Prediction of a new ground state of superhard compound B(6)O at ambient conditions

Boron suboxide B(6)O, the hardest known oxide, has an R[Image: see text]m crystal structure (α-B(6)O) that can be described as an oxygen-stuffed structure of α-boron, or, equivalently, as a cubic close packing of B(12) icosahedra with two oxygen atoms occupying all octahedral voids in it. Here we show a new ground state of this compound at ambient conditions, Cmcm-B(6)O (β-B(6)O), which in all quantum-mechanical treatments that we tested comes out to be slightly but consistently more stable. Increasing pressure and temperature further stabilizes it with respect to the known α-B(6)O structure. β-B(6)O also has a slightly higher hardness and may be synthesized using different experimental protocols. We suggest that β-B(6)O is present in mixture with α-B(6)O, and its presence accounts for previously unexplained bands in the experimental Raman spectrum.