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We search for stable compounds of boron and oxygen at pressures from 0 to 500 GPa using the ab initio evolutionary algorithm USPEX. Only two stable stoichiometries of boron oxides, namely, B 6 O and B 2 O 3 , are found to be stable, in good agreement with experiment. A hitherto unknown phase of B 6 O at ambient pressure, Cmcm-B 6 O, has recently been predicted by us and observed experimentally. For B 2 O 3 , we predict three previously unknown stable high-pressure phases-two of these (Cmc2 1 and P 2 1 2 1 2 1) are dynamically and mechanically stable at ambient pressure, and should be quenchable to ambient conditions. Their predicted hardnesses, reaching 33-35 GPa, make them harder than SiO 2-stishovite. These are the hardest known oxides (if one disregards B 6 O, which is essentially a boron-based insertion compound). Under pressure, the coordination number of boron atoms changes from 3 to 4 to 6, skipping fivefold coordination.

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Negative Poisson's ratio materials have the advantages of good shear resistance, dent resistance, and fracture resistance. Thus, they have great application potential in the manufacturing field with high requirements for material mechanical flexibility. However, negative Poisson's ratio materials are relatively rare. In this paper, the first principles calculation is used to study the single-layer P2/ m-P phosphorene. It is found that when −5% to 5% strain is applied along the zigzag (Y) direction, there is a negative Poisson's ratio effect along the Z direction, and Poisson's ratio is −0.288, which is about ten times that of the single-layer black phosphorus (the negative Poisson's ratio of the single-layer black phosphorus is −0.027). Compared with black phosphorene, it has more potential applications in wearable and impact resistant equipment.