Synthesis, Phase Evolutions, and Stabilities of Boron-Rich Tungsten Borides at High Pressure.

Boron-rich tungsten borides such as WB2+x and WB3+x have been highly expected to be superhard with many advantages over conventional superhard materials. However, because the formation of boron-rich tungsten borides is thermodynamically unfavorable at ambient pressure, their crystal structures, compositions, and properties are largely unexplored, which have impeded the rational design of functional materials in the W-B family. In this work, using unique high-pressure reactions, we report a systematic synthesis study of challenging compounds of tungsten borides including WB, WB2+x, and WB3+x. The use of pressure, combined with the controllable temperature, heating duration, and ratios of starting reactants, leads to different compositions and structures of final products with largely tunable crystallite size from nanocrystalline to single-crystal forms. In addition, the optimal conditions for the formation of WB3+x are well investigated by tuning the temperature and starting ratio of reactants, as well as by adding a solvent material. Phase diagrams and stabilities of the involved W-B compounds are also well depicted, which would provide an important guidance for future exploratory synthesis and study of the family of transition-metal borides.

Large-volume cubic press produces high temperatures above 4000 Kelvin for study of the refractory materials at pressures.

The advent of a large-volume high-pressure apparatus has led to the discovery of many new materials with exceptional properties for widespread applications such as superhard materials (e.g., diamonds). However, for most conventional devices, the pressure and temperature capabilities are often limited to 6 GPa and 2300 K, which severely impedes the study of materials at extended pressures and temperatures. In this work, we present experimental optimizations of the high-pressure cell assembly for cubic press with a focus on the improvement of its temperature capability, leading to a record temperature value of ∼4050 K and largely extended pressure conditions up to ∼10 GPa with a centimeter-sized sample volume. Pressures of the new assembly at high temperatures are investigated by the melting-point method, giving rise to a series of parallel isoforce loading lines associated with thermally induced pressure. For the first time, the high-pressure melting curve of tungsten carbide is determined up to 3800 K and 8 GPa, and single-crystal refractory materials of Mo, Ta, and WC are also grown using the optimized cell.

Phase Stability and Compressibility of 3R-MoN2 at High Pressure.

We report phase stability and compressibility of rhombohedral 3R-MoN2, a newly discovered layer-structured dinitride, using in-situ synchrotron high-pressure x-ray diffraction measurements. The obtained bulk modulus for 3R-MoN2 is 77 (6) GPa, comparable with that of typical transition-metal disulfides (such as MoS2). The axial compressibility along a axis is more than five times stiffer than that along c axis. Such strong elastic anisotropy is mainly attributed to its layered structure with loosely bonded N-Mo-N sandwich interlayers held by weak Van der Waals force. Upon compression up to ~15 GPa, a new hexagonal phase of 2H-MoN2 occurs, which is irreversible at ambient conditions. The structural transition mechanism between 3R and 2H phases is tentatively proposed to be associated with the rotation and translation of sandwich interlayers, giving rise to different layer stacking sequences in both phases. At high temperature, the decomposition of 3R-MoN2 leads to the formation of hexagonal δ-MoN and the onset degassing temperature increases as the pressure increases. In addition, the low-temperature electrical resistivity measurement indicates that 3R-MoN2 behaves as a semiconductor with an estimated band gap of Eg ≈ 0.5 eV. 3R-MoN2 also shows weak antiferromagnetic properties, which probably originates from the occurrence of magnetic zigzag edges in the structure.