Prediction of a new ground state of superhard compound B6O at ambient conditions

Boron suboxide B6O, the hardest known oxide, has an R-3m crystal structure ({\alpha}-B6O) that can be described as an oxygen-intercalated structure of {\alpha}-boron, or, equivalently, as a cubic close packing of B12 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-B6O (\b{eta}-B6O), which in all quantum-mechanical treatments that we tested (GGA, LDA, and hybrid functional HSE06) comes out to be slightly but consistently more stable. Increasing pressure and temperature further stabilize it with respect to the known {\alpha}-B6O structure. \b{eta}-B6O also has a slightly higher hardness and may be synthesized using different experimental protocols. We suggest that \b{eta}-B6O is present in mixture with {\alpha}-B6O, and its presence accounts for previously unexplained bands in the experimental Raman spectrum.