Unexpected stable phases of tungsten borides

Abstract

Tungsten borides are a unique class of compounds with excellent mechanical properties comparable to those of traditional superhard materials. However, the in-depth understanding of these compounds is hindered by the uncertainty of their phase relations and complex crystal structures. Here, we explored the W–B system systematically by ab initio variable-composition evolutionary simulations at pressures from 0 to 40 GPa. Our calculations successfully found all known stable compounds and discovered two novel stable phases, P2₁m-WB and P2₁/m-W₂B₃, and three nearly stable phases, R3m-W₂B₅, Ama2-W₆B₅, and Pmmn-WB₅, at ambient pressure and zero Kelvin. Interestingly, P2₁m-WB is much harder than the known α and β phases, while Pmmn-WB₅ exhibits the highest hardness. Furthermore, it is revealed that the much debated WB₄ becomes stable as the P6₃/mmc (2 f.u. per unit cell) phase at pressures above ∼1 GPa, not at ambient pressure as reported previously. Our findings provide important insights for understanding the rich and complex crystal structures of tungsten borides, and indicate WB₂, WB₄, and WB₅ as compounds with the most interesting mechanical properties.