Synthesis, crystal structure, and properties of stoichiometric hard tungsten tetraboride, WB 4

Tungsten tetraboride has been known so far as a non-stoichiometric compound with a variable composition (e.g. WB$_{4−x}$, WB$_{4+x}$). Its mechanical properties could exceed those of hard tungsten carbide, which is widely used nowadays in science and technology. The existence of stoichiometric WB$_4$ has not been proven yet, and its structure and crystal chemistry remain debatable to date. Here we report the synthesis of single crystals of the stoichiometric WB$_4$ phase under high-pressure high-temperature conditions. The crystal structure of WB$_4$ was determined using synchrotron single-crystal X-ray diffraction. In situ high-pressure compressibility measurements show that the bulk modulus of WB$_4$ is 238.6(2) GPa for B′ = 5.6(0). Measurements of mechanical properties of bulk polycrystalline sub-millimeter size samples under ambient conditions reveal a hardness of ∼36 GPa, confirming that the material falls in the category of superhard materials