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

Boron suboxide B₆O, the hardest known oxide, has an Rm crystal structure (α-B₆O) that can be described as an oxygen-stuffed structure of α-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 (β-B₆O), 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-B₆O structure. β-B₆O also has a slightly higher hardness and may be synthesized using different experimental protocols. We suggest that β-B₆O is present in mixture with α-B₆O, and its presence accounts for previously unexplained bands in the experimental Raman spectrum.