Developing nonprecious hydrogen evolution electrocatalysts that can work well at large current densities (e.g., at 1000 mA/cm2: a value that is relevant for practical, large-scale applications) is of great importance for realizing a viable water-splitting technology. Herein we present a combined theoretical and experimental study that leads to the identification of α-phase molybdenum diboride (α-MoB2) comprising borophene subunits as a noble metal-free, superefficient electrocatalyst for the hydrogen evolution reaction (HER). Our theoretical finding indicates, unlike the surfaces of Pt- and MoS2-based catalysts, those of α-MoB2 can maintain high catalytic activity for HER even at very high hydrogen coverage and attain a high density of efficient catalytic active sites. Experiments confirm α-MoB2 can deliver large current densities in the order of 1000 mA/cm2, and also has excellent catalytic stability during HER. The theoretical and experimental results show α-MoB2’s catalytic activity, especially at large current densities, is due to its high conductivity, large density of efficient catalytic active sites and good mass transport property.

In this work, the structure of WB2 synthesized at high pressure and high temperature (HPHT) was accurately determined by X-ray diffraction and Rietveld refinement. Its asymptotic Vickers hardness (Hv) value is 25.5 GPa which is much lower than the previous theoretical results (36–40 GPa). It is worth noting that the chemical bonds between the W layers and two different kinds of B layers show obvious polarization character based on the results obtained from X-ray photoelectron spectroscopy (XPS) and electron localization functions (ELFs), density of states (DOS), topological analysis of the static electron density and Mulliken population. This result can well clarify that WB2 is only a hard but not superhard material. Thus, a 3D network structure can not be formed between the W layers and the B layers which is previously predicted by theoretical calculations. Our results are helpful to understand the hardness mechanism and design superhard materials in TMBs.

•We provided an effective route for synthesizing high-quality bulk c-WN.•The formation mechanism of c-WN was discussed in detail.•c-WN is so far the hardest transition-metal nitrides with the sodium chloride structure.•The high hardness mechanism of c-WN was proved as strengthen p − d bonding, which was proved by XPS.Tungsten nitrides (WNs) are promising functional materials with high hardness, but the greatest challenge is to synthesize stoichiometric and bulk materials. In this paper, bulk tungsten mononitride (c-WN) with sodium chloride structure, which is a metastable phase, has been successfully synthesized at high pressure and high temperature (HPHT) using W3N4 as precursor. It is found that synergistic effect of pressure and temperature was useful to control the complete decomposition of W3N4 and to suppress further decomposing of as-synthesized c-WN. The compression ability and Vickers hardness were investigated by in situ high pressure X-ray diffraction (XRD) and Vickers microhardness tests, respectively. It is worth noting that the bulk modulus of c-WN is 422.9 ± 6.7 GPa, which is comparable to diamond. The Vickers hardness, 29 GPa obtained under an applied load of 0.49 N, is nearly 45% higher than that of TiN which is widely used as hard wear protective coatings. The excellent mechanical properties of c-WN may be ascribed to strong pd hybridization which has been further proved by XPS.Download high-res image (232KB)Download full-size image

•Two possible electronic topological transitions were uncovered in In4Se3.•The possible abnormal changes of CDW order were proposed under pressure.•The changes of thermoelectric property under pressure were discussed.•Structural changes were observed at about 7.0 and 34.2 GPa, respectively.•The compressional behaviors of the Pnnm and the Pca21 phases were all determined.High-pressure in situ angle dispersive X-ray diffraction (ADXRD) measurements were performed on the charge-density-wave (CDW) material In4Se3 up to 48.8 GPa. Pressure-induced structural changes were observed at 7.0 and 34.2 GPa, respectively. Using the CALYPSO methodology, the first high-pressure phase was solved as an exotic Pca21 structure. The compressional behaviors of the initial Pnnm and the Pca21 phases were all determined. Combined with first-principle calculations, we find that, unexpectedly, the Pnnm phase probably experiences twice electronic topological transitions (ETTs), from the initial possible CDW state to a semimetallic state at about 2.3 GPa and then back to a possible CDW state at around 3.5 GPa, which was uncovered for the first time in CDW systems. In the both possible CDW states, pressure provokes a decrease of band-gap. The observation of a bulk metallic state was ascribed to structural transition to the Pca21 phase. Besides, based on electronic band structure calculations, the thermoelectric property of the Pnnm phase under compression was discussed. Our results show that pressure play a dramatic role in tuning In4Se3's structure and transport properties.Download high-res image (528KB)Download full-size image

Nanopolycrystalline diamond (NPD) and nanotwinned diamond (NtD) were successfully synthesized in a multianvil, high-pressure apparatus at high temperatures by using the precursors of carbon onion. It was found that distinct carbon onions with hollow or multicore microstructures lead to the formation of different diamond products, namely NPD or NtD, respectively. The Vickers hardness of high-quality NtD with an average twin thickness of 6.8 nm reached as high as 180 GPa, which was measured by indentation hardness experiment. The existence of stacking faults other than various defects in the carbon onion was found to be crucial for the formation of twin boundaries in the product. The origin of the extraordinarily high Vickers hardness in the NtD sample is attributable to the high concentration of twin boundaries. Our work directly supports the argument that pursuit of nanotwinned microstructure is an effective strategy to harden materials, which is in good agreement with the well-known Hall-Petch effect.(a) HRTEM of NtD synthesized by MOC (2000 °C, 20 GPa), the insert pattern is SAED result. (b) the distribution of twin thickness.Download high-res image (520KB)Download full-size image

The phase behaviors of 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C12MIM][BF4]) had been investigated by means of Raman spectroscopy and polarized optical microscopy under pressure values up to 2.0 GPa at the temperature of 80.0 °C. Upon compression, change in the ratio of peak heights of symmetric and asymmetric CH2 stretching modes of Raman spectra in [C12MIM][BF4] (r(CH2)ss/(CH2)as) indicated that it might experience two successive phase transitions. The structure evolution of the sample, which was investigated through the image analysis from polarized optical microscopy, was found to share many of the known quantitative properties of the smectic A phase of [C12MIM][BF4]. These facts were suggestive of ionic liquid crystal induced by compression in [C12MIM][BF4] under the pressure between 0.25 GPa to 0.60 GPa, which was similar to ionic liquid crystal upon cooling from its melt.

To get a better understanding of the influence of rare-earth element doping, CaCu3Ti4O12 (CCTO) samples with a partial substitution of Ca with Eu with different compensation mechanisms were designed and prepared by solid-state reaction. All the ceramics were single phase, while the dielectric constants and thermally activated energy values for dielectric relaxation in Eu-doped ceramics were both lower than those of CCTO. Ca0.875Eu0.1Cu3Ti4O12 (CECT1) exhibited a slight decrease in both the permittivity and electric resistance of grain boundaries compared with CCTO, while Ca0.85Eu0.1Cu3Ti4O12 (CECT2) underwent a sharp decrease in permittivity associated with an abnormally large resistance. The different dielectric behavior indicates that the dielectric properties of CCTO are sensitive to the valence states of cations and defects. The variation of permittivity is related to the localization of carriers, which, according to the XPS results, should be caused by the presence of oxygen vacancies. The formation of defect complexes between cations and oxygen vacancies leads to the increase in resistance and prevents the hopping between Cu+ and Cu2+, which is an important source of the polarization in grain boundaries.

•Two electron phase transitions have been discovered in α-As2Te3.•The mechanism of the changes in thermoelectric property has been investigated.•The effects of electron phase transitions on bulk modulus have been found.•The initial α- As2Te3 transforms into γ- As2Te3 at between 17.4 and 36.3 GPa.•Our results show that the recently reported high-pressure β-As2Te3 is not existed.Using a diamond-anvil cell (DAC), we conducted in situ angle dispersive X-ray diffraction (ADXRD) up to 47.6 GPa at room temperature to evaluate the structural stability of α-As2Te3. A reversible structural transition was observed at between 17.4 and 36.3 GPa. The high-pressure structure is solved by a γ-Bi2Te3-type monoclinic phase (C2/c, γ-As2Te3). Reliable structural analyses, such as Rietveld refinement of the powder x-ray data, were provided, which show that the previously reported high-pressure β-As2Te3 phase is not existed. It is worth mentioning that twice pressure-induced electron phase transitions were revealed in α-As2Te3 at about 3 and 6 GPa, which cause the striking fluctuation in thermoelectric property at that pressures.

Small carbon nano-onions (S-CNOs) were prepared by annealing nanodiamonds (ND) in an argon atmosphere. The structure and morphology of S-CNOs were determined by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) and the average grain size of the S-CNOs was about 8 nm. In situ high pressure Raman spectra of S-CNOs were investigated by diamond anvil cell experiments at pressures up to 22.5 GPa. A reversible structural transition occurred at about 7.4 GPa, resulting from the polygonization of S-CNOs. The structural transition pressure of S-CNOs is lower than that of large CNOs (L-CNOs) and onion-like carbon (OLC) nanospheres. S-CNOs derived from the annealing of ND have a high defect density, a large number of sp3 bonds and high free energy. In addition, high pressure can be generated in the interior of S-CNOs at high temperatures. The results indicated that nanotwinned diamond (nt-diamond) may be prepared using S-CNOs derived from the annealing of NDs as a raw material below 10 GPa, which is much lower than the pressure needed for synthesizing nano-polycrystalline diamond (NPD) and nt-diamond with other carbon resources (usually more than 15 GPa).

•α-MoB2 powder synthesized under high pressure–temperature condition.•We have firstly investigated the equation of state of α-MoB2 under uniaxial compression up to 85 GPa.•The complete elastic constant tensor of α-MoB2 at high pressures up to 100 GPa are firstly calculated from density-functional theory (DFT).•We have investigated the strength of α-MoB2 under uniaxial compression up to 85 GPa.Investigations of strength and equation of state of α-MoB2 have been performed under nonhydrostatic compression up to 85 GPa using an angle-dispersive radial X-ray diffraction (RXD) techniques together with the lattice strain theory in a 2-fold panoramic diamond anvil cell at ambient temperature. The RXD data yields a bulk modulus and its pressure derivative as K0 = 323(6) GPa with K0′ = 4.59(27). The ratio of t/G is found to remain constant above ∼44 GPa, indicating that the α-MoB2 started to experience yield with plastic deformation at this pressure. Together with theoretical results on high-pressure shear modulus, our results here show that molybdenum diborides sample could support a differential stress of ∼18 GPa when it started to yield with plastic deformation at ∼44 GPa under uniaxial compression. A maximum differential stress, as high as ∼25 GPa can be supported by molybdenum diborides at the high pressure of ∼85 GPa.

The differences between the compression behaviors of nanocrystal systems and their bulk counterparts are generally attributed to the size and morphology effects. However, these effects may not be simply employed to deal with the contradictory results about In2O3 bulk and nanomaterials under high pressures. In this work, we intend to show that other than size and morphology, the effects of microstructure should play a key role in the compression behavior and phase transition routine of In2O3 cubical-shaped nanocrystals under high pressures. Two samples of In2O3 nanocubes, which have almost the same size, shape and exposed facets, are subjected to high pressure at room temperature. The In2O3 nanocubes with a lower density of microstructures undergo a first order phase transition at about 18.9 GPa, and the two phases coexist when the pressure is released. Whereas the In2O3 nanocubes with a higher density of microstructures show no sign of such a phase transition at pressures up to 33.4 GPa, a change of elastic properties at about 8.8 GPa may be observed instead. Thus, it is anticipated that controlling the microstructures of nanomaterials may be a potential route to modulate their structural and elastic behaviors under pressures.

Polycrystalline In2Ge2O7 with a monoclinic structure (thortveitite-type, T-type) and a cubic structure (pyrochlore-type, P-type) have been synthesized by using different methods. The structural stabilities and electrical transport properties of these two polymorphs under high pressure have been investigated by angle-dispersive X-ray diffraction (ADXRD) and alternate current (AC) impedance spectra. An irreversible structural phase transition from monoclinic (C2/m) to another monoclinic (P21/c) phase has been found in the T-type In2Ge2O7 above 6.6 GPa. Furthermore, the pressure dependent electrical resistance of the T-type In2Ge2O7 shows a dramatic change at 5.3 GPa, where it can be attributed to the observed pressure-induced structural phase transition. On the contrary, the P-type In2Ge2O7 with the cubic (Fdm) structure at high pressure is much more stable up to 26.5 GPa.

In this work, tungsten triboride (WB3) was successfully synthesized at high pressure and high temperature. The structure was reconfirmed to be WB3 (P63mmc), and some part has a tungsten atomic defect according to the measurement results of X-ray diffraction, high-resolution transmission electron microscopy, and Rietveld refinement. The asymptotic Vickers hardness that had eliminated influence of excess boron is 25.5 GPa for WB3. This value is in good agreement with the previous theoretic results. Proof of novel electron transfer between the tungsten atom and the boron atom was found. A deficient amount of transfer electron induces distorted sp2 hybridization of B–B bonds in WB3. The weakly directional sp2 hybridization of B–B bonds is an essential factor that can influence the hardness of WB3. Our results are helpful to design new hard and superhard materials of transition metal borides.

The high pressure behavior of α-molybdenum boride (α-MoB2, P6/mmm) and β-molybdenum boride (β-MoB2, Rm) was studied up to a pressure of 32.1 GPa and 35.5 GPa, respectively. The bulk modulus values for α-MoB2 and β-MoB2 were 317 GPa and 299 GPa respectively, which fitted the Birch-Murnaghan equation of state. That the compressibility of MoB2 mainly depends on electron concentration but is less related to structure difference was reconfirmed in this study. An anomalous second-order transition was found in β-MoB2 at 26.6 GPa, which resulted in the structure softening and changing the anisotropy of β-MoB2. The anomalous transition found in β-MoB2 under high pressure may be attributable to the limitation of the B2–B2–B2 angle in puckered boron layers. These results will promote further understanding of the mechanical properties of transition metal borides (TMBs), and will be helpful in designing hard or superhard materials with TMBs.

Molybdenum borides including α-MoB2 and β-MoB2 have been successfully synthesized from boron and molybdenum at high pressure and high temperature (HPHT). The crystalline structures are confirmed by Rietveld refinements in the hexagonal (P6/mmm) and rhombohedral (R-3m) crystal systems for α- and β-MoB2, respectively. The values of Vickers hardness (HV) are 15.2 GPa for α-MoB2, which is firstly obtained, and 22.0 GPa for β-MoB2. The hardness results for α- and β-MoB2 are in good agreement with theoretical values calculated by first-principle calculations. The difference in hardness between α- and β-MoB2 is attributed to the puckered quasi-3D (three dimensional) boron layers in β-MoB2 which is confirmed by the calculated results of the Electron Localization Function (ELF) and elastic constants. These results are helpful to understand the hardness mechanism and to design superhard transition-metal borides (TMBs).

In this paper, various cubic boron nitride (cBN) crystal morphologies were synthesized using hexagonal boron nitride (hBN) as raw materials and Li3N, LiH, and LiNH2 as catalysts/additives under high pressure and high temperature (HPHT).These crystal shapes contain thick-plate, spherical, octahedral or hex-octahedral, flat cone and flaky hexagonal morphologies. The reasons of various crystal shapes synthesized can be summarized as follows: various catalysts/additives take on distinct properties under HPHT, which have crucial effects on the cBN crystal morphologies synthesized. Catalyst Li3N tends to grow cBN with thick-plate morphology, catalyst LiH would induce the growth of cBN tending to integrated octahedral morphology, and catalyst LiNH2 would play diverse roles for cBN crystal morphologies in various systems.► The various morphologies cBN monocrystals have been successfully synthesized. ► The special shape of cBN crystal can be provided by different Li-based catalysts/additives.

Bismuth copper oxychalcogenides, BiCuChO (Ch = S, Se, Te), are facile, and rapidly synthesized by high pressure method. The Rietveld refinement of powder X-ray diffractions shows that BiCuChO compounds have a layered crystal structure with a space group of P4/nmm. All the high pressure synthesised samples show semiconductor characteristics, while BiCuTeO prepared by the conventional method displays metal conducting behavior. The conducting behavior of BiCuTeO obtained in this study originates from the low crystal defect concentrations under the effects of high pressure; evidenced by density functional theory calculations. Large Seebeck coefficient ∼600 μV/K was obtained for BiCuSO, due to its high carrier effect mass. BiCuChO exhibits extremely low thermal conductivity (<1 Wm−1K−1), which decreases with an increase in the Ch2− ion radius. The maximum figure of merit reaches 0.03, 0.31 and 0.65 for BiCuSO, BiCuSeO and BiCuTeO, respectively, values which are comparable to those for samples prepared by the conventional, complex method.

In this work, the structure of WB2 synthesized at high pressure and high temperature (HPHT) was accurately determined by X-ray diffraction and Rietveld refinement. Its asymptotic Vickers hardness (Hv) value is 25.5 GPa which is much lower than the previous theoretical results (36–40 GPa). It is worth noting that the chemical bonds between the W layers and two different kinds of B layers show obvious polarization character based on the results obtained from X-ray photoelectron spectroscopy (XPS) and electron localization functions (ELFs), density of states (DOS), topological analysis of the static electron density and Mulliken population. This result can well clarify that WB2 is only a hard but not superhard material. Thus, a 3D network structure can not be formed between the W layers and the B layers which is previously predicted by theoretical calculations. Our results are helpful to understand the hardness mechanism and design superhard materials in TMBs.