radiation are critical for achieving the necessary temporal resolution—down to 10 milliseconds in some studies—to capture fast kinetic processes in solid-state reactions and phase transformations [38]. This capability provides a direct window into reaction mechanisms, transformation pathways, and kinetic parameters that are otherwise inaccessible, forming a critical database for testing and refining theoretical models of high-temperature solid-state phenomena [38] [39].

Theoretical Background and Significance

The primary advantage of in-situ synchrotron XRD in HPHT research lies in its ability to quantify the kinetics of phase transformations. Understanding these kinetics is essential for predicting material behavior under extreme conditions and for designing synthesis protocols for novel phases. The foundation for analyzing such transformation kinetics is provided by classical kinetic theories, notably the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model [39]. This model relates the transformed fraction of a material to time through a characteristic equation, enabling researchers to extract fundamental kinetic parameters such as the Avrami exponent (n), which offers insights into the nucleation and growth mechanisms (e.g., site-saturated nucleation versus continuous nucleation). The application of this model requires high-fidelity, time-resolved data on the phase fraction evolution, which is precisely what in-situ synchrotron XRD provides [39].

The significance of this approach was demonstrated in a time-resolved study of the solid combustion reaction Ti + C → TiC, where synchrotron XRD revealed that the formation of titanium carbide was completed within 400 milliseconds of the melting of the titanium metal [38]. In contrast, the Al + Ni → AlNi reaction exhibited a more complex pathway, with the appearance and disappearance of at least one intermediate phase before the final ordered AlNi structure formed approximately 5 seconds later [38]. These findings underscore that transformation mechanisms can vary dramatically between material systems, information that is critical for tailoring synthesis processes in multianvil research.

Experimental Setup and Instrumentation

The implementation of in-situ synchrotron XRD within HPHT environments requires specialized instrumentation capable of generating and sustaining extreme conditions while allowing the passage of high-energy X-rays.

High-Pressure Devices: The Paris-Edinburgh Cell

For pressure ranges relevant to multianvil synthesis (typically up to 25 GPa and beyond), the Paris-Edinburgh (PE) press is a workhorse device [39]. This large-volume press is designed for in-situ X-ray and neutron diffraction experiments. Its compact design and ability to use toroidal anvils or other composite anvil designs allow for significant sample volumes while maintaining access for the incident and diffracted X-ray beams. The setup at beamline ID27 of the European Synchrotron Radiation Facility (ESRF) [39] is a prime example, where a PE cell is integrated with a high-intensity synchrotron source for angle-dispersive XRD measurements. This configuration is ideal for collecting high-quality diffraction patterns with sub-second time resolution, enabling the tracking of phase transitions as a function of time at fixed pressure and temperature (isobaric-isothermal conditions).

Synchrotron Beamline Configuration

Two primary XRD techniques are employed at synchrotron beamlines for these studies:

• Energy-Dispersive X-ray Diffraction (EDXRD): Utilizes a white X-ray beam and a fixed diffraction angle. A solid-state detector measures the diffracted photons by their energy. This setup is geometrically compact, making it advantageous for complex sample environments like welding setups, and has been used to achieve data acquisition rates of 5 Hz (5 spectra per second) [40].
• Angle-Dispersive X-ray Diffraction (ADXRD): Employs a monochromatic X-ray beam and records the diffraction pattern as a function of angle using two-dimensional detectors like imaging plates or CCDs. This method, as used at ID27, generally provides higher resolution patterns and is preferred for detailed structural analysis [39].

Table 1: Key Specifications of Synchrotron XRD Techniques for HPHT Studies

Feature	Energy-Dispersive XRD (EDXRD)	Angle-Dispersive XRD (ADXRD)
X-ray Beam	White beam (polychromatic)	Monochromatic (single wavelength)
Measured Quantity	Photon energy at fixed angle (2θ)	Diffraction angle (2θ)
Typical Detector	Solid-state energy-sensitive detector	2D detector (e.g., Imaging Plate, CCD)
Key Advantage	Compact geometry, fast data collection	High resolution, superior pattern quality
Representative Application	Real-time monitoring of welding transformations [40]	Kinetics of mineral phase transformations [39]

Application Notes and Protocols

This section provides a detailed, step-by-step protocol for conducting an in-situ time-resolved XRD experiment to study a phase transformation in a multianvil apparatus, based on methodologies established at leading synchrotron facilities.

Sample Preparation and Loading

• Starting Material Preparation: The sample material (e.g., a powdered mixture of elemental metals or pre-synthesized precursor phases) must be finely ground and homogenized to ensure a uniform reaction and a representative diffraction pattern.
• Capsule Assembly: Load the homogeneous powder into a suitable capsule material that is inert under the target P-T conditions, such as graphite or hexagonal boron nitride (h-BN). The capsule also serves as a pressure-transmitting medium.
• Pressure Cell Assembly: Place the capsule into the pressure cell assembly of the Paris-Edinburgh press. This assembly typically includes a gasket and anvils (often made of tungsten carbide or sintered diamond composites). The cell might also incorporate a resistive heating system (e.g., a graphite or ceramic heater) to achieve high temperatures.
• Strain Monitoring: To account for the effects of non-hydrostatic stress, it is advisable to include a pressure marker material with a well-known equation of state (e.g., gold or sodium chloride) in the sample volume. This allows for in-situ pressure determination from its lattice parameters [39].

Data Acquisition Protocol

• Beamline Alignment: Align the pressure cell with the synchrotron X-ray beam. Precisely center the sample volume in the beam path and calibrate the detector distance and orientation using a standard reference material.
• Compression and Heating: Increase the pressure to the desired isobaric value. Subsequently, initiate heating to the target isothermal temperature for the kinetic study. The rapid heating rates possible with resistive furnaces are crucial for capturing the onset of transformation.
• Triggering and Data Collection: Once stable P-T conditions are reached, begin the time-resolved data acquisition. The high flux of the synchrotron beam allows for very short exposure times—10 to 100 milliseconds—enabling the collection of a full diffraction pattern at a high repetition rate (e.g., 5-10 Hz) [38] [40]. Continue data collection throughout the transformation until no further changes in the diffraction pattern are observed.

Data Processing and Kinetic Analysis

• Data Reduction: Integrate the two-dimensional diffraction images to produce conventional one-dimensional intensity vs. 2θ (for ADXRD) or intensity vs. energy (for EDXRD) patterns. This step is often performed using software like Fit2D or similar specialized programs [39].
• Phase Identification and Rietveld Refinement: Identify the crystalline phases present in each pattern by matching the observed diffraction peaks to known crystal structures (e.g., using the ICDD database). For quantitative analysis of phase fractions and lattice parameters, perform Rietveld refinement using software such as GSAS.
• Construction of Transformation-Time Plots: For each diffraction pattern in the time series, calculate the relative abundance of the product phase(s). Plot this transformed fraction (α) as a function of time to generate the kinetic data for the transformation [39].
• Kinetic Modeling: Fit the transformation-time data to the JMAK equation: α(t) = 1 - exp(-k tⁿ), where k is the rate constant and n is the Avrami exponent. Linearizing this equation allows for the extraction of the kinetic parameters n and k, which provide insight into the transformation mechanism (e.g., nucleation and growth dimensionality).

The following workflow diagram illustrates the complete experimental and data analysis process.

The Scientist's Toolkit: Essential Research Reagents and Materials

Successful HPHT synthesis and analysis rely on a suite of specialized materials and reagents, each serving a specific function in creating, sustaining, and probing the sample environment.

Table 2: Essential Materials for HPHT In-Situ Synchrotron XRD Experiments

Item	Function	Specific Examples & Notes
Paris-Edinburgh Press	A large-volume press capable of generating simultaneous high pressure and high temperature for in-situ diffraction studies.	Allows for pressures up to ~25 GPa; compatible with synchrotron X-ray beams [39].
Anvil Materials	Create the pressure-generating thrust on the sample cell.	Tungsten Carbide (WC): For moderate pressures. Sintered Diamond Composites: For ultra-high pressures (>30 GPa).
Pressure-Transmitting Medium	Encapsulates the sample and provides a quasi-hydrostatic pressure environment.	Graphite, hexagonal Boron Nitride (h-BN), or soft salts like NaCl [39].
Heating Element	Internally heats the sample within the pressure cell to target temperatures.	Graphite or resistant metal (e.g., tantalum) furnaces; ceramic heaters for specific chemical environments.
Pressure Calibrant	A standard material with a known equation of state for in-situ pressure determination from its unit cell volume.	Gold (Au), Platinum (Pt), or Sodium Chloride (NaCl) [39].
Synchrotron Beamline	Provides the high-intensity, high-energy X-ray source required for time-resolved studies on small samples under pressure.	Beamlines optimized for high-pressure research, such as ID27 at ESRF or ID15 at APS, featuring either EDXRD or ADXRD setups [40] [39].

Representative Data and Analysis

The power of this technique is best illustrated through concrete examples. The following table synthesizes quantitative kinetic data obtained from real-time synchrotron XRD studies on different material systems, highlighting the variability in transformation behavior.

Table 3: Kinetic Data from Time-Resolved Synchrotron XRD Studies

Material System	Transformation	Key Kinetic Observation	Experimental Conditions	Citation
Ti + C	Solid combustion to TiC	Formation completed within 400 ms of Ti melting.	Combustion wavefront, time resolution down to 10 ms.	[38]
Al + Ni	Solid combustion to AlNi	Final AlNi formed ~5 seconds after wavefront; intermediate phases observed.	Combustion synthesis, sub-second time resolution.	[38]
10% Cr-10% Ni LTT Alloy	Austenite to Martensite	Martensite start (Ms) temperature determined in real-time during welding.	Simulated welding, EDXRD at 5 Hz acquisition rate.	[40]
Mg2SiO4 Olivine	α (olivine) to γ (ringwoodite) polymorph	Transformation kinetics followed using JMAK model; rates depend on presence of water.	High P/T in multianvil press, ~15 GPa, ~1000°C.	[39]

Troubleshooting and Best Practices

Even with a well-designed experiment, challenges can arise. Adherence to best practices is essential for generating reliable data.

• Managing Non-Hydrostatic Stress: Differential stress within the sample can significantly alter transformation kinetics and mechanisms [39]. Using a soft, well-characterized pressure-transmitting medium and including a pressure calibrant are critical steps to monitor and mitigate these effects.
• Achieving Adequate Time Resolution: The required time resolution dictates the minimum exposure time per pattern. A balance must be struck between temporal resolution and pattern statistics (quality). Pilot experiments can help determine the shortest feasible exposure time that still yields a usable signal-to-noise ratio for the phases of interest.
• Data Treatment Consistency: The entire time-series dataset must be processed consistently. This includes using identical integration parameters, background subtraction routines, and refinement constraints for all patterns to ensure that changes in refined parameters genuinely reflect sample changes and not processing artifacts [39].
• Interpretation of Kinetic Parameters: The extracted Avrami exponent n should be interpreted with caution. While it provides clues about the transformation mechanism (e.g., diffusion-controlled growth, interface-controlled growth), it is not always unambiguous. The microstructure of the starting material, the presence of impurities, and the specific P-T path can all influence the observed kinetics [39]. Correlating diffraction data with post-experiment electron microscopy of the quenched sample is highly recommended.

Calibration, Pressure Limits, and Overcoming Experimental Challenges

Pressure calibration is a cornerstone of high-pressure science, essential for ensuring the accuracy and reproducibility of experiments in fields ranging from solid-state physics to geochemistry and materials synthesis. This document provides detailed application notes and protocols for using the well-characterized phase transitions of Bismuth (Bi), Zinc Sulfide (ZnS), Gallium Arsenide (GaAs), and Gallium Phosphide (GaP) as reliable pressure calibrants. The content is framed within the context of multianvil high-pressure high-temperature (HPHT) syntheses research, a methodology critical for discovering and synthesizing novel materials. These calibrants are particularly valued for their sharp, reproducible phase transitions, which provide fixed points that can be cross-referenced against other pressure standards like the ruby fluorescence scale. The following sections summarize their quantitative transition data and provide detailed experimental methodologies for their use.

The following table summarizes the characteristic phase transition pressures of the primary calibrants discussed in this document. These values serve as fixed points for the calibration of high-pressure apparatus.

Table 1: Characteristic Phase Transition Pressures of Common Calibrants at Room Temperature

Calibrant	Initial Crystal Structure	High-Pressure Crystal Structure	Transition Pressure (GPa)	Key References
Bismuth (Bi)	Rhombohedral (Bi-I)	Monoclinic (Bi-II)	2.55	[41]
	Monoclinic (Bi-II)	Incommensurate Composite (Bi-III)	2.7	[41]
	Incommensurate Composite (Bi-III)	Body-Centered Cubic (Bi-V)	7.7	[41]
Zinc Sulfide (ZnS)	Zinc Blende (B3)	Rock Salt (B1)	15.5 - 15.7	[42]
Gallium Arsenide (GaAs)	Zinc Blende	Orthorhombic (Cmcm)	17.3 (at room temperature)	[43]
Gallium Phosphide (GaP)	Zinc Blende	Not Specified in Detail	~2 (onset of metallization)	[44]

Note: The transition pressure for GaAs has a negative temperature dependence, described by the boundary equation P (GPa) = 18.0 – 0.0025 × T (K). The value for GaP is associated with a drastic resistance drop indicative of a metal-insulator transition [44].

Experimental Protocols for Pressure Calibration

General Workflow for Pressure Calibration Using Fixed Points

The following diagram outlines the general logical workflow for calibrating pressure using fixed-point phase transitions.

Protocol 1: Calibration Using Bismuth (Bi) Phase Transitions

Bismuth is a premier low-pressure calibrant due to its multiple, well-defined transitions below 10 GPa.

• Principle: Monitor changes in electrical resistance or unit cell volume (via X-ray diffraction) to detect the phase transitions between Bi-I, Bi-II, Bi-III, and Bi-V [41].
• Materials:
  • Bismuth Sample: High-purity (≥99.998%) polycrystalline Bi [41].
  • Pressure Transmitting Medium: Often omitted due to Bi's softness, but a medium like NaCl or methanol-ethanol can be used for improved hydrostaticity [41].
  • Electrodes: For 4-probe resistance measurements (if applicable).
• Equipment:
  • Diamond Anvil Cell (DAC) or Multi-anvil Press.
  • Resistance measurement system (e.g., LCR meter) or Synchrotron X-ray source for in-situ XRD.
• Step-by-Step Procedure:
  • Loading: For a DAC, load a small piece of Bi foil or powder into the sample chamber alongside a ruby sphere for in-situ fluorescence pressure measurement [41].
  • Compression: Increase pressure gradually while continuously monitoring the sample's electrical resistance or collecting XRD patterns at set intervals.
  • Data Collection: Record the pressure value (from the ruby scale or actuator load) at which abrupt changes in resistance or XRD patterns occur.
  • Identification: Identify the specific transitions:
    • A sharp resistance drop at ~2.55 GPa signifies the Bi-I to Bi-II transition.
    • The transition to Bi-III occurs at ~2.7 GPa.
    • The transition to the bcc Bi-V phase occurs at ~7.7 GPa [41].
  • Calibration: Correlate the known transition pressures with the corresponding apparatus parameters (e.g., oil pressure in a multi-anvil press) to create a calibration curve.

Protocol 2: Calibration Using Zinc Sulfide (ZnS) Phase Transition

The B3 (zinc blende) to B1 (rock salt) transition in ZnS provides a robust calibration point in the mid-pressure range.

• Principle: The transition is marked by a change in crystal structure and a volume collapse, detectable by X-ray diffraction [42].
• Materials:
  • Zinc Sulfide: High-quality, fully-dense polycrystalline CVD ZnS with porosity <0.1% is ideal to ensure a sharp transition [42].
• Equipment:
  • Diamond Anvil Cell or Multi-anvil Press.
  • In-situ X-ray diffraction system.
• Step-by-Step Procedure:
  • Loading: Mix a small amount of fine ZnS powder with a pressure standard (e.g., Au or NaCl) or a fluorescence marker (ruby) and load into the pressure cell.
  • Compression & Monitoring: Increase pressure while collecting X-ray diffraction patterns.
  • Identification: Observe the disappearance of the zinc blende (B3) diffraction peaks (e.g., (111)) and the appearance and growth of the rock salt (B1) peaks (e.g., (200)). The onset of this transition occurs at 15.5-15.7 GPa [42].
  • Calibration: Note the apparatus parameter at the transition onset and use this as a fixed point for calibration.

Protocol 3: Calibration Using Gallium Arsenide (GaAs) and Gallium Phosphide (GaP)

These III-V semiconductors undergo pressure-induced phase transitions accompanied by dramatic electronic property changes.

• Principle: The zincblende to orthorhombic (Cmcm) transition in GaAs, and a similar metal-insulator transition in GaP, can be detected by resistance measurements or XRD [44] [43].
• Materials:
  • GaAs/GaP Sample: Single crystal or high-purity polycrystalline material.
• Equipment:
  • DAC or multi-anvil with resistance measurement capabilities.
  • Heating system for temperature-dependent studies (for GaAs).
• Step-by-Step Procedure for GaAs:
  • Loading: Load the GaAs sample with electrodes for resistance measurement.
  • Compression & Monitoring: Increase pressure while monitoring electrical resistance.
  • Identification: A sharp, orders-of-magnitude drop in resistance is observed at the transition. At room temperature, this occurs at 17.3 GPa [43].
  • Temperature Dependence: For non-ambient temperature work, use the determined phase boundary: P (GPa) = 18.0 – 0.0025 × T (K) [43].
• Procedure for GaP:
  • The methodology is similar, with a characteristic resistance drop occurring around 2 GPa, associated with a metal-insulator transition [44].

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 2: Key Materials and Reagents for High-Pressure Calibration Experiments

Item	Function / Application	Specification / Notes
Bismuth (Bi)	Primary low-pressure calibrant.	High purity (99.998+%). Provides multiple fixed points at 2.55, 2.7, and 7.7 GPa [41].
Zinc Sulfide (ZnS)	Mid-pressure calibrant.	High-quality, low-porosity CVD polycrystal preferred. Sharp B3-B1 transition at ~15.5 GPa [42].
Gallium Arsenide (GaAs)	High-pressure calibrant.	Used for transition at 17.3 GPa (room temperature). Essential for studies with negative dP/dT slope [43].
Ruby Spheres (α-Al2O3:Cr³⁺)	In-situ pressure standard.	Fluorescence R1 line shift with pressure is the most common primary standard in DACs.
Diamond Anvil Cell (DAC)	Primary device for generating static pressure.	Capable of reaching >100 GPa. Used for establishing fixed points.
Multi-anvil Press	Apparatus for HPHT synthesis.	Used for bulk sample synthesis. Calibrated using fixed points determined in DACs.
Methanol-Ethanol Mixture	Hydrostatic pressure-transmitting medium.	4:1 ratio. Provides good hydrostaticity up to ~10 GPa.
NaCl	Pressure medium and secondary standard.	Quasi-hydrostatic medium also used as an internal pressure standard via its EOS.

The accurate calibration of pressure is fundamental to the success of multianvil HPHT syntheses research. The phase transitions in Bismuth, Zinc Sulfide, Gallium Arsenide, and Gallium Phosphide provide a series of reliable and well-characterized fixed points that span a useful pressure range for materials discovery and geophysical simulation. By adhering to the detailed protocols and utilizing the specified materials outlined in this document, researchers can ensure their high-pressure apparatus is precisely calibrated, thereby validating the conditions under which novel phases, such as the high-entropy metal chalcogenides or new carbon allotropes, are synthesized and studied [44] [45].

In multianvil high-pressure high-temperature (HPHT) syntheses research, achieving and maintaining target pressures is fundamental to studying material behavior, synthesizing novel phases, and modeling geological processes. A significant and often practically limiting phenomenon encountered in these experiments is the pressure plateau—a point during compression where a substantial increase in applied force yields a negligible increase in the sample-pressure within the assembly [46]. This plateau defines the useful upper-pressure limit for a given experimental configuration and is intrinsically linked to the deformation behavior of the gasketing materials used to contain the pressure [46]. Understanding this relationship is critical for researchers in designing experiments, accurately interpreting results, and pushing the boundaries of attainable pressure conditions for applications ranging from the synthesis of lower mantle minerals to the discovery of new pharmaceutical polymorphs.

This application note details the underlying mechanisms of the pressure plateau, provides quantitative data on its manifestation across standard assemblies, and outlines standardized protocols for system calibration and phase analysis to ensure reliable and reproducible research outcomes.

The Pressure Plateau Mechanism and Gasketing Role

The pressure plateau occurs when the mechanical work applied by the hydraulic press is primarily consumed by the permanent deformation (flow) of the gasketing material, rather than in further compressing the sample volume. Each incremental increase in pressure requires a reduction in the volume of the high-pressure assembly. When the gasketing material begins to flow plastically, it accommodates this required volume change by deforming, thereby preventing further compression of the sample chamber. This results in a state where the application of considerably higher loads does not produce a significant increase in the sample-pressure [46].

The gasket, typically made from materials like Re or other strong alloys in diamond anvil cells, or softer materials in multianvil systems, serves the critical function of creating a sealed chamber that contains the pressurized sample and pressure medium. Its mechanical properties and behavior under extreme stress are therefore the primary determinants of the plateau pressure (P_max). The finite-element calculation mentioned in one study highlights that the design of the support system around the anvils can significantly reduce tensile stresses, indirectly influencing the efficiency of force transmission and the point at which gasket failure initiates the plateau [46].

Table 1: Characteristics of Pressure Plateau in Various Multianvil Assemblies

Assembly (Octahedron Edge Length/ Truncation, mm)	Useful Upper-Pressure Limit, P_max (GPa)	Key Observation
25/17	~10 GPa (at 1500 t)	Pressure reaches a plateau defining its maximum useful value [46].
18/11	~20 GPa	Unlikely to significantly exceed this pressure regardless of press size [46].
18/8	~22-26 GPa	A doubling of the applied load raises pressure by only 4.2 GPa [46].

Figure 1: Workflow of pressure plateau formation. Applied force initially compresses the sample, but is eventually consumed by gasket deformation, leading to a pressure plateau.

Experimental Protocols

Protocol: Calibration of a Multianvil System

Objective: To establish a reliable pressure-load relationship for a given assembly and identify its pressure plateau.

• Assembly Preparation: Select an appropriate assembly size (e.g., 18/11, 18/8). Insert a calibration material, such as Bi, GaP, or GaAs, into the octahedral chamber alongside the pressure medium [46].
• Pressurization: Place the assembly into the multianvil guide blocks and load it into the hydraulic press. Increase the press load in controlled increments at room temperature.
• Transition Detection: At each load increment, use in-situ resistance measurements or post-mortem analysis to detect known pressure-induced phase transitions in the calibration material (e.g., Bi I-II at 2.55 GPa, Bi III-V at 7.7 GPa, GaP at 22.5 GPa) [46].
• Data Recording & Plateau Identification: Record the press load at which each transition occurs. Plot the measured pressure against the applied load. The plateau is identified as the region where the slope of the curve drastically decreases, meaning a large increase in load results in a minimal pressure increase [46].
• High-Temperature Calibration (Optional): For high-temperature work, repeat using materials with known pressure-temperature phase boundaries (e.g., CaGeO₃ garnet-perovskite transition at 6.2 GPa and 1600 K) [46].

Protocol: Micro-texture Analysis for Melting Detection

Objective: To accurately determine melting temperatures in laser-heated diamond anvil cell (LHDAC) experiments and avoid confusion with other high-temperature phenomena like gasket deformation.

• HPHT Experiment: Conduct the laser-heating experiment on the sample (e.g., MgO) within the DAC, monitoring temperature via thermal radiation spectroscopy [47].
• Laser Power-Temperature Profiling: Gradually increase the laser power while recording temperature. Observe the relationship for two distinct plateaus: a lower-temperature plateau often associated with significant sample/gasket deformation, and a higher-temperature plateau indicative of true melting [47].
• Quenching and Recovery: After observing a plateau, rapidly quench the sample and carefully decompress the DAC to recover the sample.
• Cross-sectioning: Prepare a cross-section of the quenched sample using focused ion beam (FIB) milling or other precise techniques.
• Microscopy and Diffraction:
  • Use Scanning Electron Microscopy (SEM) to image the micro-texture. Look for a chilled margin of fine-grained crystals and inward-growing columnar crystals, which are characteristic of solidification from a melt [47].
  • Use Transmission Electron Microscopy (TEM) and selected area electron diffraction to confirm the crystallinity and phase of the different texture domains.
  • Identify the presence of spheroidal voids within crystals, which are remnants of trapped melt and further confirm melting and solidification [47].

The Scientist's Toolkit: Key Research Reagent Solutions

Table 2: Essential Materials for High-Pressure Synthesis and Calibration

Item Name	Function/Application	Examples & Notes
Tungsten Carbide (WC) Inner-Anvils	Generate high pressure on the sample via truncated corners.	Standard for pressures up to ~25-28 GPa. Larger truncations allow larger sample volumes [46].
Sintered Diamond Inner-Anvils	Extend the accessible pressure range beyond WC limits.	Enables experiments in the ~50 GPa range and up to 100 GPa with temperature limitations [47].
Gasketing Materials	Contain the pressurized sample and medium; critical for defining pressure plateau.	Rhenium (Re) in LHDAC; other composites in multianvil systems. Properties dictate P_max [46].
Pressure Calibration Materials	Provide fixed points for accurate pressure determination.	Bi (2.55, 7.7 GPa), ZnS (15.5 GPa), GaAs (18.8 GPa), GaP (22.5 GPa) [46].
Hydrostatic Pressure Media	Transmit pressure uniformly to the sample, reducing shear stresses.	Argon, NaCl, or other noble gases/salts. Essential for clean diffraction data and equilibrium conditions.
Diamond Anvil Cell (DAC)	Generate ultra-high pressures for spectroscopic and diffraction studies.	Merrill-Bassett type; allows in-situ X-ray, Raman, and IR analysis [48].

Figure 2: Logical relationship showing how gasketing enables a stable sample environment for accurate high-pressure analysis.

The pressure plateau, governed by the mechanical limits of gasketing materials, represents a fundamental practical boundary in high-pressure syntheses research. A thorough understanding of this phenomenon, combined with rigorous calibration and modern analytical techniques like micro-texture analysis, allows researchers to design experiments more effectively, accurately interpret their results, and reliably push the frontiers of high-pressure science. This knowledge is indispensable for fields as diverse as deep Earth geophysics, materials science, and pharmaceutical development, where the precise control and measurement of pressure is paramount.

In multianvil, high-pressure high-temperature (HP-HT) synthesis, the precise configuration of the anvil geometry and the surrounding pressure medium is a critical determinant of experimental success. The Truncation Edge Length (TEL) of the second-stage anvils and the Octahedron Edge Length (OEL) of the pressure-transmitting medium (PTM) form a core relationship in the Kawai-type multianvil apparatus [5]. Optimizing the size matching between these components is essential for achieving high pressure-generation efficiency, maintaining excellent sealing performance, and ensuring the hydrostatic conditions required for synthesizing novel materials and probing deep Earth processes [49]. This application note details the quantitative relationships, provides validated experimental protocols, and outlines essential reagent solutions for optimizing these parameters.

Quantitative Data on TEL and OEL

The following tables summarize key quantitative data for configuring Kawai-type multianvil apparatuses. The choice of TEL and corresponding OEL directly dictates the attainable pressure and temperature conditions.

Table 1: Performance of Second-Stage Anvils by Material and TEL [5]

Anvil Material	Typical TEL (mm)	Maximum Pressure (GPa)	Maximum Temperature (K)
Tungsten Carbide (nominally hard)	1.5 - 2.0	27	3040
Tungsten Carbide (hardest grade)	1.5 - 2.0	44	2000
Sintered Diamond	Information Missing	106	1200
Sintered Diamond	Information Missing	109 (Room Temperature)	Not Specified

Table 2: Common TEL and OEL Combinations and Their Applications [5]

Truncation Edge Length (TEL)	Octahedron Edge Length (OEL) of PTM	Typical Pressure Range	Common Application Context
0.5 mm	Not Specified	Very High (>20 GPa)	Specialist high-pressure studies
1.0 mm	Not Specified	High	General high-pressure synthesis
8 mm, 11 mm, 17 mm	Not Specified	Lower	Large-volume synthesis

Table 3: Pressure-Generation Efficiency and Sealing vs. PTM Size [49] This data is for a cubic press with 23.5 mm first-stage anvils, demonstrating the critical nature of size matching.

PTM Edge Length	Pressure-Generation Efficiency	Sealing Performance	Overall Recommendation
33.5 mm	Highest	Worst	Not recommended
32.5 mm	High	Best	Best overall performance
30.5 - 31.5 mm	Lower	Good	Less efficient

Experimental Protocols

Protocol: Optimizing TEL and OEL for Target P-T Conditions

This protocol describes the procedure for selecting and assembling a multianvil geometry to achieve specific pressure and temperature conditions for material synthesis [5] [49].

• Objective: To achieve a target pressure of 20 GPa at 1700 K for the synthesis of lower mantle mineral analogs.
• Materials Required: Refer to Section 5, "The Scientist's Toolkit," for a detailed list.
• Pre-experiment Preparation:
  • Anvil Selection: Select the hardest grade of tungsten carbide second-stage anvils with a TEL of 1.5 mm [5].
  • PTM Machining: Machine an octahedral PTM from a semi-sintered magnesia-based material to an OEL of 32.5 mm, designed for optimal performance with the 23.5 mm first-stage anvils [49].
  • Cell Assembly Fabrication: Within the PTM, assemble a cylindrical graphite furnace, an insulating MgO sleeve, and a LaCrO₃ heater if temperatures exceeding 2000 K are required. Place the sample material (e.g., a glass of eclogitic composition) inside a ceramic capsule (e.g., MgO or Al₂O₃) and position it at the center of the furnace [50].
• Experimental Procedure:
  • Assembly Loading: Load the complete cell assembly into the center of the eight second-stage anvils. Place the anvil assembly inside the first-stage anvils and guide blocks of the hydraulic press [5].
  • Compression: Gradually increase the load on the hydraulic ram(s). The six first-stage anvils transmit force to the eight second-stage anvils, which compress the octahedral PTM.
  • Heating: Once the target load (proxy for pressure) is stabilized, apply power to the graphite furnace to heat the sample to the target temperature (e.g., 1700 K). Monitor temperature using a W/Re thermocouple placed next to the sample capsule.
  • Synthesis and Quenching: Maintain the HP-HT conditions for a duration sufficient for synthesis or phase stabilization (e.g., 60 minutes [50]). Subsequently, quench the sample by cutting power to the furnace before decompressing the press.
• Post-experiment Analysis:
  • Recover the cell assembly and extract the synthesized sample.
  • Characterize the run products using techniques such as Electron Microprobe Analysis (EMPA) for chemical composition [50] [51], and single-crystal or powder X-ray Diffraction (XRD) for phase identification and crystal structure determination [50] [51].

Protocol: In-situ HP-HT Synthesis and Characterization in a Diamond Anvil Cell (DAC)

For synthesis at pressures exceeding 50 GPa, Diamond Anvil Cells (DACs) are required. This protocol outlines the procedure for laser-heated synthesis and in-situ characterization [27] [52] [45].

• Objective: To synthesize novel carbon allotropes from methanol precursor at 15 GPa.
• Materials Required: Bragg-Mini DAC, diamond anvils with 0.5 mm culets, Inconel 718 gasket, methanol, ruby chips for pressure calibration [52].
• Pre-experiment Preparation:
  • Gasket Preparation: Pre-indent a metal gasket and drill a micro-compartment (e.g., 300 µm diameter) to serve as the sample chamber [52].
  • Sample Loading: Load the sample chamber with the methanol precursor and a few ruby chips for pressure measurement.
• Experimental Procedure:
  • Compression: Tighten the DAC to compress the sample to the target pressure (e.g., 15 GPa). Monitor pressure in real-time via the shift in the ruby fluorescence spectrum [52].
  • Laser-Heated Synthesis: Focus a continuous-wave 1064 nm laser beam (20 µm spot size) into the sample chamber to locally heat the precursor and induce a chemical reaction.
  • In-situ Characterization: Simultaneously use confocal Raman spectroscopy to analyze the reaction products (e.g., diamond, carbon nanotubes) formed in the reaction zone in real-time [52].
• Post-experiment Analysis:
  • Decompress the cell and recover the sample.
  • Analyze the solid deposits using ex-situ techniques like scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to characterize microstructure and morphology [52].

Workflow and Relationship Diagrams

The Scientist's Toolkit

Table 4: Essential Research Reagent Solutions for Multianvil HP-HT Synthesis

Item	Function & Application	Specific Examples & Notes
Second-Stage Anvils	Generate ultra-high pressure on the sample. Material limits max P-T.	Tungsten Carbide (to 44 GPa), Sintered Diamond (to 109 GPa) [5].
Pressure-Transmitting Medium (PTM)	Transmits pressure hydrostatically to the sample. OEL is critical.	Semi-sintered MgO-based octahedron; OEL must be matched to anvil TEL for efficiency [49].
Furnace/Heater	Internally heats the sample within the pressurized cell.	Graphite (moderate T), LaCrO₃ (high T > 2000 K) [50].
Thermocouple	Monitors sample temperature in real-time during synthesis.	Type D (W/Re) thermocouples for high-temperature compatibility [50].
Insulating Sleeves	Thermally and electrically isolate the heater from the PTM.	MgO or Al₂O₃ sleeves [50].
Sample Capsule	Contains the precursor materials, preventing reaction and contamination.	MgO, Al₂O₃, or Pt capsules, depending on sample chemistry [50].
Gasket Material	Provides lateral support to anvils and contains the PTM, improving seal.	Pyrophyllite is commonly used in multianvil systems [5].
Pressure Calibrant	Allows for accurate determination of pressure inside the cell.	Ruby chips for DAC (fluorescence) [52]; fixed points for large-volume press.

The pursuit of materials synthesis under extreme high-pressure and high-temperature (HP-HT) conditions is a cornerstone of modern condensed matter physics, geology, and materials science. Within this domain, sintered diamond anvils represent a transformative material advancement that significantly enhances the performance and capabilities of multianvil apparatus. Unlike single-crystal diamond anvils, which are limited by crystallographic cleavage planes and anisotropic hardness, sintered diamond anvils are engineered from nanoscale diamond grains fused under high pressure, resulting in a superhard material with superior mechanical properties including suppressed anisotropy, enhanced fracture toughness, and exceptional compressive strength [53]. These characteristics make them ideally suited for large-volume presses (LVP) used in multianvil systems, where they enable access to previously unattainable pressure-temperature (P-T) conditions while maintaining generous sample volumes essential for meaningful materials synthesis and characterization.

The significance of this advancement is profound within the context of multianvil high-pressure syntheses research. Sintered diamond anvils have drastically extended the practical operating range of conventional large-volume presses. Whereas traditional tungsten carbide anvils typically reach their operational limit at approximately 30 GPa, the integration of sintered diamond anvils has pushed these boundaries dramatically, with reported achievements exceeding 90 GPa at room temperature while preserving sample volumes in the cubic millimeter scale [32]. This expanded P-T space opens new frontiers for synthesizing and studying novel materials, including superhard compounds, advanced semiconductors, and exotic phases relevant to planetary interiors, thereby solidifying sintered diamond anvils as an indispensable tool in the high-pressure researcher's arsenal.

Quantitative Performance Enhancements

The transition from traditional anvil materials like tungsten carbide to sintered diamonds has yielded measurable and substantial improvements in high-pressure apparatus performance. The tables below summarize key comparative data and application-specific performance metrics.

Table 1: Comparative Anvil Material Properties and Performance

Material Property / Performance Metric	Tungsten Carbide	Single-Crystal Diamond	Sintered Diamond
Typical Pressure Limit in LVP	~30 GPa [32]	Not commonly used in LVP	>90 GPa (with 14 mm cubes) [32]
Hardness	High	Extreme (but anisotropic)	Superior & Isotropic [53]
Fracture Toughness	Moderate	Low (due to cleavage)	High [53]
Anvil Size Availability	Large (e.g., 14 mm+ cubes)	Limited by crystal size	Large (e.g., 14 mm cubes) [32]
Primary Advantage	Cost, Durability	Ultimate static pressure in DACs	Combines high pressure & large volume

Table 2: Performance of Sintered Diamond Anvils in Specific Applications

Anvil Type / Configuration	Reported Maximum Pressure	Sample Volume / Scale	Key Application or Experiment
Sintered Diamond Cubes (14 mm) in 6-8 type Multianvil	>90 GPa [32]	Large volume (mm³ scale)	General high-pressure synthesis & Earth science [32]
Single-Toroidal Sintered Diamond Anvils (Paris-Edinburgh Press)	14 GPa [54]	38 mm³ [54]	Neutron diffraction (8.7 GPa / 1380 K) [54]
Sintered Diamond Anvils in Drickamer Cell	30 GPa [55]	Not Specified	High-pressure research
Sintered Diamond Anvils in DIA Apparatus	18 GPa [55]	Not Specified	High-pressure research

The data illustrates that sintered diamond anvils are not merely incremental improvements but represent a paradigm shift, particularly in large-volume press technology. The ability to generate pressures above 90 GPa [32] in a multianvil apparatus bridges the gap between conventional large-volume presses and diamond anvil cells (DACs), enabling in-situ synchrotron studies of materials under conditions corresponding to the Earth's lower mantle. Furthermore, specialized designs like the single-toroidal sintered diamond anvil demonstrate remarkable stability at simultaneous high pressure and temperature (8.7 GPa and 1380 K), which is critical for reliable synthesis and phase stability studies [54].

Experimental Protocols for High-Pressure Synthesis

The following section outlines detailed methodologies for utilizing sintered diamond anvils in multianvil systems for high-pressure materials synthesis, incorporating both assembly preparation and in-situ characterization techniques.

Assembly Preparation and Anvil Configuration

Protocol 1: Cell Assembly for Synthetic Exploration

• Anvil Selection and Preparation: Select sintered diamond anvils with appropriate truncation sizes (e.g., 2-3 mm) based on the target pressure [55]. Ensure anvil faces are clean and free of particulate matter to prevent pressure points that could lead to failure.
• Gasketing and Pressure Medium: Employ a gasketing system, typically a pre-indented metal foil (e.g., rhenium, tungsten, or stainless steel). The pressure-transmitting medium (e.g., NaCl, MgO, or neon gas for higher quasi-hydrostaticity) must be carefully selected based on the desired P-T conditions and the need for X-ray transparency [27] [53].
• Sample Loading: The starting material (chemical precursors in powder, pellet, or thin-film form) is loaded into the chamber within the pressure medium. For in-situ X-ray diffraction, a pressure marker (e.g., gold, platinum, or ruby) is included adjacent to the sample [27].
• Multianvil Assembly: The prepared cell assembly is meticulously centered within the sintered diamond anvils, which are then aligned in the multianvil module (e.g., 6-8 type system). Proper alignment is critical for generating uniform pressure and avoiding shear stresses that can damage the anvils [32].

In-situ High-Pressure Synthesis and Characterization

Protocol 2: Real-Time Synthesis Monitored by Synchrotron X-Ray Diffraction

This protocol leverages the in-situ "watch-anvil" technique, which has superseded the traditional, time-consuming "cook and look" methodology [27].

• Beamline Setup: The prepared multianvil apparatus is integrated with a synchrotron X-ray beamline (e.g., GSECARS 13ID-D). The high flux and brilliance of synchrotron radiation are necessary to penetrate the high-pressure assembly [27] [56].
• Compression and Heating: The pressure is increased gradually to the target synthesis range (e.g., 3-8 GPa, the "industrial pressure range"). Simultaneously, the sample is heated to high temperatures (often >1000 K) using an internal heater (e.g., a graphite or resistive metal heater) to enhance chemical reactivity and crystallization [27].
• Real-Time Diffraction Monitoring: During the compression and heating cycles, energy-dispersive or angle-dispersive X-ray diffraction patterns are collected continuously or at fine intervals. The diffraction data allows researchers to:
  • Determine the Pressure: From the known equation of state of the pressure marker.
  • Observe Reaction Onset: Identify the (P,T) conditions at which new crystalline phases form from the precursors.
  • Optimize Synthesis Parameters: Monitor phase purity and crystallinity in real-time to adjust pressure, temperature, or dwell time for optimal product yield [27].
• Quenching and Recovery: After the synthesis is complete, the temperature is rapidly quenched while maintaining pressure to preserve the high-pressure phase. Subsequently, the pressure is slowly released, and the synthesized sample is recovered for further ex-situ characterization.

Diagram 1: In-situ HP-HT Synthesis Workflow. This flowchart outlines the protocol for real-time synthesis optimization using sintered diamond anvils and synchrotron X-ray diffraction.

The Scientist's Toolkit: Essential Research Reagents and Materials

Successful high-pressure synthesis with sintered diamond anvils relies on a carefully selected suite of materials and reagents. The table below details the key components of a high-pressure cell assembly and their critical functions.

Table 3: Essential Materials for HP-HT Synthesis with Sintered Diamond Anvils

Item / Reagent	Function / Role	Examples & Technical Notes
Sintered Diamond Anvils	Primary pressure-generating components; their high hardness and toughness allow for high pressures on large samples.	6-8 type cubes, Drickamer anvils; Truncation size (e.g., 2-3 mm) is selected based on target pressure [32] [55].
Pressure Transmitting Medium	Encapsulates the sample to provide quasi-hydrostatic conditions, minimizing shear stresses.	NaCl, MgO, BN, or noble gases (e.g., Ne); Choice affects hydrostaticity and X-ray transparency [27] [53].
Gasket Material	Confines the pressure medium and sample, providing lateral support to the anvils.	Rhenium, Tungsten, Stainless Steel; Pre-indented to initial thickness of ~30 μm [53].
Internal Heater	Generates high temperatures within the pressurized sample volume.	Graphite, Tungsten, or resistive metal sleeves/coils; Material must be stable and not react with the sample or assembly at high T [27].
Chemical Precursors	Starting materials for the synthesis of the target compound.	High-purity powders, foils, or pre-reacted pellets; composition depends on the target material [27].
Pressure Calibrant	Allows for accurate in-situ pressure determination via its known Equation of State (EoS).	Gold, Platinum, Ruby (Al₂O₃:Cr³⁺), NaCl; Must be chemically inert and have a well-characterized EoS [27] [53].
Synchrotron X-Ray Beam	Probe for in-situ characterization via X-ray diffraction, providing real-time structural data.	High-flux, high-brilliance beam; May be focused to a micron-scale spot for mapping [27] [56].

Advanced Configurations and Implementation Workflow

The performance of sintered diamond anvils can be further enhanced through innovative design configurations. The single-toroidal design represents a significant advancement for achieving remarkable stability and large sample volumes at simultaneous high pressure and temperature. This design features a toroidal (donut-shaped) groove on the culet face, which, when combined with a gasketing strategy, enhances pressure confinement and efficiency. This configuration has proven highly effective in the Paris-Edinburgh press, enabling neutron diffraction experiments—a technique that requires particularly large sample volumes—at pressures up to 8.7 GPa and temperatures of 1380 K [54].

Another groundbreaking configuration involves the use of second-stage micro-anvils. In this approach, tiny semi-balls or anvils fabricated from superhard nanodiamond (grain size below ~50 nm) are placed within the primary pressure chamber of a conventional diamond anvil cell. These micro-anvils act as secondary pressure generators, effectively creating a double-stage compression system. This configuration has demonstrated the capability to reach extreme static pressures exceeding 600 GPa (6 Mbar), drastically extending the achievable pressure range in static compression experiments [53].

Diagram 2: Pressure Generation Mechanism. This diagram illustrates how force is intensified and transmitted through the sintered diamond anvil and gasket system to generate extreme pressures on the sample.

The implementation of these advanced anvil systems follows a logical decision workflow. The researcher must first define the experimental goal, which dictates the choice between a large-volume press (LVP) for synthesis or a diamond anvil cell (DAC) for ultimate pressure. If an LVP is chosen and the target pressure exceeds the capability of tungsten carbide (~30 GPa), sintered diamond anvils become the necessary choice. The final configuration—whether standard cubes, single-toroidal, or a double-stage system—is then selected based on the specific requirements for pressure, volume, and analytical technique. This systematic approach ensures that the unique properties of sintered diamond anvils are fully leveraged to meet the demanding needs of modern high-pressure research.

In multianvil high-pressure high-temperature (HPHT) synthesis, achieving and maintaining precise thermal conditions is paramount for successful solid-state chemistry reactions. This technique, which enables sample synthesis at pressures up to 25 GPa, requires exceptional control over thermal gradients to ensure sample homogeneity and reaction reproducibility [57]. The management of these thermal profiles becomes increasingly complex in large assemblies where spatial temperature variations can significantly impact material properties and reaction pathways. This application note details advanced protocols for temperature measurement and control specifically tailored for HPHT research environments, providing researchers with methodologies to mitigate thermal gradient effects that compromise experimental integrity.

Research Reagent Solutions and Essential Materials

The following table catalogues critical equipment and materials required for implementing robust temperature measurement and control systems in HPHT research environments.

Table 1: Essential Research Reagents and Materials for Temperature Management in HPHT Research

Item	Function	Application Notes
Two-Color IR Pyrometer (e.g., IGAR 6 SMART)	Non-contact temperature measurement	Wide range (100-2000°C); measures through dust/smoke; no emissivity adjustment needed; ideal for high-temperature environments [58].
Fiber Optic IR Pyrometer (e.g., M322-18F)	Temperature measurement in confined spaces	Fiber optic cable with miniature head; suitable for complex assembly geometries [58].
Microheater Arrays	Localized heating and temperature sensing	Provides fine spatial resolution for adverse flow conditions; requires careful fabrication to avoid breakage [59].
Fluorescent Polymeric Nanothermometers	Intracellular temperature mapping	Enables subcellular thermal monitoring; reveals metabolic activity via temperature variations (e.g., 2.4°C increase in mitochondria) [60].
Thermal Insulation Wedges/Waveguides	Protects measurement equipment from extreme heat	Built with low thermal conductivity materials (e.g., polyetherimide); guides ultrasound signals while providing thermal protection [61].
Quantum Dots (QDs)	High-resolution surface temperature mapping	Spray or spin-coated onto surfaces; emission spectrum shifts with temperature; enables submicron spatial resolution [59].

Quantitative Data on Temperature Measurement Technologies

The selection of appropriate temperature monitoring technologies is critical for accurate thermal gradient management. The following table summarizes key performance metrics for various measurement systems relevant to HPHT applications.

Table 2: Performance Comparison of Temperature Measurement Techniques

Technique	Temperature Range	Spatial Resolution	Response Time	Key Advantages	Primary Limitations
Two-Color IR Pyrometry [58]	250-2550°C	2-3 mm spot size	2 ms (adjustable)	Unaffected by dust/smoke; no emissivity adjustment	Requires optical access to target
Fiber Optic IR Pyrometry [58]	500-1800°C	1.2-6 mm spot size	1 ms (adjustable)	Miniature fiber head for confined spaces; immune to EMI	Fiber may be delicate in high-pressure environments
Microheater Arrays [59]	Application-dependent	Sub-micron to micron scale	~1 ms (est.)	Can simultaneously heat and sense at multiple points	Intrusive; fabrication challenges; Joule heating
Fluorescent Nanothermometers [60]	Biologically relevant	Subcellular	Seconds to minutes	Maps intracellular thermal dynamics	Potential phototoxicity; requires complex calibration
Quantum Dots [59]	-50 to 975°C	Submicron (via visible light)	Seconds	Fine spatial resolution; tunable emission	Coating required; calibration sensitive to environment
Ultrasonic NDT [61]	>80°C (specimen)	Millimeter scale	Microseconds	Can inspect internal flaws; works on metals	Signal distortion from thermal gradients

Experimental Protocols

Protocol: Non-Contact Temperature Monitoring and Control for HPHT Assemblies

This protocol utilizes infrared pyrometry for real-time temperature monitoring and gradient control in multianvil systems.

Materials and Reagents:

• Two-color infrared pyrometer (e.g., IGAR 6 SMART Assembly, 1ASM-700-216-00)
• Dual-axis mounting bracket (1ASM-700-221-00)
• Water cooling system (if ambient temperature exceeds 85°C)
• Data acquisition system with control algorithms
• Calibration source traceable to national standards

Procedure:

• Sensor Positioning and Mounting
  • Mount the pyrometer sensor using the dual-axis bracket to allow precise vertical and horizontal angle adjustment [58].
  • Position the sensor at a distance that ensures the measurement spot size (e.g., 2 mm at 210 mm for IGAR 6) adequately covers the target area of interest.
  • Ensure the laser targeting system accurately aligns with the measurement location.

• System Integration

  • Connect the pyrometer to the control system using the appropriate connection cable (typically 2m/6.5ft for Ultraflex systems).
  • Integrate the pyrometer output with the heating control system to enable closed-loop temperature regulation.
  • Implement data logging capabilities to record temperature profiles for each experiment.

• Calibration and Validation

  • Perform calibration against a reference standard at multiple temperature points across the intended operating range.
  • Verify measurement accuracy by comparing with thermocouple readings at stable temperature points (where feasible).
  • Confirm response time settings (adjustable from 2 ms to 10 s) based on process dynamics [58].

• Operational Monitoring

  • Monitor temperature continuously during ramp-up, stabilization, and cool-down phases.
  • Implement automatic heating adjustments when temperatures deviate from set values by predetermined thresholds.
  • For large assemblies, consider multiple pyrometers at strategic locations to map spatial temperature variations.

• Data Analysis and Reporting

  • Analyze temperature logs to identify thermal gradients across the assembly.
  • Correlate thermal profiles with experimental outcomes to optimize temperature parameters.
  • Document any thermal excursions and corresponding system responses.

Protocol: Mitigation of Ultrasonic Signal Distortion in High-Temperature NDT

This protocol addresses the correction of ultrasonic non-destructive test signals distorted by thermal gradients, which is crucial for accurate flaw detection in HPHT equipment.

Materials and Reagents:

• Phased-array ultrasonic testing system
• Polyetherimide wedges or other thermal protection structures
• Heat-resistant coupling medium
• Reference specimens with known flaw characteristics
• Focal law correction software tool

Procedure:

• Pre-Inspection Thermal Modeling
  • Model the heat propagation through the wedge and specimen using transient heat transfer equations.
  • Calculate the expected temperature distribution along the ultrasonic path prior to inspection.

• Sound Speed Characterization

  • Determine the temperature dependence of sound speed for the specific material using the relationship: V = V₀ + α·T, where V is velocity, V₀ is baseline velocity, α is the material-specific coefficient, and T is temperature [61].
  • Establish a linear approximation of sound speed versus temperature for the relevant temperature range.

• Focal Law Correction

  • Apply ray tracing concepts to calculate the distorted ultrasonic beam path through the thermal gradient.
  • Compute corrected focal laws that account for the variation in sound speed and beam steering effects.
  • Program the phased-array system with the temperature-corrected focal laws.

• Validation and Verification

  • Inspect reference specimens with known flaw characteristics at both ambient and elevated temperatures.
  • Compare flaw detection and sizing accuracy with and without focal law correction.
  • Optimize correction parameters until positioning errors are reduced by at least 68% as demonstrated in prior research [61].

• In-Service Implementation

  • Continuously monitor specimen temperature during inspection.
  • Apply real-time focal law adjustments based on measured temperature profiles.
  • Document inspection results with and without thermal compensation for quality assurance.

Workflow and System Diagrams

Effective management of thermal gradients in large assemblies for multianvil HPHT synthesis requires integrated approach combining advanced measurement technologies, robust experimental protocols, and computational correction methods. The implementation of non-contact temperature monitoring using IR pyrometry provides critical real-time data for thermal control systems, while specialized protocols for ultrasonic NDT enable accurate flaw detection despite challenging thermal conditions. Through the systematic application of these methodologies, researchers can significantly improve experimental reproducibility and material quality in high-pressure solid-state chemistry synthesis. Future developments in nanoscale temperature mapping and real-time thermal compensation algorithms will further enhance capabilities in this critical research domain.

Characterizing Synthesis Products and Cross-Technique Comparisons

In multianvil high-pressure high-temperature (HP-HT) synthesis research, comprehensive post-synthesis analysis is crucial for determining the success of experiments and characterizing the resulting materials. These analytical techniques confirm the crystallinity, chemical composition, and microstructural features of new materials synthesized under extreme conditions, linking their atomic structure to physical properties. This document provides detailed application notes and protocols for X-ray diffraction (XRD), electron microprobe analysis (EMPA), and transmission electron microscopy (TEM), framed within the context of HP-HT syntheses common in geophysical and advanced materials research [51] [62] [63].

In HP-HT synthesis research, post-synthesis analysis typically follows an integrated workflow where techniques are applied sequentially to extract complementary information about a material. The workflow progresses from initial phase identification and chemical composition analysis to detailed structural and microstructural characterization.

The following table summarizes the primary applications and key capabilities of each technique in HP-HT research:

Table 1: Analytical Techniques for HP-HT Synthesis Research

Technique	Primary Applications in HP-HT Research	Key Capabilities	Spatial Resolution
XRD	Phase identification, unit cell determination, purity assessment	Bulk analysis, crystal structure solution, phase quantification	Bulk powder: ~mm; Single crystal: ~μm
EMPA	Quantitative chemical composition, homogeneity assessment, zoning detection	Elemental quantification, micron-scale mapping, low Z-element analysis	~1 μm
TEM	Nanoscale crystallography, defect analysis, microstructure characterization	Atomic-resolution imaging, local structure analysis, combined chemical/structural data	Imaging: <0.1 nm; Diffraction: ~100 nm

X-Ray Diffraction (XRD) Analysis

Application Notes

XRD serves as the primary technique for initial phase identification and crystal structure determination in HP-HT synthesized materials. Single-crystal XRD (SC-XRD) has successfully determined structures of novel HP-HT phases such as the high-pressure Co₃TeO₆ phase (space group R3, a = 519.37(6) pm, c = 1382.4(2) pm) [64] and the iron sulfide phase Fe₄₊ₓS₃ (space group Pnma) relevant to Martian core studies [62]. For microcrystalline products, powder XRD (PXRD) provides essential data on phase composition and stability, as demonstrated in high-temperature PXRD studies of HP-Co₃TeO₆ showing exceptional stability up to 1070 K [64].

When crystal sizes are sub-micrometer, three-dimensional electron diffraction (3D ED) has emerged as a powerful alternative to SC-XRD, enabling ab initio structure determination from nanoscale crystals [65] [51]. The automated pipeline Instamatic-solve has demonstrated successful structure solution of diverse materials within 2 minutes, provided data meets critical quality criteria (completeness ≥50% and resolution better than 1.0 Å) [65].

Protocols for XRD Analysis

Single-Crystal XRD Protocol

Sample Preparation:

• Select single crystals (≥1 μm) under optical microscope or SEM
• Mount suitable crystal on micromesh loop with paratone oil
• Crystal size ideally 0.1-0.3 times absorption length for optimal data quality

Data Collection:

• Use micro-focused X-ray source (Mo/Kα, λ = 0.71073 Å or Ag/Kα, λ = 0.56087 Å)
• Collect full sphere of reciprocal space with 0.5° frame width
• Determine unit cell and space group from auto-indexing
• For structures with severe absorption (e.g., heavy elements), apply numerical absorption correction

Data Processing and Structure Refinement:

• Integrate data using Bruker SAINT or CrysAlisPro
• Correct for Lorentz, polarization, and absorption effects
• Solve structure by direct methods (SHELXT) or charge flipping
• Refine using full-matrix least-squares (SHELXL) with all non-hydrogen atoms anisotropic
• Validate structure with IUCr checkCIF

Powder XRD Protocol for HP-HT Products

Sample Preparation:

• Gently crush sample in agate mortar to avoid preferred orientation
• Load powder into glass capillary (0.3-0.5 mm diameter) for transmission geometry
• Alternatively, spread on zero-background silicon wafer for reflection geometry

Data Collection:

• Use transmission geometry with Debye-Scherrer configuration
• Employ synchrotron radiation for high resolution or small samples
• Collect data over 5-90° 2θ range with step size ≤0.01°
• For phase identification, use laboratory source with position-sensitive detector

Data Analysis:

• Identify phases by comparison with ICDD database
• For structure refinement, use Rietveld method (TOPAS, GSAS)
• Refine scale factor, background, unit cell, and atomic parameters sequentially

Electron Microprobe Analysis (EMPA)

Application Notes

EMPA provides quantitative chemical composition data essential for verifying stoichiometry and assessing homogeneity of HP-HT synthesized materials. This technique has been successfully applied to characterize various HP-HT phases, including the new high-pressure polymorph parabreyite (CaSiO₃ system) [51] and the boron-rich terbium hydroxyborate Tb₃B₁₂O₁₉(OH)₇ synthesized at 6 GPa and 650°C [66]. Wavelength-dispersive spectroscopy (WDS) is preferred over energy-dispersive spectroscopy (EDS) for superior spectral resolution and sensitivity for light elements, which is crucial for accurate quantification of borates, silicates, and other complex oxides [51] [66].

EMPA Protocol

Sample Preparation:

• Embed synthesized products in epoxy resin
• Polish to mirror finish using diamond suspensions (final 0.25 μm)
• Apply carbon coating (15-20 nm thickness) for conductivity
• For non-conductive samples, use thicker carbon coating or alternative substrates

Instrument Setup and Calibration:

• Operate at 15 kV accelerating voltage, 15 nA beam current [51]
• Use wavelength-dispersive spectrometers with appropriate analyzing crystals
• Calibrate using well-characterized mineral standards (e.g., wollastonite for Ca, Si; albite for Na, Al; synthetic oxides for transition metals)
• For boron analysis, use dravite or other borate standards [66]

Quantitative Analysis:

• Use beam size of 1-5 μm to minimize damage, especially for hydrous phases or borates
• Collect counting statistics for 10-20 seconds per element
• Apply ZAF (atomic number-absorption-fluorescence) or φρz (phi-rho-z) matrix corrections
• Analyze multiple points (n≥5) per phase to assess homogeneity
• Report averages with standard deviations for composition variability

Table 2: EMPA Standards and Detection Limits for Common Elements in HP-HT Materials

Element	Recommended Standard	Analyzing Crystal	Practical Detection Limit (wt%)
B	Dravite	LDE1/TAP	0.05
C	Synthetic diamond	LDE1/LS1	0.10
O	Willemite	LDE2/TAP	0.15
Si	Wollastonite	TAP	0.05
Ca	Wollastonite	PET	0.03
Fe	Hematite	LIF	0.10
Tb	TbF₃	LIF	0.15

Transmission Electron Microscopy (TEM)

Application Notes

TEM provides nanoscale structural and chemical information that is complementary to bulk techniques, particularly valuable for heterogeneous samples, polyphase assemblages, or nanocrystalline products common in HP-HT syntheses. Conventional high-resolution TEM (HRTEM) reveals lattice fringes and defects, while advanced techniques like 3D electron diffraction (3D ED) enable complete structure solution from nanocrystals as demonstrated for parabreyite, where 3D ED combined with synchrotron SC-XRD solved the structure of this new high-pressure CaSiO₃ polymorph (space group P1̄, a=8.1911(10) Å, b=9.3441(9) Å, c=10.4604(10) Å, α=73.901(8)°, β=89.814(9)°, γ=77.513(9)°) [51].

For complex materials with structural modulations, such as 2D van der Waals materials or incommensurate structures, combined electron diffraction and dark-field (DF) imaging can resolve stacking sequences, domain boundaries, and local symmetry breaking [67]. Automated diffraction tomography and continuous-rotation ED methods have dramatically improved the reliability of nanocrystal structure analysis [65].

TEM Protocols

Sample Preparation for HP-HT Materials

Mechanical Thinning:

• Embed crushed powder in epoxy resin
• Prepare ultrathin sections (100-200 nm) using ultramicrotome with diamond knife
• Transfer sections to lacey carbon Cu grids

Focused Ion Beam (FIB) Milling:

• Identify region of interest using SEM
• Deposit protective Pt or W layer (1-2 μm)
• Lift-out thin lamella (10-15 μm × 5 μm × 2 μm) using micromanipulator
• Attach to TEM grid and thin to electron transparency (≤100 nm) with decreasing ion energy
• Use low-energy (2-5 kV) final cleaning to minimize damage

3D Electron Diffraction Protocol

Data Collection:

• Use parallel beam mode with defocused beam (≈150 nm for LaB₆, ≈30 nm for FEG) [51]
• Select crystal using nanoprobe or microdiffraction mode
• Collect continuous-rotation ED data with 0.1-0.5° frame width over ≥90° range
• Use hybrid pixel detector (e.g., ASI Timepix, CheeTah) for single-electron detection [51]

Data Processing with Instamatic-solve:

• Automate processing using Instamatic-solve pipeline [65]
• Index patterns with XDS package [65]
• Solve structure using SHELXT with preliminary chemical composition [65]
• Validate with figures of merit (Rint, completeness, resolution)

Advanced Analysis:

• For complex modulations, combine with dark-field imaging to correlate local structure with diffraction features [67]
• For beam-sensitive materials (e.g., borates, hydrous phases), use low-dose techniques and cryo-cooling

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Essential Materials for HP-HT Synthesis and Analysis

Item	Function	Application Notes
MgO Octahedra	Pressure medium in multianvil assemblies	Standard 25M assembly (25 mm edge length) for 4-8 GPa range [51]
Graphite Furnaces	Heating element in HP cells	Compatible with various capsule materials; enables temperatures >2000°C
Gold Capsules	Sample encapsulation	3 mm diameter typical; inert, minimal Fe loss; suitable for silicate, borate systems [51]
PtRh Thermocouples	Temperature monitoring	Type S (PtRh30/PtRh6); junction near crystallization capsule; accuracy ±20°C [51]
Diamond Knives	Ultramicrotomy sample preparation	Sectioning hardened HP-HT samples to 100-200 nm thickness for TEM
Lacey Carbon Grids	TEM sample support	Minimal background for ED; suitable for nanopowders and FIB sections
Reference Minerals	EMPA standardization	Wollastonite (Ca, Si), hematite (Fe), dravite (B) for quantitative analysis [51] [66]

Integrated Workflow for Automated Structure Solution

The Instamatic-solve pipeline represents a significant advancement for automated, real-time structure solution from 3D electron diffraction data, demonstrating successful application across diverse material systems including zeolites, metal-organic frameworks, and pharmaceutical compounds [65].

This automated workflow enables rapid structural characterization of HP-HT synthesized materials, significantly accelerating the research cycle in high-pressure materials discovery.

Validating Phase Purity and Crystal Structure of Recovered Samples

In multianvil high-pressure high-temperature (HP-HT) syntheses research, the accurate validation of phase purity and crystal structure is paramount. The complex physicochemical environment within multianvil apparatuses often produces metastable phases or polytype mixtures, making rigorous postsynthesis characterization a critical step [51]. This protocol details comprehensive methodologies for establishing the phase purity and structural integrity of samples recovered from HP-HT experiments, with specific application to novel silicate polymorphs such as parabreyite synthesized in the CaSiO3 system [51]. These procedures ensure researchers can confidently correlate synthesized material properties with their crystallographic characteristics, forming a reliable foundation for subsequent material performance evaluation in fields ranging from mineral physics to advanced materials design.

Experimental Protocols for Phase Purity Assessment

X-Ray Diffraction (XRD) Analysis

XRD serves as the primary technique for initial phase identification and purity assessment by providing a fingerprint comparison between the measured diffraction pattern and reference data [68].

Procedure:

• Sample Preparation: Gently grind the recovered sample to a fine powder using an agate mortar and pestle. Avoid excessive pressure to prevent introducing strain or amorphization. For samples potentially containing nanoscale crystallites, consider side-loading into a holder to minimize preferential orientation.
• Data Collection: Conduct powder XRD measurements using a diffractometer with Cu Kα radiation (λ = 1.5418 Å). Typical parameters include a 2θ range of 5–90°, a step size of 0.02°, and a counting time of 1–2 seconds per step.
• Phase Identification: Compare the obtained diffraction pattern with reference patterns from the International Centre for Diffraction Data (ICDD) database. For newly discovered phases without database entries, use a simulated pattern generated from a single-crystal structure solution [68] [51].
• Purity Verification: Scrutinize the baseline for broad humps indicating amorphous content and the diffraction peaks for minor shoulders or peaks corresponding to impurity phases. Common impurities in HP-HT syntheses include unreacted starting materials or decomposition products.

Interpretation: High phase purity is confirmed when all observable diffraction peaks can be indexed to the target phase, with no detectable peaks from impurities above the background noise level. Quantitative analysis using Rietveld refinement can provide an estimated percentage of crystalline impurity phases.

Raman Spectroscopy

Raman spectroscopy complements XRD by probing local bonding environments and vibrational modes, making it highly sensitive to structural anomalies and minute crystalline or amorphous impurities [68] [51].

Procedure:

• Sample Setup: Place a representative fragment of the recovered sample on a microscope slide. Ensure a clean, flat surface for analysis to optimize signal quality.
• Spectral Acquisition: Using a confocal Raman microscope, focus a laser beam (common wavelengths: 532 nm or 785 nm) onto the sample surface. Collect spectra from multiple spots (recommended: 10–20 spots for a heterogeneous sample) to assess homogeneity.
• Parameter Settings: Use a grating appropriate for the desired spectral resolution, typically covering a wavenumber range of 100–1200 cm⁻¹ for silicate frameworks. Adjust laser power to avoid damaging or thermally altering the sample.
• Data Comparison: Compare the acquired spectra with reference spectra from phase-pure standards or ab initio simulated phonon data, if available [68].

Interpretation: Consistent Raman spectra across different sample regions indicate homogeneity. The presence of extra peaks or shifts in characteristic peaks suggests impurities or structural deviations. As demonstrated in parabreyite studies, distinct Raman fingerprints can differentiate it from other polymorphs like breyite, even when XRD patterns appear similar [51].

Protocols for Crystal Structure Determination

Synchrotron Single-Crystal X-Ray Diffraction (SC-XRD)

SC-XRD is the definitive method for determining the atomic structure of crystalline phases, providing precise lattice parameters, atomic coordinates, and thermal displacement parameters [51].

Procedure:

• Crystal Selection: Under an optical microscope, isolate a single crystal of suitable size (typically 5–100 μm for synchrotron radiation) and quality from the crushed synthesis product.
• Mounting: Carefully mount the selected crystal on a MiTeGen loop or a glass fiber.
• Data Collection: Collect a complete diffraction dataset at a synchrotron beamline. A high-energy, monochromatic X-ray beam (e.g., λ = 0.4–0.7 Å) is essential for penetrating small crystals and minimizing absorption. Automatically collect a series of diffraction images by rotating the crystal.
• Data Reduction: Process the diffraction images to determine the unit cell and integrate reflection intensities using software like CrysAlisPro or APEX3.
• Structure Solution and Refinement: Solve the structure using direct methods or intrinsic phasing algorithms in programs such as SHELXT or OLEX2. Subsequently, refine the structural model against the F² data using least-squares methods in SHELXL or OLEX2.

Application in HP-HT Research: This protocol enabled the full structural characterization of parabreyite, confirming its triclinic (P1̄) symmetry and unique configuration of threefold tetrahedral rings distinct from breyite [51].

3D Electron Diffraction (3D ED)

For crystallites too small for even synchrotron SC-XRD (typically < 1 μm), 3D ED (also known as microcrystal electron diffraction) provides an powerful alternative for ab initio structure determination [51].

Procedure:

• Sample Preparation: Gently crush the sample and disperse the powder onto a lacey carbon-coated TEM grid.
• Screening: Use a Transmission Electron Microscope (TEM) operating at 200 kV to identify isolated, well-formed nanocrystals.
• Data Collection: Using a parallel electron beam (∼30-150 nm diameter), collect a tomographic series of diffraction patterns by continuously tilting the crystal around a single axis. Hybrid pixel detectors (e.g., Timepix, CheeTah) are used for high-efficiency data collection [51].
• Data Processing: Index the diffraction patterns and reconstruct the 3D reciprocal lattice using dedicated software (e.g., XDS, APEX3). Merge data to produce a structure factor list.
• Structure Solution: Solve the crystal structure using standard crystallographic software, similar to SC-XRD analysis.

Application in HP-HT Research: 3D ED was instrumental in solving the crystal structure of parabreyite nanocrystals, which were unsuitable for conventional SC-XRD analysis [51].

Workflow Visualization

The following diagram illustrates the integrated workflow for validating phase purity and crystal structure, from sample recovery to final reporting.

Data Presentation and Analysis

Quantitative Phase and Structural Data

The following tables summarize exemplary quantitative data obtained from the validation of a new high-pressure polymorph, as reported in recent studies.

Table 1: Crystallographic Data for Parabreyite (a High-Pressure CaSiO3 Polymorph) [51]

Parameter	Value
Crystal System	Triclinic
Space Group	P1̄
a (Å)	8.1911(10)
b (Å)	9.3441(9)
c (Å)	10.4604(10)
α (°)	73.901(8)
β (°)	89.814(9)
γ (°)	77.513(9)
Volume (ų)	748.7(1)
Bulk Modulus, K₀ (GPa)	90.7(5)

Table 2: Analytical Techniques for Phase Purity and Structure Validation

Technique	Key Application	Detected Impurities/Features	Sample Requirements
Laboratory XRD [68]	Bulk phase identification, initial purity check	Competing crystalline phases (e.g., ZrO₂, V₂O₅ in ZrV₂O₇)	Powder, ~100 mg
Synchrotron SC-XRD [51]	Definitive atomic structure determination	N/A (requires single crystal)	Single crystal, 5-100 µm
Raman Spectroscopy [68] [51]	Local structure, bonding, amorphous content	Different polymorphs, amorphous phases, local defects	Solid surface, minimal fluorescence
3D Electron Diffraction [51]	Structure of nano-crystalline material	N/A (targets single nanocrystal)	Nanocrystals (< 1 µm)

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential Materials and Reagents for HP-HT Synthesis and Validation

Item	Function/Application	Example from Context
Multianvil Apparatus	Generates simultaneous high pressure and high temperature for synthesis.	Used to stabilize parabreyite at 4–5 GPa and 600–800 °C [51].
High-Purity Oxide Precursors	Starting materials for solid-state reactions.	ZrO₂ and V₂O₅ for ZrV₂O₇ synthesis [68].
Stoichiometric Gels	Provides atomic-level mixing for enhanced reaction kinetics in wet-chemistry synthesis.	CaSiO₃ gel used as a precursor for high-pressure syntheses [51].
Diamond Anvil Cell (DAC)	Enables in situ high-pressure studies, such as compressibility measurements.	Used for in situ SC-XRD on parabreyite (0–10 GPa) [51].
Synchrotron Radiation	High-intensity X-ray source for probing micro-crystals and high-pressure phenomena.	Essential for SC-XRD data collection from small crystals of high-pressure phases [51].
Reference Crystal Structures	Simulated XRD patterns and Raman spectra for comparison and phase identification.	Ab initio simulated phonon data used to interpret Raman spectra of ZrV₂O₇ [68].

Comparing Multianvil Presses with Piston-Cylinder and Diamond Anvil Cell Techniques

High-pressure, high-temperature (HPHT) techniques are indispensable tools in geoscience and materials science for simulating planetary interiors, synthesizing novel materials, and studying the properties of matter under extreme conditions. The three predominant static compression devices—piston-cylinder, multianvil press, and diamond anvil cell—each occupy a distinct region in pressure-temperature (P-T) space and offer different advantages regarding sample volume, diagnostic access, and experimental feasibility. Framed within a broader thesis on multianvil high-pressure syntheses, this application note provides a structured comparison of these techniques, detailed experimental protocols, and a curation of essential research reagents. Understanding the capabilities and limitations of each apparatus is fundamental to designing experiments that probe the deep Earth or create new materials with unique properties.

Technical Comparison of HPHT Apparatuses

The following table summarizes the fundamental characteristics of the three primary high-pressure apparatuses, providing a direct comparison of their operational ranges and typical applications [1] [69] [70].

Table 1: Key Characteristics of Piston-Cylinder, Multianvil, and Diamond Anvil Cell Apparatuses

Feature	Piston-Cylinder	Multianvil Press (Kawai-type)	Diamond Anvil Cell (DAC)
Typical Pressure Range	Up to ~3-5 GPa [70]	~30 GPa (WC anvils), up to ~90 GPa (SD anvils) [32] [1]	>100 GPa, up to ~400 GPa [1]
Typical Temperature Range	Up to ~2200°C	Up to ~3000°C (resistive heating) [1]	Up to ~6000 K (with laser heating) [1]
Sample Volume	Large (cm³ to mm³)	Moderate (mm³ scale)	Extremely Small (nl to pl) [71]
Key Advantages	Large sample volume, good T homogeneity, relatively simple operation.	Large sample volume, homogeneous heating, suitable for synthesis & physical property measurements.	Highest static pressures, broad optical access, versatile for in-situ spectroscopy.
Primary Limitations	Limited pressure range.	High cost, complex assembly, pressure limited by anvil strength.	Tiny sample volume, challenging temperature homogeneity.
Common Applications	Phase equilibrium studies, mineral synthesis, petrology.	In-situ X-ray/neutron studies, synthesis of novel materials (e.g., nano-polycrystalline diamond), physical property measurements [1].	Studies of deep planetary interiors, material physics at extreme conditions, novel states of matter.

Beyond these core characteristics, the principle of operation differentiates these devices. The piston-cylinder apparatus is a uniaxial device where a single piston compresses a cylindrical sample volume. The Kawai-type multianvil apparatus (KMA) employs a double-stage system where first-stage anvils (or guide blocks) drive a set of eight second-stage anvils to quasi-isostatically compress an octahedral sample volume [1]. The diamond anvil cell (DAC) uses two opposed brilliant-cut diamond anvils with small culet faces to generate immense pressures on a microscopic sample, which is often contained by a pre-indented metal gasket [71] [1].

Table 2: Anvil Materials and Pressure Generation in Different Apparatuses

Apparatus	Common Anvil Materials	Impact of Material on Pressure
Piston-Cylinder	Hardened Steel, Tungsten Carbide (WC)	Tungsten carbide anvils allow operation at the higher end of the device's pressure range.
Multianvil Press	Tungsten Carbide (WC), Sintered Diamond (SD)	Using SD anvils, which are much harder than WC, has pushed KMA pressures from ~30 GPa to over 90 GPa [32] [1].
Diamond Anvil Cell	Single-Crystal Diamond	The unparalleled hardness and strength of diamond are the sole enablers of the multi-hundred GPa pressure regime [1].

Decision Workflow for Apparatus Selection

The choice of an appropriate high-pressure technique is governed by the scientific goal's specific pressure, temperature, and sample size requirements. The following workflow diagram outlines the logical decision process for selecting the most suitable apparatus.

Diagram 1: Apparatus Selection Workflow

Detailed Experimental Protocols

Protocol: Synthesis of Pseudo-Ternary Phosphides via Multianvil Press

This protocol details the high-pressure synthesis of a homogeneous pseudo-ternary transition-metal phosphide, (V,Cr,Mo)P₄, as described in recent literature [31]. The KMA is ideal for this synthesis as it provides the necessary P-T conditions (4-8 GPa, 900-1000°C) and a large enough volume to produce sufficient material for subsequent analysis.

1. Sample Preparation:

• Reactant Preparation: Use vanadium (V), chromium (Cr), and molybdenum (Mo) metal powders. To ensure a homogeneous product, first create a pre-alloyed V-Cr-Mo metal alloy. Weigh and mix the powders in a 1:1:1 molar ratio.
• Phosphorus Addition: Combine the metal alloy with red phosphorus in a stoichiometric ratio to yield the desired (V,Cr,Mo)P₄ composition.
• Capsule Loading: Load the mixed powder into a durable, inert capsule, such as one made from hexagonal boron nitride (hBN) or a noble metal like gold. Seal the capsule securely to prevent phosphorus loss during processing.

2. High-Pressure Assembly:

• Pressure Medium: Assemble the sample capsule within an octahedral pressure medium, such as MgO or a semi-sintered zirconia-based ceramic.
• Anvil Configuration: Place the octahedral assembly into the center of a Kawai-type eight-cube anvil array. For pressures of ~4 GPa, tungsten carbide (WC) anvils with appropriate truncations are suitable.
• Heating Element: Incorporate a cylindrical heater (e.g., a graphite or rhenium foil) around the sample capsule within the pressure medium. Add thermal insulation sleeves (e.g., made from ZrO₂) to minimize heat loss.

3. High-Pressure Experiment:

• Compression: Apply the press load gradually to compress the assembly to the target pressure of 4 GPa.
• Sintering: Once the target pressure is stabilized, heat the sample to 900-1000°C. Maintain these pressure and temperature conditions for a dwell time of approximately 60 minutes to allow for complete reaction and crystallization.
• Quenching: After the dwell time, cut power to the heater to rapidly quench the sample, preserving the high-pressure phase.
• Decompression: Slowly release the press load and decompress the assembly to ambient pressure over several hours to avoid damaging the anvils or the synthesized sample.

4. Product Recovery & Analysis:

• Recovery: Carefully disassemble the pressure medium and extract the sample capsule.
• Characterization: Analyze the run product using techniques such as:
  • Synchrotron X-ray Diffraction (XRD): For crystal structure identification and determination of lattice parameters.
  • Scanning Electron Microscopy (SEM) with EDS: To examine microstructure and confirm homogeneous distribution of V, Cr, and Mo.
  • Scanning Transmission Electron Microscopy (STEM): For nanoscale analysis of composition and crystal structure.

Protocol: Metal-Silicate Partitioning Studies for Planetary Geochemistry

This protocol outlines the use of large-volume presses, including multianvil and piston-cylinder apparatuses, to determine the distribution (partitioning) of highly volatile elements (H, C, N, S) between metallic core-forming and silicate mantle-forming liquids, simulating planetary differentiation [69].

1. Experimental Charge Design:

• Starting Materials: Prepare a homogeneous powder mixture of silicate mineral oxides (e.g., MgO, SiO₂, Fe₂O₃, Al₂O₃) and metals (e.g., Fe, Ni, C, FeS) that matches a model bulk planetary composition.
• Containment: Use single-crystal MgO or sapphire capsules to contain the sample, as they offer excellent chemical inertness at high temperatures.

2. High-Pressure Experiment:

• Assembly: Load the capsule into the high-pressure assembly, ensuring proper thermal insulation (e.g., ZrO₂ sleeves) and a graphite or LaCrO₃ heater.
• Run Conditions: Compress the assembly to the target pressure (e.g., 1-25 GPa for multianvil). Heat the sample to superliquidus temperatures (e.g., >1800-2400°C) to ensure complete melting of both metal and silicate phases, allowing them to equilibrate.
• Oxygen Fugacity (fO₂) Control: The fO₂ is a critical parameter. It can be controlled by the choice of capsule material and starting composition or fixed by using solid oxygen buffers (e.g., Ni-NiO).
• Quench & Decompress: After a sufficient time for equilibration (minutes to hours), quench the sample to glass and metal alloy, then decompress.

3. Analytical Procedure:

• Sample Preparation: Recover the experimental charge and mount it in epoxy resin. Polish the mount to expose the interior for analysis.
• Phase Identification: Use a scanning electron microscope to identify quenched metal spheres within the silicate glass matrix.
• Quantitative Analysis:
  • Electron Probe Microanalysis (EPMA): Perform high-precision, high-spatial-resolution major element analysis on both metal and silicate phases.
  • Secondary Ion Mass Spectrometry (SIMS) or NanoSIMS: Measure the concentrations of trace volatile elements (H, C, N) in the different phases with high sensitivity.
  • Laser Ablation ICP-MS (LA-ICP-MS): For trace element analysis of a wider range of siderophile and lithophile elements.

The Scientist's Toolkit: Key Research Reagents & Materials

Successful high-pressure experimentation relies on a suite of specialized materials for anvils, pressure media, heaters, and insulation.

Table 3: Essential Materials for High-Pressure Research

Material	Primary Function	Key Properties & Notes
Tungsten Carbide (WC)	Second-stage anvils in KMA; parts in piston-cylinder.	High hardness and compressive strength. Pressure generation is limited by its strength (~30-50 GPa in KMA) [1].
Sintered Diamond (SD)	Second-stage anvils in KMA for ultrahigh-pressure.	Much harder than WC, enabling pressures >90 GPa in KMA [32] [1].
Single-Crystal Diamond	Anvils in Diamond Anvil Cells.	Extreme hardness and transparency to X-rays and light; essential for the highest pressures [1].
MgO + Cr₂O₃ / ZrO₂	Octahedral pressure medium in KMA.	Semi-sintered ceramics that transmit pressure quasi-isostatically and provide thermal and electrical insulation.
Rhenium (Re)	Gasket material in DAC; heater in KMA.	High strength at high T; used as a gasket to confine samples in DACs [71]. Excellent refractory properties.
Graphite	Heating element in KMA & piston-cylinder.	Good electrical conductivity and high thermal stability in inert environments.
Hexagonal Boron Nitride (hBN)	Capsule/sleeve material.	Chemically inert, high-temperature stability; ideal for containing reactive samples like phosphides [31].
CsCl / CsBr	Thermal insulation sleeve in HPHT cells.	High electrical resistivity and good thermal insulation properties; helps reflect heat inward [72].
Pyrophyllite	Gasket and insulation material.	A hydrous aluminum silicate; used as a gasket and secondary insulation material in HPHT cells [72].

The synthesis of materials at high-pressure and high-temperature (HP-HT) conditions represents a frontier in modern materials science, enabling the creation of substances with exceptional properties not found in nature. This field has revolutionized our ability to design and produce superhard materials, defined by a Vickers hardness exceeding 40 GPa, for demanding industrial applications ranging from precision machining to extreme environment electronics [73] [74]. Within this domain, cubic boron nitride (c-BN) stands out as a material of paramount importance. Despite the superior hardness of diamond, its practical application is limited by poor thermal stability in air above 800°C and a tendency to react chemically with ferrous metals. c-BN, with its unique zincblende crystal structure, addresses these limitations by offering exceptional thermal stability (up to 1400°C in inert atmospheres) and chemical inertness toward iron-based alloys, making it irreplaceable for machining hard ferrous materials [75] [76] [77]. Furthermore, the pursuit of novel materials has extended beyond single-phase substances to complex nanocomposites and heterostructures, such as C(_2)-BN and cubic-hexagonal diamond composites, which are engineered at the nanoscale to overcome the traditional trade-off between hardness and toughness [74] [78]. This case study, framed within a broader thesis on multianvil HP-HT syntheses research, provides a detailed property analysis and experimental protocols for these advanced materials.

Experimental Protocols for HP-HT Synthesis and Processing

High-Pressure High-Temperature (HP-HT) Synthesis of c-BN

The synthesis of cubic boron nitride primarily relies on the HP-HT method, which mimics the natural conditions of diamond formation but with precise control over parameters and catalysts.

• Primary Starting Material: The process typically begins with hexagonal Boron Nitride (h-BN) powder as the precursor material. For high-quality synthesis, the purity of the h-BN powder should be ≥99.5%, with an average grain size of 1-3 µm [79].
• Catalyst System: The phase transformation from the hexagonal to the cubic form is facilitated by catalyst-solvent systems. Alkali and alkaline earth metal compounds, such as Lithium Nitride (Li(3)N), Magnesium Boron Nitride (Mg(3)BN(_2)), and other nitrides, are commonly employed. These catalysts lower the required pressure and temperature for the transformation, making the process more feasible on an industrial scale [76].
• HP-HT Conditions: The h-BN and catalyst mixture is sealed in a capsule, often made of pyrophyllite, and placed in a press. Industrial synthesis is typically conducted at pressures of 5–7 GPa (approximately 50,000-70,000 atmospheres) and temperatures between 1400–1800°C [76].
• Apparatus: The process can be carried out in various types of large-volume presses, including belt-type presses and multi-anvil presses (e.g., a six-anvil apparatus), which are capable of generating the required uniform ultra-high pressure [79] [76].
• Post-Synthesis Processing: After the HP-HT treatment, the resulting compact contains c-BN crystals bonded by the catalyst metal. The product is subsequently subjected to acid washing (e.g., with hydrochloric or nitric acid) to remove residual metallic catalyst and by-products, yielding pure c-BN microcrystals or aggregates [76].

Sintering of Polycrystalline c-BN (PcBN) Compacts

For application in cutting tools, c-BN grains are sintered into polycrystalline compacts (PcBN) with the addition of binders.

• cBN Powder Preparation: c-BN grains, obtained from the synthesis process, are used. The grain size significantly influences the final properties; for example, smaller grains (e.g., 1-3 µm) generally contribute to higher hardness [79] [77].
• Binder Systems: Metallic and ceramic binders are mixed with the cBN powder to facilitate sintering and impart specific properties.
  • Metallic Binders: Examples include Aluminum (Al), Cobalt (Co), Nickel (Ni), Titanium (Ti), and Molybdenum (Mo). These binders often improve fracture toughness and thermal conductivity but may have lower high-temperature softening points [79].
  • Ceramic Binders: These include Titanium Carbide (TiC), Titanium Nitride (TiN), Titanium Carbonitride (Ti(C,N)), and Aluminum Nitride (AlN). Ceramic binders enhance high-temperature hardness and stability, as well as oxidation resistance, but can be more brittle [79] [77].
• Mixing and Pre-compaction: The initial powders are mixed into a uniform slurry, often using a liquid plasticizer, and then dried to form spherical granules to ensure a uniform charge. This powder mixture is then pre-compacted at a lower pressure (e.g., ~420 MPa) into a "pre-shape" [79].
• Sintering HP-HT Cycle: The pre-shape is sintered using a multi-anvil HP-HT apparatus. A representative protocol involves subjecting the compact to a pressure of 4.5–7.5 GPa and a temperature of 1400–2273°C for a specific dwell time, typically 2 to 5 minutes [79] [74]. During this process, chemical reactions occur (e.g., Al melting at 660°C and reacting with cBN to form AlN and AlB(_{12})), leading to densification and the formation of a solid compact [79].
• Post-Processing: The sintered PcBN compact is then cut into specific tool shapes, such as SNGN 1204 inserts, using electro-discharge machining (EDM) and ground to final dimensions [79].

Synthesis of Advanced Superhard Composites

Recent research has focused on creating composite materials that integrate multiple ultra-hard phases.

• C(_2)-BN Nanocomposite Synthesis: This involves reacting nanodiamond (average grain size ~50 nm) with pre-synthesized cubic Boron Nitride (average grain size ~250 nm) in a stoichiometric ratio [74].
• HP-HT Consolidation: The mixed powders are sintered in a large-volume press at conditions of 7.5 GPa and 2273 K (≈2000°C) for 2 hours [74]. This process results in a nanocomposite where nanotwinned diamond and cBN domains are "stitched" together by sp(^3)-hybridized C-B and C-N bonds at their interfaces.
• Cubic-Hexagonal Diamond Heterostructure Synthesis: A novel, bioinspired approach involves using engineered curved graphite precursors to create localized stress concentrations [78].
• HP-HT Transformation: These curved graphite precursors are processed at 15 GPa and 2300 K (≈2027°C). The localized stress fields promote the simultaneous nucleation of hexagonal diamond (HD) within a cubic diamond (CD) matrix, forming a coherent CD-HD heterostructure [78].

The workflow for the synthesis and characterization of these superhard materials is systematized in the following protocol.

Property Analysis and Data Presentation

Mechanical and Physical Properties of Superhard Materials

The properties of HP-HT synthesized materials are characterized by a combination of extreme hardness, thermal stability, and unique functional characteristics. The data below provides a comparative analysis.

Table 1: Comparative Properties of HP-HT Synthesized Superhard Materials

Material	Crystal Structure / Type	Vickers Hardness (GPa)	Fracture Toughness (MPa·m(^{1/2}))	Thermal Stability in Air	Key Characteristics & Bonding
c-BN (Cubic Boron Nitride)	Sphalerite (cubic), sp(^3) [76]	45 - 50 [75]	5.0 - 7.0 (varies with binder)	Up to ~1200-1400°C [75] [77]	Chemically inert with ferrous metals; wide bandgap (~6.4 eV) [76]
PcBN (High cBN content)	Polycrystalline composite with metallic/ceramic binders [79]	41.7 (at 5 kgf) [79]	7.6 ± 0.5 [79]	Similar to c-BN, binder-dependent	High fracture toughness; used for rough, non-continuous cutting [79]
PcBN (Low cBN content)	Polycrystalline composite with higher binder volume [79]	37.3 (at 5 kgf) [79]	6.4 ± 0.5 [79]	Similar to c-BN, binder-dependent	Soother cutting; used for continuous finishing [79]
C(_2)-BN Nanocomposite	Nanotwinned diamond and cBN domains stitched by sp(^3) C-B and C-N bonds [74]	85 ± 2 [74]	Not explicitly stated	High (exceeds sintered cBN) [74]	p-type semiconductor; domains ~50-250 nm [74]
Cubic-Hexagonal Diamond Heterostructure	Interlocked cubic (CD) and hexagonal (HD) diamond [78]	169 [78]	15.7 [78]	High	Bioinspired synthesis; overcomes hardness-toughness trade-off [78]
w-BN (Wurtzite BN)	Wurtzite (hexagonal), sp(^3) [76]	~9.0 (Mohs) [76]	Not explicitly stated	< 1200°C [76]	Metastable phase; high thermal conductivity [76]

Functional Performance in Applications

The ultimate test for superhard materials is their performance in real-world applications, particularly in machining and cutting tools.

Table 2: Performance of cBN Tools with Different Binders in High-Speed Turning of Compacted Graphite Iron (CGI) [77]

cBN Tool Grade (Binder Type)	cBN Content / Grain Size	Predominant Wear Mechanism(s)	Key Performance Findings
TiC Ceramic Binder	~50-90% content	Adhesion, Abrasion, Diffusion	Formation of a protective oxide layer (e.g., Al(2)O(3) from Al in binder) on the tool flank face can act as a diffusion barrier, delaying wear.
Al Metallic Binder	~50-90% content	Adhesion, Abrasion	The compatibility between the binder and the workpiece material significantly influences tool life and wear rate.
TiN Ceramic Binder	~50-90% content	Adhesion, Abrasion	Tools with TiC and Al binders can achieve higher material removal rates and extended tool life compared to other binders.

Characterization Techniques and Data Interpretation

A robust characterization protocol is essential for linking synthetic parameters to the structural and functional properties of HP-HT materials.

• X-ray Diffraction (XRD): Used for phase identification and to detect reaction products. For example, XRD can confirm the consumption of metallic Al binder and the formation of new phases like AlN and AlB(_2) during sintering [79]. Broadening of diffraction peaks can indicate a reduction in crystallite size or the presence of microstrain, as observed in nanocomposites and curved graphite precursors [74] [78].
• Scanning Electron Microscopy (SEM): Provides information on grain morphology, distribution, and porosity. SEM in Back-Scattered Electron (BSE) mode is particularly useful for distinguishing between different phases (e.g., cBN grains vs. binder regions) in a PcBN compact [79]. It is also used to examine tool wear morphologies, such as flank wear and crater wear, after machining tests [77].
• Transmission Electron Microscopy (TEM/HRTEM): This is a critical technique for analyzing nanoscale microstructure. HRTEM can reveal details such as nanotwinning within diamond and cBN domains (1.5-8 nm wide), the ~1-2 nm thick "sutures" between domains in a C(_2)-BN composite, and the dislocations and stacking faults that accommodate lattice mismatch [74] [78].
• Spectroscopic Techniques:
  • Raman Spectroscopy: Probes vibrational modes and disorder. The D and G bands in graphite-derived precursors indicate defect density, which can be correlated to synthesis conditions [78]. A sharp peak at 1332 cm(^{-1}) confirms the presence of diamond [74].
  • Electron Energy-Loss Spectroscopy (EELS) & Inelastic X-ray Scattering (IXS): These techniques analyze local chemical bonding and hybridization. They can definitively prove the presence of sp(^3)-hybridized B-C-N bonds at the interfaces in C(_2)-BN composites, distinguishing them from graphitic (sp(^2)) carbon [74].
• Mechanical Property Measurement:
  • Vickers Hardness: Measured using a diamond pyramid indenter. The calculation is ( H_v = 1.8544 \frac{P}{d^2} ), where ( P ) is the load and ( d ) is the diagonal of the indent. The "indentation size effect" must be considered, where hardness values measured at low loads (e.g., 2 kgf) can be significantly higher than those at standard loads (e.g., 5 kgf) [79].
  • Fracture Toughness: Often calculated from the cracks emanating from Vickers indentations using equations like Niihara's, which incorporates Young's modulus, hardness, load, and crack length [79].

The Scientist's Toolkit: Research Reagent Solutions

Successful HP-HT research relies on a suite of specialized materials and reagents, each serving a specific function in the synthesis and processing pipeline.

Table 3: Essential Materials and Reagents for HP-HT Superhard Material Research

Reagent / Material	Function in HP-HT Research	Specific Examples & Notes
h-BN Powder (High Purity)	Primary precursor for c-BN synthesis.	Purity ≥99.5%; grain size 1-3 µm [79].
Nanodiamond Powder	Starting material for diamond composites and sintering.	Average grain size ~50 nm for C(_2)-BN synthesis [74].
Catalyst-Solvents	Lower the pressure/temperature required for h-BN to c-BN transformation.	Li(3)N, Mg(3)BN(_2), other alkali/alkaline earth metal nitrides [76].
Metallic Binders	Facilitate sintering of PcBN; can improve fracture toughness.	Al, Co, Ni, Ti, Mo. Al melts during sintering, aiding densification and forming new ceramic phases (AlN, AlB(_{12})) [79].
Ceramic Binders	Enhance high-temperature hardness and oxidation resistance of PcBN.	TiC, TiN, Ti(C,N), AlN. Titanium compounds react with cBN to form TiN and TiB(_2) [79] [77].
Graphite Precursors	Starting material for diamond synthesis.	Can be engineered with curvature (e.g., "graphitic mimosa") to promote hexagonal diamond nucleation [78].
Pyrophyllite Capsules	Pressure-transmitting medium and sample container in multi-anvil presses.	Provides thermal and electrical insulation; machined into custom shapes [79].
Acid Etchants	Post-synthesis removal of catalyst residues and metallic binders.	Hydrochloric acid (HCl), Nitric acid (HNO(_3)) [76].

This case study underscores the critical role of precise HP-HT synthesis protocols in tailoring the microstructure and properties of advanced superhard materials. The analysis confirms that while single-phase c-BN remains a cornerstone for machining ferrous alloys, the future of the field lies in the rational design of composite and heterostructured systems. Materials like the C(_2)-BN nanocomposite and the bioinspired cubic-hexagonal diamond heterostructure demonstrate that it is possible to transcend historical property trade-offs, achieving simultaneous gains in hardness and toughness through nanoscale engineering of interfaces and phases [74] [78]. The continued development of these materials, guided by deep microstructural characterization and an understanding of structure-property relationships, promises to unlock new capabilities for industrial machining, extreme-environment electronics, and high-pressure science. The experimental frameworks and property databases presented here provide a foundational resource for ongoing research within the broader context of multianvil HP-HT syntheses.

Assessing Sample Volume and Hydrostaticity for Different Research Needs

High-pressure high-temperature (HP/HT) synthesis serves as a fundamental tool in material science and chemistry, enabling the discovery and production of novel functional materials that are unattainable under ambient conditions. By applying mechanical compression force, HP/HT techniques decrease atomic volume and increase electron density of reactants, leading to unusual and interesting properties through structural transformations or the formation of new chemical bonds [80] [8]. The parameter of hydrostaticity—the uniform transmission of pressure through a medium—is crucial for achieving reproducible results and preventing sample failure, while the available sample volume directly impacts the potential applications and scalability of synthesized materials. Within the broader context of multianvil HP/HT research, understanding the interplay between these factors—pressure range, temperature capability, sample volume, and hydrostaticity—is essential for selecting the appropriate apparatus for specific research goals, from exploratory solid-state chemistry to the green synthesis of organic compounds and pharmaceuticals [81] [80].

High-Pressure Apparatus and Technical Specifications

The selection of a high-pressure apparatus represents a critical decision point in experimental design, dictated by the target pressure, required sample volume, and the necessary degree of hydrostaticity.

Table 1: Comparison of High-Pressure Apparatus and Their Capabilities

Apparatus Type	Maximum Pressure (GPa)	Typical Sample Volume	Hydrostaticity & Common Media	Primary Research Applications
Piston-Cylinder	1 - 3 [81] [8]	1 - 1000 cm³ [8]	Good; gases, liquid salts [8]	General solid-state synthesis, inorganic materials [8]
Belt Apparatus	Up to 10 [81]	~1 cm³ (at >5 GPa) [8]	Non-hydrostatic; solid media (e.g., pyrophyllite)	Superhard materials (e.g., diamond, cBN) [8]
Multianvil (Walker-type)	25 [81]	Not specified in results	Non-hydrostatic; solid media	Extended P/T conditions for solid-state chemistry [81]
Bridgman Anvil (Hard Alloy)	15 - 20 [8]	Small (decreases with pressure)	Non-hydrostatic; solid media	Material property studies, phase transitions [8]
Diamond Anvil Cell (DAC)	100 - 300 [8]	Extremely small (micrometers)	Hydrostatic (if liquid medium used); gases, liquids	Ultra-high-pressure physics, spectroscopy [8]
High Hydrostatic Pressure (HHP) Reactors	2 - 20 kbar (0.2 - 2 GPa) [80]	Scalable to industrial volumes [80]	Excellent; water, other fluids	Green organic synthesis, barochemistry, food processing [80]

Key Concepts: Hydrostaticity and Sample Volume

• Hydrostaticity: This refers to a state where pressure is applied uniformly from all directions to a sample. It is typically achieved using fluid pressure-transmitting media like water, oils, or noble gases. Non-hydrostatic conditions, created by solid media, can lead to shear stresses and sample failure but are necessary for the highest pressures [80] [8].
• Sample Volume: A fundamental trade-off exists in high-pressure technology; as the target pressure increases, the achievable sample volume decreases significantly. This is a key limitation of ultra-high-pressure devices like Diamond Anvil Cells (DACs), whereas large-volume presses (e.g., piston-cylinder) are restricted to more moderate pressures [8].
• Pressure Cycling: A technique used in barochemistry where the system is repeatedly pressurized and decompressed. This action causes periodic volume changes that can improve reaction kinetics and yield, potentially by enhancing mass transfer and molecular re-alignments [80].

Application-Specific Experimental Protocols

The choice of HP/HT method is dictated by the end goal, whether it is synthesizing a new perovskite, a superconducting material, or a pharmaceutical intermediate.

Protocol 1: Synthesis of Magnetic Perovskite Oxides

This protocol details the synthesis of BiCu₀.₄Mn₀.₆O₃, a representative complex perovskite stabilized under high pressure [82].

• Objective: To stabilize a metastable, magnetic perovskite oxide with potential multifunctional properties.
• Materials:
  • Starting Materials: High-purity Bi₂O₃, CuO, and MnO₂ powders.
  • Pressure Medium: A solid medium assembly, typically pyrophyllite or a similar ceramic.
  • Sample Capsule: A noble metal capsule (e.g., platinum, gold) to isolate the sample from the medium.
• Apparatus: Walker-type multianvil press or equivalent.
• Procedure:
  • Pre-compaction: Finely grind the precursor powders and stoichiometrically mix them in an inert atmosphere.
  • Loading: Load the mixed powder into the metal capsule and seal it. Assemble the capsule within the solid pressure medium inside the multianvil system.
  • Compression: Apply isotropic pressure to the system until the target of 4 GPa is reached.
  • Heating: While maintaining pressure, heat the sample to 1000 °C for 30 minutes. The short duration is critical to prevent phase degradation.
  • Quenching: Rapidly cool the sample to room temperature while maintaining pressure.
  • Decompression: Slowly release the pressure to ambient conditions over several hours to retain the metastable perovskite phase.
• Notes: The obtained samples are often characterized by structural defectivity, leading to broad peaks in diffraction patterns. The choice of space group for structural analysis should consider this inherent defectivity to avoid over-interpretation [82].

Protocol 2: Green Organic Synthesis via Barochemistry

This protocol outlines the use of High Hydrostatic Pressure (HHP) for solvent-free or catalyst-free organic synthesis, such as Diels-Alder reactions or cyclizations [80].

• Objective: To activate chemical reactions efficiently, achieving higher yields and selectivity under green chemistry principles.
• Materials:
  • Reactants: Pure organic starting materials.
  • Pressure Transmitting Fluid: Water is recommended due to its low compressibility, non-toxicity, non-flammability, and low cost. In some cases, silicone oil or other fluids may be used.
  • Vessel Liner: An inert, flexible container (e.g., Teflon, polypropylene) to hold the reactants and separate them from the water.
• Apparatus: Industrially available HHP reactor (batch or stopped-flow).
• Procedure:
  • Loading: Weigh the reactants and place them inside the flexible container. Seal the container to prevent contamination by the pressure fluid.
  • Assembly: Place the sealed container into the high-pressure vessel and fill the remaining volume with water.
  • Pressurization: Close the vessel and pressurize it to the target range of 2 - 20 kbar (0.2 - 2 GPa). This can be done as:
    • Static Pressure: Hold at a constant pressure for a defined period.
    • Pressure Cycling: Cycle between high pressure and ambient (or lower) pressure for a set number of cycles (see Figure 1b). This often yields superior results.
  • Reaction: Maintain the target pressure at room temperature for the required reaction time (minutes to hours).
  • Work-up: Depressurize the system, retrieve the product container, and proceed with standard work-up procedures.
• Notes: The green benefits of this method are multifaceted, including the elimination of catalysts and solvents, higher atom economy, and energy efficiency, as the system requires no continuous energy input once pressurized [80].

Diagram 1: Apparatus selection based on pressure, volume, and hydrostaticity needs.

The Scientist's Toolkit: Research Reagent Solutions

Table 2: Essential Materials for HP/HT Experiments

Item	Function / Purpose	Examples & Notes
Solid Pressure Media	Transmits force to create quasi-hydrostatic conditions for solid-state synthesis.	Pyrophyllite, MgO-doped alumina, zirconia. Choice affects pressure efficiency and thermal insulation [8].
Fluid Pressure Media	Provides true hydrostatic conditions for barochemistry.	Water (most common), silicone oil, noble gases (for ultra-high pressures). Must be non-compressible and inert [80].
Sample Encapsulation	Isolates the sample from the pressure medium to prevent contamination.	Noble metal capsules (Pt, Au) for solid-state synthesis; Teflon/Polymer liners for organic synthesis in HHP [80] [82].
Resistance Heating Assembly	Provides the high temperature required for synthesis and sintering.	Graphite or metal (e.g., tungsten, molybdenum) furnaces placed within the pressure chamber.
Thermocouples	Monitors and controls the in-situ temperature during the experiment.	Type K, S, or R, depending on temperature range. Placement is critical for accurate measurement.

The strategic assessment of sample volume and hydrostaticity requirements is a cornerstone of successful multianvil HP/HT research. As this guide has detailed, the landscape of high-pressure apparatus offers a spectrum of tools, each with distinct advantages. The large sample volumes of piston-cylinder devices are well-suited for initial synthetic explorations, while the extreme pressures of multianvil and diamond anvil systems are indispensable for stabilizing novel, metastable phases like the BiCu₀.₄Mn₀.₆O₃ perovskite [82]. Conversely, the excellent hydrostaticity provided by HHP reactors with water as a fluid medium opens a vast field for green organic synthesis, enabling reactions with higher yields and selectivity under catalyst- and solvent-free conditions [80]. The provided protocols, selection logic, and reagent tables offer a foundational framework for researchers to align their specific scientific objectives with the most appropriate and effective high-pressure techniques, thereby driving innovation in the synthesis of next-generation functional materials and chemicals.

Conclusion

Multianvil HP-HT synthesis stands as an indispensable technology, bridging our understanding of planetary interiors with the creation of next-generation materials. The stable, large-volume conditions it provides are crucial for synthesizing millimeter-sized single crystals of mantle minerals and bulk volumes of superhard materials like nanocrystalline diamond. Future directions point toward integrating more robust in-situ characterization techniques, pushing pressure generation beyond 100 GPa with advanced anvil materials, and further exploring the synthesis of metastable phases with tailored properties. For biomedical and clinical research, the implications are profound, ranging from the development of ultra-hard materials for surgical tools to novel compounds for drug delivery systems and diagnostic platforms, all enabled by this unique high-pressure dimension.

References

• 1. Kawai-type multianvil ultrahigh-pressure technology - PMC [https://pmc.ncbi.nlm.nih.gov/articles/PMC11105972/]
• 2. Kawai-type multianvil ultrahigh-pressure technology [https://www.jstage.jst.go.jp/article/pjab/100/3/100_pjab.100.013/_article/-char/en]
• 3. The Multi-Anvil Apparatus - SERC [https://serc.carleton.edu/NAGTWorkshops/mineralogy/mineral_physics/multi_anvil.html]
• 4. Anvil press [https://en.wikipedia.org/wiki/Anvil_press]
• 5. I. Multianvil Apparatus (Cubic anvil, Kawai-type apparatas) [https://home.hiroshima-u.ac.jp/kawazoe/html/Kawazoe04-Method-EN.html]
• 6. A Breakthrough in Pressure Generation by a Kawai-Type ... [https://www.sciencedirect.com/science/article/pii/S2095809918306787]
• 7. High-pressure generation in the Kawai-type multianvil ... [https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/j.crte.2018.07.004/]
• 8. High-Pressure High-Temperature (HPHT) Synthesis of ... [https://www.intechopen.com/chapters/58807]
• 9. Development of Static High-Pressure Techniques and the ... [https://www.cambridge.org/core/books/static-and-dynamic-high-pressure-mineral-physics/development-of-static-highpressure-techniques-and-the-study-of-the-earths-deep-interior-in-the-last-50-years-and-its-future/79B068CCA7559FDA636987EBFCE7333F]
• 10. HPHT Diamond Manufacturing: Complete Technical Guide ... [https://labrilliante.com/education/how-are-diamonds-grown/hpht]
• 11. United States HPHT Diamond Cubic Press Market Size 2026 [https://www.linkedin.com/pulse/united-states-hpht-diamond-cubic-press-market-size-fygof]
• 12. High Pressure (HP) in Spark Plasma Sintering (SPS) ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC9318362/]
• 13. Analysis of the High-Pressure High-Temperature (HPHT) ... [https://www.sciencedirect.com/science/article/abs/pii/S0022024823000763]
• 14. Homogeneous spin-dephasing time of NV − centre in ... [https://www.nature.com/articles/s43246-025-00782-7]
• 15. The Multi-Anvil Apparatus [https://serc.carleton.edu/msu_nanotech/methods/Multi_Anvil.html]
• 16. 12 - Synthesis of High-Pressure Silicate Polymorphs Using ... [https://www.cambridge.org/core/books/static-and-dynamic-high-pressure-mineral-physics/synthesis-of-highpressure-silicate-polymorphs-using-multianvil-press/34FBB30EF5E63B88C2FC46DE066615D4]
• 17. Calibration of a multi-anvil high-pressure apparatus to ... [https://epjtechniquesandinstrumentation.springeropen.com/articles/10.1140/epjti/s40485-018-0047-z]
• 18. Techniques - high-pressure synthesis | Core Facilities [https://cores.research.asu.edu/materials/techniques-high-pressure-synthesis]
• 19. Multianvil high-pressure/high-temperature synthesis and ... [https://pubs.rsc.org/en/content/articlehtml/2021/tc/d0tc05210h]
• 20. Floating zone growth of large tetragonal Ruddlesden ... [https://www.nature.com/articles/s42005-025-02340-6]
• 21. Toroidal diamond anvil cell for detailed measurements ... [https://www.nature.com/articles/s41467-018-05294-2]
• 22. MASTer: A high-performance stable temperature controller ... [https://www.researching.cn/articles/OJ303f0737be4e6d34]
• 23. High-pressure high-temperature synthesis of NdRe2 [https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2024.1259032/full]
• 24. 4 - Development of Large-Volume Diamond Anvil Cell for ... [https://www.cambridge.org/core/books/static-and-dynamic-high-pressure-mineral-physics/development-of-largevolume-diamond-anvil-cell-for-neutron-diffraction-the-neutron-diamond-anvil-cell-project-at-ornl/FA88B7B37F2DD236F10BF1DB8EFC90C5]
• 25. Laser heated diamond anvil cell facility for high ... [https://link.springer.com/article/10.1007/s12648-018-1237-x]
• 26. Multianvil Presses [https://highpressure.ethz.ch/facilities/experimental/multianvil-presses.html]
• 27. In Situ High-Pressure Synthesis of New Outstanding Light ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC8348659/]
• 28. In Situ High-Pressure Synthesis of New Outstanding Light ... [https://www.mdpi.com/1996-1944/14/15/4245]
• 29. One-Atmosphere Gas Mixing Furnaces [https://highpressure.ethz.ch/facilities/experimental/one-atmosphere-gas-mixing-furnaces.html]
• 30. High pressure and high temperature synthesis of a new ... [https://www.sciencedirect.com/science/article/abs/pii/S0038109824003296]
• 31. High-pressure synthesis, crystal structure and anisotropic ... [https://www.sciencedirect.com/science/article/abs/pii/S0022459624004754]
• 32. Recent advances of high-pressure generation in a ... [https://www.sciencedirect.com/science/article/pii/S167498711000037X]
• 33. Phase relations of bridgmanite, the most abundant mineral ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11787361/]
• 34. Thermodynamic Properties of Fe-Bearing Wadsleyite and ... [https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2022.879678/full]
• 35. Constraining composition and temperature variations in the ... [https://www.nature.com/articles/s41467-022-28709-7]
• 36. Amorphous diamond embedded in dense boron nitride ... [https://www.oaepublish.com/articles/microstructures.2023.54]
• 37. (PDF) Simultaneous crystallization of diamond and cubic ... boron [https://www.academia.edu/85309362/Simultaneous_crystallization_of_diamond_and_cubic_boron_nitride_from_the_graphite_relative_boron_carbide_nitride_BC2N_under_high_pressure_high_temperature_conditions]
• 38. Time-Resolved X-ray Diffraction Study of Solid Combustion ... [https://pubmed.ncbi.nlm.nih.gov/17812168/]
• 39. Kinetics of high-pressure mineral phase transformations ... [https://www.cambridge.org/core/journals/mineralogical-magazine/article/kinetics-of-highpressure-mineral-phase-transformations-using-in-situ-timeresolved-xray-diffraction-in-the-parisedinburgh-cell-a-practical-guide-for-data-acquisition-and-treatment/D7CA5DFD2B8F3460EF07B0FF4C02F6E4]
• 40. Real time monitoring of phase transformation and strain ... [https://www.sciencedirect.com/science/article/abs/pii/S0924013614002313]
• 41. Phase transition of solid bismuth under high pressure [https://cpb.iphy.ac.cn/article/2016/1849/cpb_25_10_108103.html]
• 42. Phase transition and EOS of zinc sulfide under shock ... [https://www.sciencedirect.com/science/article/abs/pii/S002236979800328X]
• 43. Phase transformation of GaAs at high pressures and ... [https://www.sciencedirect.com/science/article/pii/S0022369717315226]
• 44. Using high pressure to investigate the stability of a ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11882918/]
• 45. High-pressure chemistry [https://www.nature.com/articles/s42004-025-01559-2]
• 46. A new large-volume multianvil system [https://www.sciencedirect.com/science/article/abs/pii/S0031920104001359]
• 47. Melting temperatures of MgO under high pressure by micro ... [https://www.nature.com/articles/ncomms15735]
• 48. A Review on High Pressure Experiments for Study of ... [https://www.sciencedirect.com/science/article/abs/pii/S0022354920302707]
• 49. The effect of size matching between anvils and the pressure ... [https://colab.ws/articles/10.1063%2F5.0018188]
• 50. High-pressure and high-temperature single-crystal X-ray ... [https://link.springer.com/article/10.1007/s00269-021-01139-5]
• 51. EJM - Crystal structure and elastic properties of parabreyite: a new... [https://ejm.copernicus.org/articles/37/13/2025/]
• 52. High-Pressure Laser Reactive Synthesis Within Diamond ... [https://www.mdpi.com/2073-4352/15/4/292]
• 53. Implementation of micro-ball nanodiamond anvils for high- ... [https://www.nature.com/articles/ncomms2160]
• 54. New single-toroidal sintered diamond anvil and assembly ... [https://www.sciencedirect.com/science/article/pii/S2773183924000429]
• 55. High pressure and high temperature generation using ... [https://asu.elsevierpure.com/en/publications/high-pressure-and-high-temperature-generation-using-sintered-diam-2/]
• 56. Poster Session & Reception - Meetings and Events Calendar [https://schedule.aps.org/sccm/2025/events/F00/21]
• 57. Multianvil high-pressure / high-temperature synthesis in .. ... [https://www.degruyterbrill.com/document/doi/10.1524/zkri.219.6.330.34633/html?lang=en&srsltid=AfmBOopc2RvCO-ghbsKseR7OS7XbPTZrnOheunHJSYaCRhNzHZsfuSI0]
• 58. Temperature Control and Monitoring [https://ultraflexpower.com/induction-products/optional-equipment/temperature-monitoring/]
• 59. Review of Temperature Measurement Techniques [https://biomedres.us/fulltexts/BJSTR.MS.ID.006465.php]
• 60. Exploring the Frontiers of Cell Temperature Measurement ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11714221/]
• 61. Effects of Thermal Gradients in High-Temperature ... [https://www.mdpi.com/1424-8220/22/7/2799]
• 62. The structure and stability of Fe4+xS3 and its potential to ... [https://www.nature.com/articles/s41467-025-56220-2]
• 63. Multianvil high-pressure / high-temperature synthesis in ... [https://www.bohrium.com/paper-details/multianvil-high-pressure-high-temperature-synthesis-in-solid-state-chemistry/812347524550492160-99049]
• 64. Multianvil high-pressure/high-temperature synthesis and ... [https://pubs.rsc.org/en/content/articlelanding/2021/tc/d0tc05210h]
• 65. Automated and real-time structure solution using 3D electron ... [https://journals.iucr.org/j/issues/2025/06/00/te5157/]
• 66. High-pressure/high-temperature synthesis of the new ... [https://www.sciencedirect.com/science/article/pii/S1293255823002650]
• 67. Electron diffraction and dark-field TEM for structural analysis of ... [https://appmicro.springeropen.com/articles/10.1186/s42649-025-00117-3]
• 68. Synthesis and phase purity of the negative thermal expansion ... [https://pubs.rsc.org/en/content/articlehtml/2025/tc/d4tc04095c]
• 69. Review of experimental and analytical techniques to ... [https://progearthplanetsci.springeropen.com/articles/10.1186/s40645-024-00629-8]
• 70. Multianvil high-pressure / high-temperature synthesis in ... [https://www.scilit.com/publications/fd8ee944efd2d2fd50fc3d0088002a7f]
• 71. NMR at pressures up to 90 GPa [https://www.sciencedirect.com/science/article/abs/pii/S1090780718301320]
• 72. US20130263748A1 - High pressure high temperature cell [https://patents.google.com/patent/US20130263748A1/en]
• 73. Synthesis of superhard materials [https://www.sciencedirect.com/science/article/pii/S1369702105711597]
• 74. Ultrahard stitching of nanotwinned diamond and cubic ... [https://www.nature.com/articles/srep30518]
• 75. Boron Nitride Guide: Properties, Structure & Applications [https://www.samaterials.com/content/boron-nitride-guide-properties-structure-applications.html]
• 76. HBN, CBN, and WBN: A Comparative Analysis of Boron ... [https://www.samaterials.com/content/hbn-cbn-wbn-boron-nitride-polymorphs-comparison.html]
• 77. Wear behaviors of cubic boron nitride tools with various ... [https://www.sciencedirect.com/science/article/abs/pii/S0043164822001764#!]
• 78. Curved graphite precursors enable cubic-hexagonal diamond ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC12577702/]
• 79. Reactions and Phase Transformations at Sintering of Cubic ... [https://www.sciepublish.com/article/pii/293]
• 80. application of high pressure in organic synthesis [https://pubs.rsc.org/en/content/articlehtml/2025/gc/d5gc01075f]
• 81. Multianvil high - pressure / high - temperature in solid state... synthesis [https://www.degruyterbrill.com/document/doi/10.1524/zkri.219.6.330.34633/html]
• 82. High-pressure high-temperature synthesis of magnetic ... [https://www.nature.com/articles/s43246-025-00800-8]