Unexpectedly hard and highly stable WB3 with a noncompact structure

Tungsten borides are currently a topic of promising superhard materials, but their structures are still not fully resolved. Here, we identify a highly stable WB3 in hR24 symmetry (hR24-WB3) by first-principles calculations. It can be derived from well-known hP3-WB2 by removing one third of tungsten atoms systematically so that the remaining tungsten atoms form three cycled layers of open hexagons with each a layer displaced by one atom. Furthermore, such a noncompact and metallic system has an unexpectedly high hardness, which originates from its three-dimensional covalent framework consisting of hexagonal boron planes interconnected with strong zigzag W–B bonds.