•Pressure-induced phase transition was observed in α-BiNbO4.•Phase transition routine in the phase evolution of BiNbO4 was explored.•The effect of pressure for phase transition of BiNbO4 was studied.In the present work, the orthorhombic (α) BiNbO4 have been investigated with the in-situ angle-dispersive x-ray diffraction (ADXRD) measurements under high pressure. Pressure-induced phase transition was observed in α-BiNbO4, and compressibility of the unit cell axes (a, b and c axes) of α-BiNbO4 shows an anisotropic with pressure increasing. The phase transition routine and the effect of pressure in the phase evolution of BiNbO4 were also explored. The results indicate that pressure-induced phase transition is reversible, in contrast, pressure and temperature-induced phase transition is irreversible in system of BiNbO4, but the phase remains stable without pressure. High pressure plays a critical role and provides a driving force for phase transition of BiNbO4.

Nano-polycrystalline diamond (NPD) can be obtained by direct conversion of various carbons at high temperature and high pressure (HPHT). However, the ultra-high synthesis condition is an obstacle for large-scale production of NPD. This work aimed to reduce the synthesis condition. Annealed precursors fabricated by annealing detonation nano-diamond in vacuum were used for synthesizing NPD at HPHT conditions of 1600–1800 °C, 10–15 GPa and holding time of 1–10 min. High-purity NPD with high Vickers hardness of 147 ± 17 GPa was synthesized at 1800 °C and 10 GPa for 10 min. The synthesis conditions were much lower than that using graphite precursors; in particular, the pressure was dramatically reduced to 10 GPa from 15 GPa. X-ray diffraction and transmission electron microscopy analyses suggested that most carbon particles in the annealed precursors contain a diamond core and the residual diamond core is the key for the reduction in synthesis pressure. Meanwhile, the onion-like carbon became metastable at HPHT because of the compression from the surrounding carbon particles and then transformed to diamond or graphite. Our findings proved that pure NPD with high hardness can be synthesized in proximity to industrial conditions.

•Systematically studying the grain size dependence of mechanical properties and microstructure of cBN-Si composites.•The cBN-Si compacts are more denser and have better mechanical performances.•The uniformity of the overall performance of cBN-Si composites.•The cBN-cBN bonds appeared extensively in the sintered samples.•The deformation occurred more easily for the coarse grain cBN than the fine-grained cBN at high pressure and temperature.The grain size dependence of the mechanical properties of cBN-Si composites prepared using the high pressure infiltration method has been investigated. Indentation testing indicates that cBN-Si composites have hardness values of 38–43 GPa, which increase with increasing grain size and are harder than traditional polycrystalline cBN composites (PcBNs). Thermostability analyses display that cBN-Si composites with a grain size of > 9 μm also possess a higher temperature of oxidation, compared to traditional PcBNs, and the thermostability increases with increasing cBN grain size. Fracture toughness tests show that almost no cracks appear on the polished cBN-Si samples when the loading forces are increased to 294 N and the fracture toughness is better than for commercial samples. Scanning electron microscopy illustrates that deformations and close pores occurred easily between coarse BN grains, leading to denser cBN-Si compacts with better mechanical performances.

•ZrO2 whiskers are synthesized at 15 GPa, 1800 °C.•As-prepared ZrO2 whiskers are colorless and transparent.•They exhibited some regular and irregular shaped cross sections.•The aspect ratio of ZrO2 whiskers could be from 10 to 100.ZrO2 whiskers have been successfully synthesized at temperature of 1800 °C and high pressure of 15 GPa, using ZrO2 powder as the starting material. After a high pressure and high temperature (HPHT) process, a bunch of long and idiomorphic ZrO2 whiskers are obtained. The grown whiskers look like needles, and have silky luster. They are colorless and transparent, and exhibit hexagonal, triangular, quadrilateral, and irregular shaped cross sections. Through this HPHT method, whiskers with various diameters from 10 to 20 μm, and length from 200 to 1000 μm can be produced. The aspect ratio of length to diameter can be from 10 to 100. Growth mechanism of the ZrO2 whiskers prepared under HPHT conditions are discussed, and the ZrO2 whiskers growth may undergo a vapor–solid process.

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.

•Phase transition of MoS2 was systematically investigated under high P–T conditions.•Sulfur-catalyzed hexagonal–rhombohedral transition was observed in MoS2.•Phase-transition mechanisms were proposed for phase transitions.We report phase transition and stability of MoS2 with and without the presence of sulfur melt under high-pressure and high-temperature conditions. Rhombohedral (3R) phase is found to be a high-temperature phase of MoS2 at high pressures. Excess sulfur melt catalyzes the hexagonal (2H) to rhombohedral (3R) phase transformation and lowers the conversion temperature by more than 280 K. Boundary between 2H and 3R phases has been delineated with a negative slope. Based on experimental observations, sulfur-catalyzed 2H→3R transformation mechanisms are proposed involving atomic exchange between MoS2 and sulfur, which is different from the case of without excess sulfur that proceeds through rotation and translation of the S–Mo–S sandwich layers.

•Phase transition of PbNiO3 from LiNbO3-type to perovskite-type occurred between 4.4 and 9.5 GPa in the compression process.•The compression behavior of lattice parameters of the LiNbO3-type and perovskite-type PbNiO3 has also been analyzed.•The equation of state (EOS) at room temperature of the two types of PbNiO3 has been acquired.The phase transition of PbNiO3 from LiNbO3 structure to the perovskite structure has been observed between 4.4 and 9.5 GPa, which was investigated by high-pressure synchrotron x-ray powder diffraction at room temperature. Compression behavior of the two phases has been analyzed, and the c axis is the soft direction for LiNbO3-type PbNiO3. The bulk modulus of the LiNbO3-type PbNiO3 is K0=184(3) GPa, K0′=4.5(9), and the perovskite type is K0=166(8) GPa, K0′=4.9(8).

Superhard materials are solids whose Vickers hardness is beyond 40 GPa. They have wide applications in industry such as cutting and polishing tools, wear-resistant coatings. Most preparations of superhard materials are conducted under extreme pressure and temperature conditions, not only for scientific investigations, but also for the practical applications. In this paper, we would introduce the recent progress on the design and preparations of novel superhard materials, mainly on nanopolycrystalline diamond, B–C–N superhard solid solutions, and cubic-Si3N4/diamond nanocomposites prepared under ultrahigh pressure and high temperature (HPHT), using multi-anvil apparatus based on the hinged-type cubic press. Bulk materials of all these superhard phases have been successfully synthesized and are systematically tested. We emphasize that ultra-HPHT method plays an important role in the scientific research and industrial production of superhard materials. It provides the driving forces for the light elements forming novel superhard phases as well as the way for sintering high-density nanosuperhard materials.

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.

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 AxMyOz (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 WxN) 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 transitions of both α-BiNbO4 and β-BiNbO4 were investigated under high pressure and high temperature (HPHT) (0–5 GPa and 300–1800 °C, respectively) using X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscope (EDS). The stable regimes for the BiNbO4 phases were also determined. Experiments showed that both α-BiNbO4 and β-BiNbO4 could convert to a new high pressure phase HP-BiNbO4 above 3 GPa, and the transition temperatures decreased with increasing pressures. Below 3 GPa, β-BiNbO4 keeps stable up to 1800 °C, while α-BiNbO4 converts to β-BiNbO4 when temperature is higher than 1150 °C. By heating the metastable HP-BiNbO4 phase at room pressure, HP-BiNbO4 converts to α-BiNbO4 at 600 °C. Above 1150 °C, HP-BiNbO4 would transform to β-BiNbO4. Phase relationships of Bi2O3, Nb2O5, α-BiNbO4, β-BiNbO4 and HP-BiNbO4 were also summarized.Highlights► A new high pressure phase (HP-BiNbO4) was discovered above 3 GPa. ► HP transformation boundaries of BiNbO4 were defined up to 5 GPa, 1800 °C. ► The quenched HP-BiNbO4 would convert to the starting phases when heating. ► Phase relations of Bi2O3, Nb2O5, α, β and HP-BiNbO4 are summarized.

Well-crystallized graphite and high energy ball-milled graphite were treated under 16 GPa and temperatures up to 2500 °C. Our experiments indicated that the ball-milled graphite can completely transform to nano-polycrystalline diamond (NPD) at 2100 °C, while the well-crystallized graphite required 2500 °C. The samples were characterized by X-ray diffraction, optical microscope, scanning electron microscope, and Vickers indenter hardness test. We found that high pressure not only made the conversion of graphite to NPD thermodynamically preferred, but also reduced the needed activation energy barrier. By analyzing thermodynamics and kinetics of the transformation, we suggested a low boundary for NPD forming region based on the phase diagram of carbon, and the formation mechanisms of Nano-polycrystalline diamond.Highlights► Under higher pressure NPD could form with lower temperature. ► Ball milling lowered the formation temperature of nano-polycrystalline diamond. ► Pressure–temperature region of nano-polycrystalline diamond formation was suggested. ► NPD formation mechanisms are different with graphite and milled graphite.

Among transition metal nitrides, tungsten nitrides possess unique and/or superior chemical, mechanical, and thermal properties. Preparation of these nitrides, however, is challenging because the incorporation of nitrogen into tungsten lattice is thermodynamically unfavorable at atmospheric pressure. To date, most materials in the W–N system are in the form of thin films produced by nonequilibrium processes and are often poorly crystallized, which severely limits their use in diverse technological applications. Here we report synthesis of tungsten nitrides through new approaches involving solid-state ion exchange and nitrogen degassing under pressure. We unveil a number of novel nitrides including hexagonal and rhombohedral W2N3. The final products are phase-pure and well-crystallized in bulk forms. For hexagonal W2N3, hexagonal WN, and cubic W3N4, they exhibit elastic properties rivaling or even exceeding cubic-BN. All four nitrides are prepared at a moderate pressure of 5 GPa, the lowest among high-pressure synthesis of transition metal nitrides, making it practically feasible for massive and industrial-scale production.Keywords: high-pressure synthesis; ion exchange; tungsten nitride;

High pressure in situ differential thermal analysis (DTA) was applied to study phase transitions of Na0.5Bi0.5TiO3 (NBT) up to 5 GPa. DTA analysis at room temperature with pressure as the variable indicated NBT samples have two pressure induced phase transformations at about 0.7 GPa and 2.5 GPa respectively. Below 0.7 GPa, two temperature induced phase transitions could be observed at around 220 °C and 520 °C as it was reported at ambient pressure. But under a pressure higher than 0.7 GPa, these two temperature induced phase transitions would not happen. In situ electrical resistance measurements showed that the resistance of NBT changes smoothly, and the tendency is similar to that of electrical insulators like pyrophyllite. We suggest that the pressure induced phase transitions at 0.7 GPa and 2.5 GPa are both insulator to insulator phase transitions.Highlights► Using DTA with pressure as variable to detect phase transitions. ► Pressure induced phase transitions of NBT happened at 0.7 Gpa and 2.5 GPa. ► Insulator to insulator phase transitions occurred at both 0.7 GPa and 2.5 GPa. ► Temperature induced phase transitions at 220 °C and 520 °C happened only below 0.7 GPa.

Powder mixture of graphite and silicon nitride with hexagonal structure (α/β-Si3N4) was ball-milled in nitrogen and compressed under ~ 16 GPa/1800 °C and ~ 18 GPa/2000 °C, to investigate the possibility for replacing Si atoms by C atoms in cubic spinel silicon nitride (c-Si3N4). The sintered samples were characterized by X-ray diffraction (XRD), hardness measurement, optical microscope, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The results show that the well-sintered compacts were composed of nanocrystalline c-Si3N4 and nanocrystalline diamond, and neither C3N4 and CSi2N4 nor other reaction products of Si3N4 and C were observed. Vickers hardness test shows that the average hardness of the samples is 41–42 GPa. As a comparison, phase-pure c-Si3N4 bulks also have been synthesized and their hardness was confirmed to be about 31 GPa. The pure c-Si3N4 sintered bulks exhibit a high frangibility, but their fracture toughness could be largely improved by adding carbon (nanocrystalline diamond) in the sintered compacts.Highlights► Well-sintered compacts of c-Si3N4 and diamond composite have been synthesized. ► Nanodiamond improved the sintered compacts in hardness and fracture toughness. ► Vickers hardness of c-Si3N4 was confirmed to be 31 GPa by synthesized pure bulks. ► Si atoms in Si3N4 cannot be replaced by C atoms up to 18 GPa and 2000 °C.

GaN crystals are successfully obtained through solid-state metathesis (SSM) reaction between sodium gallium oxide (NaGaO2) and boron nitride (BN) under high pressure and high temperature. X-ray diffraction (XRD) pattern indicates that the attained GaN crystals possess a hexagonal wurtzite-type structure. Scanning electron microscopy (SEM) is used to estimate the size and morphology of GaN crystals, and results show that GaN grains with the size over 100 μm can be prepared at 5 GPa and 1600 °C. Moreover, pressure–temperature (P–T) formation region of GaN has been discussed. Our results suggest a promising novel route for synthesizing GaN crystals from SSM reactions under high pressure.

The Vickers hardness of pressure synthesized WB compacts was measured using a diamond indentation method. Under the applied load of 4.9 N, our test gave a maximum Vickers hardness of 30.1 GPa for WB compacts. The hardness of WC-8 wt.% Co composites was also measured under the same load for comparison, which was found to be much lower than that of WB. The Young's modulus, bulk modulus, and shear modulus of WB and WC-8 wt.% Co were further investigated by ultrasonic measurements. The results suggest that WB might be a promising hard alloy material for industrial applications.