High-Pressure Coupling Reactions to Produce a Spherical Bulk RexN/Fe3N Composite.

We report a novel high-pressure coupling (HPC) reaction that couples the nitridation of Re with high-pressure solid-state metathesis (HPSSM) of Fe3N to produce a spherical bulk RexN/Fe3N composite. Compared with conventional methods, upon coupling of the HPSSM reactions, the synthetic pressure for Re nitridation was successfully reduced from 13 to 10 GPa (for Re3N) and from 20 to 15 GPa (for Re2N). The product RexN species would be surrounded by product Fe3N, resulting in a spherical bulk RexN/Fe3N composite (x = 2 or 3). The composite exhibits a soft magnetic behavior, and the content of nitrogen in RexN (x = 2 or 3) was controlled by adjusting the P-T conditions. The HPC reaction establishes a new approach for the bulk synthesis of 5d transition metal nitride.

Elucidating the Phase Transformation and Metallization Behavior of Zinc Phosphide under High Pressure.

Among the family of II3V2-type compounds, zinc phosphide (Zn3P2) occupies a unique position. As one of the most promising semiconductors well-suited for photovoltaic applications, Zn3P2 has attracted considerable attention. The stability of its structure and properties are of great interest and importance for science and technology. Here, we systematically investigate the pressurized behavior of Zn3P2 using in situ synchrotron radiation angle-dispersive X-ray diffraction (ADXRD) and in situ electrical resistance measurement under high pressure. The ADXRD experiment shows that Zn3P2 undergoes an irreversible structural phase transition under high pressure, beginning at 11.0 GPa and being completed at ∼17.7 GPa. Consistently, the high-pressure electrical resistance measurement reveals a pressure-induced semiconductor-metal transition for Zn3P2 near 11.0 GPa. The kinetics of the phase transition is also studied using in situ electrical resistance measurement and can be well described by the classical Avrami model. What's more, the new high-pressure structure of Zn3P2 is refined to be orthorhombic with space group Pmmn; the lattice parameters and bulk modulus of this high-pressure phase are determined as a = 3.546 Å, b = 5.004 Å, c = 3.167 Å, and B0 = 126.3 GPa. Interestingly, we also predict a possible structural phase transformation of orthorhombic phase (Pmmn) to cubic phase (P4232) during the decompression process; this cubic Zn3P2 is metastable at ambient conditions. These experimental results reveal the unexpected high-pressure structural behaviors and electrical properties of Zn3P2, which could help to promote the further understanding and the future applications of Zn3P2 as well as other II3V2 compounds.

Reciprocating Compression of ZnO Probed by X-ray Diffraction: The Size Effect on Structural Properties under High Pressure.

Zinc oxide, ZnO, an important technologically relevant binary compound, was investigated by reciprocating compress the sample in a diamond anvil cell using in situ high-pressure synchrotron X-ray diffraction at room temperature. The starting sample (∼200 nm) was compressed to 20 GPa and then decompressed to ambient condition. The quenched sample, with average grain size ∼10 nm, was recompressed to 20 GPa and then released to ambient condition. The structural stability and compressibility of the initial bulk ZnO and quenched nano ZnO were compared. Results reveal that the grain size and the fractional cell distortion have little effect on the structural stability of ZnO. The bulk modulus of the B4 (hexagonal wurtzites structure) and B1 (cubic rock salt structure) phases for bulk ZnO under hydrostatic compression were estimated as 164(3) and 201(2) GPa, respectively. Importantly, the effect of pressure in atomic positions, bond distances, and bond angles was obtained. On the basis of this information, the B4-to-B1 phase transformation was demonstrated to follow the hexagonal path rather than the tetragonal path. For the first time, the detail of the intermediate hexagonal ZnO, revealing the B4-to-B1 transition mechanism, was detected by experimental method. These findings enrich our knowledge on the diversity of the size influences on the high-pressure behaviors of materials and offer new insights into the mechanism of the B4-to-B1 phase transition that is commonly observed in many other wurzite semiconductor compounds.

Unusual Mott transition in multiferroic PbCrO3.

The Mott insulator in correlated electron systems arises from classical Coulomb repulsion between carriers to provide a powerful force for electron localization. Turning such an insulator into a metal, the so-called Mott transition, is commonly achieved by "bandwidth" control or "band filling." However, both mechanisms deviate from the original concept of Mott, which attributes such a transition to the screening of Coulomb potential and associated lattice contraction. Here, we report a pressure-induced isostructural Mott transition in cubic perovskite PbCrO3. At the transition pressure of ∼3 GPa, PbCrO3 exhibits significant collapse in both lattice volume and Coulomb potential. Concurrent with the collapse, it transforms from a hybrid multiferroic insulator to a metal. For the first time to our knowledge, these findings validate the scenario conceived by Mott. Close to the Mott criticality at ∼300 K, fluctuations of the lattice and charge give rise to elastic anomalies and Laudau critical behaviors resembling the classic liquid-gas transition. The anomalously large lattice volume and Coulomb potential in the low-pressure insulating phase are largely associated with the ferroelectric distortion, which is substantially suppressed at high pressures, leading to the first-order phase transition without symmetry breaking.

Structures, Phase Transformations, and Dielectric Properties of BiTaO4 Ceramics.

Low (α)- and high-temperature (ß) forms of BiTaO4 have attracted much attention due to their dielectric and photocatalytic properties. In the present work, a third form, the so-called HP-BiTaO4, was synthesized at high temperature and pressure. The phase evolution, phase transformations, and dielectric properties of α- and ß-BiTaO4 and HP-BiTaO4 ceramics are studied in detail. ß-BiTaO4 ceramics densified at 1300 °C with the microwave permittivity εr ≈ 53, the microwave quality factor Qf ≈ 12070 GHz, and the temperature coefficient of resonant frequency τf ≈ -200 ppm/°C. HP-BiTaO4 ceramics were synthesized at 5 GPa and 1300 °C followed by annealing at 600 °C. In contrast with the α phase, HP-BiTaO4 exhibited εr ≈ 195 at 1 kHz to 10 MHz, accompanied by a low dielectric loss of ∼0.004. The relation between structure and dielectric properties is discussed in the context of Shannon's additive rule and bond theory.

A new molybdenum nitride catalyst with rhombohedral MoS2 structure for hydrogenation applications.

Nitrogen-rich transition-metal nitrides hold great promise to be the next-generation catalysts for clean and renewable energy applications. However, incorporation of nitrogen into the crystalline lattices of transition metals is thermodynamically unfavorable at atmospheric pressure; most of the known transition metal nitrides are nitrogen-deficient with molar ratios of N:metal less than a unity. In this work, we have formulated a high-pressure route for the synthesis of a nitrogen-rich molybdenum nitride through a solid-state ion-exchange reaction. The newly discovered nitride, 3R-MoN2, adopts a rhombohedral R3m structure, isotypic with MoS2. This new nitride exhibits catalytic activities that are three times more active than the traditional catalyst MoS2 for the hydrodesulfurization of dibenzothiophene and more than twice as high in the selectivity to hydrogenation. The nitride is also catalytically active in sour methanation of syngas with >80% CO and H2 conversion at 723 K. Our formulated route for the synthesis of 3R-MoN2 is at a moderate pressure of 3.5 GPa and, thus, is feasible for industrial-scale catalyst production.

Revisit of Pressure-Induced Phase Transition in PbSe: Crystal Structure, and Thermoelastic and Electrical Properties.

Lead selenide, PbSe, an important lead chalcogenide semiconductor, has been investigated using in-situ high-pressure/high-temperature synchrotron X-ray diffraction and electrical resistivity measurements. For the first time, high-quality X-ray diffraction data were collected for the intermediate orthorhombic PbSe. Combined with ab initio calculations, we find a Cmcm, InI-type symmetry for the intermediate phase, which is structurally more favorable than the anti-GeS-type Pnma. At room temperature, the onset of the cubic-orthorhombic transition was observed at ∼3.5 GPa with a ∼3.4% volume reduction. At an elevated temperature of 1000 K, the reversed orthorhombic-to-cubic transition was observed at 6.12 GPa, indicating a positive Clapeyron slope for the phase boundary. Interestingly, phase-transition induced elastic softening in PbSe was also observed, which can be mainly attributed to the loosely bonded trigonal prisms along the b-axis in the Cmcm structure. In a comparison with the cubic phase, orthorhombic PbSe exhibits a large negative pressure dependence of electrical resistivity. In addition, thermoelastic properties of orthorhombic PbSe have been derived from isothermal compression data, such as the temperature derivative of bulk modulus and thermally induced pressure.

Synthesis of large-sized spherical Co-C alloys with soft magnetic properties though a high-pressure solid-state metathesis reaction.

In this work, we report a novel high-pressure solid-state metathesis (HSM) reaction to produce spherical bulk (diameters 2-4 mm) Co-C alloys (Co3C and Co1-xCx). At 2-5 GPa and 1300 °C, C atoms preferentially occupy the interstitial sites of the face-centered cubic (fcc) Co lattice, leading to the formation of metastable Pnma Co3C. The Co3C decomposes above 1400 °C at 2-5 GPa, C atoms infiltrate the interstitial sites of the fcc Co lattice, saturating the C content in Co, forming an fcc Co1-xCx solid solution while the C atoms in excess are found to precipitate in the form of graphite. The Vickers hardness of the Co-C alloys is approximately 6.1 GPa, representing a 19.6% increase compared to hexagonal close-packed (hcp) Co. First-principles calculations indicate that the presence of C atoms in the Pnma Co3C structure leads to a relative decrease in the magnetic moments of the two distinct Co atom occupancies. The Co-C alloys exhibited a soft magnetic behavior with saturation magnetization up to 93.71 emu g-1 and coercivity of 74.8 Oe; coercivity increased as the synthesis pressure rises.

Synthetic route to metal nitrides: high-pressure solid-state metathesis reaction.

We report a general synthetic route to well-crystallized metal nitrides through a high-pressure solid-state metathesis reaction (HPSSM) between boron nitride (BN) and ternary metal oxide A(x)M(y)O(z) (A = alkaline or alkaline-earth metal and M = main group or transition metal). On the basis of the synthetic metal nitrides (Fe3N, Re3N, VN, GaN, CrN, and W(x)N) and elemental products (graphite, rhenium, indium, and cobalt metals), the HPSSM reaction has been systematically investigated with regard to its general chemical equation, reaction scheme, and characteristics, and its thermodynamic considerations have been explored by density functional theory (DFT) calculations. Our results indicate that pressure plays an important role in the synthesis, which involves an ion-exchange process between boron and the metal ion, opening a new pathway for material synthesis.

Phase-transition induced elastic softening and band gap transition in semiconducting PbS at high pressure.

We have investigated the crystal structure and phase stability, elastic incompressibility, and electronic properties of PbS based on high-pressure neutron diffraction, in-situ electrical resistance measurements, and first-principles calculations. The refinements show that the orthorhombic phase is structurally isotypic with indium iodide (InI) adopting a Cmcm structure (B33). The cubic-to-orthorhombic transition occurs at ∼2.1(1) GPa with a 3.8% volume collapse and a positive Clausius-Clapeyron slope. Phase-transition induced elastic softening is also observed, which is presumably attributed to the enhanced metallic bonding in the B33 phase. On the basis of band structure simulations, the cubic and orthorhombic phases are typical of direct and indirect semiconductors with band gaps of 0.47(1) and 1.04(1) eV, respectively, which supports electrical resistivity measurements of an abrupt jump at the structural transition. On the basis of the resolved structure for B33, the phase transition paths for B1→B33→B2 involve translation of a trigonal prism in B1 and motion of the next-nearest neighbor Pb atom into {SPb7} coordination and subsequent lattice distortion in the B33 phase.

Rediscovering the intrinsic mechanical properties of bulk nanocrystalline indium arsenide.

Is the inverse Hall-Petch relation in ceramic systems the same as that in metal systems? The premise to explore this subject is the synthesis of a dense bulk nanocrystalline material with clean grain boundaries. By using the reciprocating pressure-induced phase transition (RPPT) technique, compact bulk nanocrystalline indium arsenide (InAs) has been synthesized from a single crystal in a single step, while its grain size is controlled by thermal annealing. The influence of macroscopic stress or surface states on the mechanical characterization has been successfully excluded by combining first-principles calculations and experiments. Unexpectedly, nanoindentation tests show a potential inverse Hall-Petch relation in the bulk InAs with a critical grain size (Dcri) of 35.93 nm in the experimental scope. Further molecular dynamics investigation confirms the existence of the inverse Hall-Petch relation in the bulk nanocrystalline InAs with Dcri = 20.14 nm for the defective polycrystalline structure, with its Dcri significantly affected by the intragranular-defect density. The experimental and theoretical conclusions comprehensively reveal the great potential of RPPT in the synthesis and characterization of compact bulk nanocrystalline materials, which provides a novel window to rediscover their intrinsic mechanical properties, for instance, the inverse Hall-Petch relation of bulk nanocrystalline InAs.

Synthesis of stoichiometric and bulk CrN through a solid-state ion-exchange reaction.

Chromium mononitride (CrN) exhibits interesting magnetic, structural, and electronic properties for technological applications. Experimental reports on these properties are often inconsistent owing to differences in the degree of nonstoichiometry in CrN(x). To date, the preparation of bulk and stoichiometric CrN has been challenging; most products are in the form of a thin film produced by non-equilibrium processes, and are often nonstoichiometric and poorly crystallized. In this work, we formulated a solid-state ion-exchange route for the synthesis of CrN under high pressure. The final CrN product is phase-pure, stoichiometric, and well-crystallized in the bulk form. Near-stoichiometric and well-crystallized CrN can be synthesized using the same route at atmospheric pressure, making massive and industrial-scale production technologically feasible. The successful synthesis of stoichiometric and bulk CrN is expected to open new opportunities in diverse areas of fundamental research.

Squeezing indium arsenide single crystal to ultrafine nanostructured compact bulk.

Reciprocating pressure-induced phase transition (RPPT) has been proposed as a new approach to synthesize nanostructured bulk materials with clean grain boundary interfaces for structures that undergo reversible pressure-induced phase transitions. The modulation effects on grain size under different cycle numbers of RPPT for InAs were investigated and the initial single-crystal bulk, with a dimensional size of about 30 µm, was transformed into a nanostructure with an average grain size of 7 nm by the utilization of the in situ high-pressure diamond anvil cell (DAC) technique. To verify the DAC findings, compact nanostructured bulk InAs with grain sizes ranging from 2-20 nm (average = 8 nm) and large dimensions (3.2 mm × 3.2 mm × 0.5 mm) was successfully synthesized from single-crystal InAs using a large volume press (LVP). The smaller work function (3.86 eV) and larger bandgap energy (2.64 eV) of the compact nanostructured bulk InAs phase compared to those of single-crystal InAs demonstrated that the nanostructure affected the macroscopic properties of InAs. The findings confirm the feasibility of synthesizing nanostructured bulk materials via RPPT.

Powder conductor for pressure calibration applied to large volume press under high pressure.

Pressure is the core of high-pressure science and technology, and the accuracy of pressure calibration is of much importance for high-pressure experiments and production. Although the pressure limit of the large volume press (LVP) continues to increase, there are no well solutions for in situ pressure calibration. In this study, using in situ high-pressure electrical performance measurement technology, two ideal calibration standard materials in powder conductors, cadmium phosphide (Cd3P2) and zinc telluride (ZnTe) with stable physical and chemical properties and obvious resistance change, are applied to pressure calibration in the LVP. In situ high-pressure synchrotron radiation x-ray diffraction was used to verify the phase transition pressure point of Cd3P2. The introduction of powder conductors for pressure calibration commits to establish a pressure system, which is safer, more stable to operate, and more accurate in experimental measurements for the LVP.

Ultrastrong catalyst-free polycrystalline diamond.

Diamond is the hardest naturally occurring material found on earth but single crystal diamond is brittle due to the nature of catastrophic cleavage fracture. Polycrystalline diamond compact (PDC) materials are made by high pressure and high temperature (HPHT) technology. PDC materials have been widely used in several industries. Wear resistance is a key material property that has long been pursued for its valuable industrial applications. However, the inevitable use of catalysts introduced by the conventional manufacturing process significantly reduces their end-use performance and limits many of their potential applications. In this work, an ultra-strong catalyst-free polycrystalline diamond compact material has been successfully synthesized through innovative ultra-high pressure and ultra-high temperature (UHPHT) technology. These results set up new industry records for wear resistance and thermal stability for PDC cutters utilized for drilling in the oil and gas industry. The new material also broke all single-crystal diamond indenters, suggesting that the new material is too hard to be measured by the current standard single-crystal diamond indentation method. This represents a major breakthrough in hard materials that can expand many potential scientific research and industrial applications.

The effect of size matching between anvils and the pressure transmitting medium on the pressure-generation efficiency and sealing performance for a large volume cubic pressure cell.

Size matching between anvils and the pressure transmitting medium (PTM) is a key factor that affects pressure generation and sealing for a large volume cubic press. In this work, we studied the influence of PTM sizes from 30.5 mm to 34.5 mm at a fixed anvil geometry dimension (23.5 mm) on the pressure efficiency and sealing performance by measuring the pressure of the gasket and cell simultaneously at room temperature. Wires made of Bi, Tl, Ba, or Manganin were used for pressure calibration experiments within a pressure range of up to 6 GPa. It was found that a PTM with an edge length of 33.5 mm had the highest pressure-generation efficiency, but its sealing performance was the worst. Furthermore, it was confirmed that a PTM with an edge length of 32.5 mm had the best overall performance for a 23.5 mm anvil when both efficiency and sealing were considered. The results show that the pressure-generation efficiency and sealing performance are highly sensitive to PTM size. It is less rigorous to gauge the performance of the assembly only by the pressure-generation efficiency. This work provides practical guidelines and contributes to optimizing the design of the high-pressure assembly.

A new pressurization-insulation and pre-sealing system to improve pressure in cubic press from 6 GPa to 12 GPa.

In this paper, a pressurization-insulation and pre-sealing (PIPS) system is designed to increase the cell pressure of the widely used large volume cubic press without sacrificing cell volume. The sample chamber was sandwiched between a pair of tungsten carbide anvils used as the pressurization system. Ultra-high pressure in the cavity was up to about 12 GPa, and the pressure limit had increased by 100% in contrast with that of an anvil-gasket (AG) system. Furthermore, the confining pressure around the sample chamber was supported by grade 304 stainless steel and a zirconia-calcium oxide solid solution before a press load of 2.8 MN was applied as well as by four surrounding anvils. The relationship between the sample chamber pressure and the press load for this system was calibrated at room temperature using transitions in zinc telluride. With samples of similar volumes, the proposed system retained not only stability but also uniform pressure and temperature fields, in contrast with the AG system and the anvil-preformed gasket cubic press pressurization system. The results of more than 20 experiments show that the proposed PIPS system can operate stably under a press load of 4.2 MN, corresponding cell pressure of 10 GPa, and temperature in the cell exceeding 1800 °C by using graphite as a heater.

Hardness of Polycrystalline Wurtzite Boron Nitride (wBN) Compacts.

Wurtzite boron nitride (wBN), due to its superior properties and many potential practical and scientific applications, such as ideal machining/cutting/milling ferrous and carbide materials, especially as an ideal dielectric substrate material for optical, electronic, and 2-D graphene-based devices, has recently attracted much attention from both academic and industrial fields. Despite decades of research, there is an ongoing debate about if the single-phase wBN is harder than diamond because of the difficulty to make pure wBN material. Here we report the successful synthesis of pure single-phase polycrystalline wurtzite-type boron nitride (wBN) bulk material by using wBN powder as a starting material with a well-controlled process under ultra-high pressure and high temperature. The cubic boron nitride (cBN) was also successfully prepared for the first time from wBN starting material for comparison and verification. The X-ray diffraction (XRD) and TEM clearly confirmed that a pure single-phase wBN compact was produced. The microstructure and mechanical properties including Vickers hardness, fracture toughness, and thermal stability for the pure single-phase wBN was first evaluated.

Experimental study on the pressure-generation efficiency and pressure-seal mechanism for large volume cubic press.

Measuring the pressure of a gasket (Pg) and cell (Pc) in situ is the key point to understanding the mechanism of pressure-generation and pressure-seal for the widely used large volume cubic press. However, it is a challenge to measure Pg due to the large deformation in the gasket zone and the complex rheological behavior of the pressure transmitting medium. Herein, a method of in situ electric resistance measurement has been developed to measure Pg. The open circuit failure in electric resistance measurement was avoided by using powder electrodes which could match the mould-pressed pyrophyllite cube in rheological behavior during compression. The relationships between press-load vs. Pc and press-load vs. Pg were obtained through in situ electric resistance measurements of bismuth, thallium, barium, and manganin. It was found that Pg exceeded Pc at around 5 GPa and Pc generated in the large volume cubic press was limited to the rapid rise of Pg above 5 GPa. Furthermore, the maximum ΔP (ΔP = Pc - Pg) above 0.9 GPa has been observed when Pc was released to a pressure range of 3-4 GPa, and this also leads to a large probability of high pressure cavity seal failure.

In situ high-pressure measurement of crystal solubility by using neutron diffraction.

Crystal solubility is one of the most important thermo-physical properties and plays a key role in industrial applications, fundamental science, and geoscientific research. However, high-pressure in situ measurements of crystal solubility remain very challenging. Here, we present a method involving high-pressure neutron diffraction for making high-precision in situ measurements of crystal solubility as a function of pressure over a wide range of pressures. For these experiments, we designed a piston-cylinder cell with a large chamber volume for high-pressure neutron diffraction. The solution pressures are continuously monitored in situ based on the equation of state of the sample crystal. The solubility at a high pressure can be obtained by applying a Rietveld quantitative multiphase analysis. To evaluate the proposed method, we measured the high-pressure solubility of NaCl in water up to 610 MPa. At a low pressure, the results are consistent with the previous results measured ex situ. At a higher pressure, more reliable data could be provided by using an in situ high-pressure neutron diffraction method.