Congruent melting of tungsten phosphide at 5 GPa and 3200 ℃ for growing its large single crystals

As one of important members of refractory materials, tungsten phosphide (WP) holds great potential for fundamental study and industrial applications in many fields of science and technology, due to its excellent properties such as superconductivity and as-predicted topological band structure. However, synthesis of high-quality WP crystals is still challenging using tradition synthetic methods, because the required synthesis temperature for growing its large crystals is very stringent as high as 3000℃, which is far beyond the temperature capability of most laboratory-based devices for crystal growth. In addition, high temperature often induces decomposition of metal phosphides, leading to off-stoichiometric samples based on which the materials' intrinsic properties cannot be explored. In this work, we report a high-pressure synthesis of single-crystal WP through a direct crystallization from cooling the congruent W-P melts at 5 GPa and~3200℃. In combination of x-ray diffraction, electron microscope, and thermal analysis, the crystal structure, morphology, and stability of recovered samples are well investigated. The final product is phase-pure and nearly stoichiometric WP in a single-crystal form with a large grain size in excess of one millimeter, making it feasible for most experimental measurements, especially for that the large crystal is required. Success in synthesis of high-quality WP crystals at high pressure would offer great opportunities for determination of its intrinsic properties and also spurs more study efforts on the family of transition-metal phosphides.