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 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.