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

La2Sn2O7 is a transparent conducting oxide (TCO) material and shows a strong near-infrared fluorescent at ambient pressure and room temperature. By in situ high-pressure research, pressure-induced visible photoluminescence (PL) above 2 GPa near 2 eV is observed. The emergence of unusual visible PL behavior is associated with the seriously trigonal lattice distortion of the SnO6 octehedra, under which the Sn–O1–Sn exchange angle θ is decreased below 22.1 GPa, thus enhancing the PL quantum yield leading to Sn 3P1 1S0 photons transition. Besides, bandgap closing followed by bandgap opening and the visible PL appearing at the point of the gap reversal, which is consistent with high-pressure phase decomposition, are discovered. The high-pressure PL results demonstrate a well-defined pressure window (7–17 GPa) with flat maximum PL yielding and sharp edges at both ends, which may provide a great calibration tool for pressure sensors for operation in the deep sea or at extreme conditions.

•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

The geometrically frustrated pyrochlore Eu2Sn2O7 is an insulator with slight trigonal lattice distortion at ambient condition. High pressure is applied to this system to investigate the responses of structural evolution, optical emission and electrical transport properties. In situ high pressure synchrotron X-ray diffraction, Raman spectroscopy, and photoluminescence studies are performed in Eu2Sn2O7 up to 31.2 and 34.1 GPa, respectively. The abrupt change of the oxygen atomic position without breaking the crystal symmetry is accompanied by disappearing of Raman mode involving SnO6 octahedron distortion around 17.8 GPa. It indicates a pressure-induced second-order iso-structural transition, which suppresses the trigonal distortion in the SnO6 octahedron but enhances the local symmetry distortion of EuO8 hexahedron. Anomalous luminescence of the Eu3+ 4f–4f transition is observed, which confirms the enhancement of EuO8 hexahedral distortion at high pressure region. In situ high-pressure electrical transport property is measured by alternating current (AC) impedance spectroscopy up to 32.5 GPa. A rapid increase in resistance with gain of 4 orders of magnitude by applied pressure is observed until 16.6 GPa, and it is followed by a slight decreasing to the highest pressure measured here. All these observations indicate a pressure-enhanced trigonal lattice distortion before the transition pressure, and thus it will enlarge an opening gap at the Fermi energy, followed by releasing distortion at higher 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.

The structural and electrical properties of ZnV2O6 under high pressure have been studied using Raman spectroscopy, in situ angle dispersive X-ray diffraction (ADXRD), and alternating current (AC) impedance spectroscopy. The results of Raman spectra indicate that ZnV2O6 undergoes a reversible structural change around 16.6 GPa, as evidenced by the appearance of new peaks. The results of Rietveld refinements from in situ ADXRD data indicate that the monoclinic symmetry (C2/m) is retained up to 16.0 GPa and the C2 phase comes to coexist between 16.0 and 16.9 GPa. Above 16.9 GPa, the high-pressure phase can be distinguished only as the C2 structure. The transformation process from the C2/m phase to the C2 phase is mainly caused by the more distorted ZnO6 octahedra and VO6 octahedra at higher pressures. The equal bond distances Zn–O2 and V–O3 in the C2/m phase become unequal in the C2 phase. Furthermore, the measurements of the AC impedance spectroscopy of ZnV2O6 reveal obvious changes in its electrical transport properties at 14.1 GPa which could correspond to the observed phase transition in the Raman and ADXRD measurements. The combined analyses of experimental results suggest the occurrence of a reversible structural phase transition of ZnV2O6 around 16.0 GPa.

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).

One-dimensional silicon carbide–carbon nanotube composite was fabricated using a direct solid reaction process between pure silicon powders and carbon nanotubes at temperatures ranging from 1300 to 1400 °C. Scanning electron microscopy, X-ray diffraction, Raman scattering, and cathodoluminescence techniques were used to characterize their structures. The as-prepared composite products have shown pronounced luminescent properties with two emission bands centered at 2.3 eV and 2.9 eV by room-temperature cathodoluminescence.Research Highlights► SiC-C nanotube composite was directly fabricated from silicon powders and CNTs. ► The outer diameter of the composite nanotubes is approximately 60-100 nm. ► The SiC-C composites have shown two emission bands centered at 2.3 eV and 2.9 eV.

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.