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 (ß-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. ß-B6O also has a slightly higher hardness and may be synthesized using different experimental protocols. We suggest that ß-B6O is present in mixture with α-B6O, and its presence accounts for previously unexplained bands in the experimental Raman spectrum.

A novel phase of beryllium fluoride at high pressure.

A previously unknown thermodynamically stable high-pressure phase of BeF2 has been predicted using the evolutionary algorithm USPEX. This phase occurs in the pressure range 18-27 GPa. Its structure has C2/c space group symmetry and contains 18 atoms in the primitive unit cell. Given the analogy between BeF2 and SiO2, silica phases have been investigated as well, but the new phase has not been observed to be thermodynamically stable for this system. However, it is found to be metastable and to have comparable energy to the known metastable phases of SiO2, suggesting a possibility of its synthesis.