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

Abstract

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.