Superhard phases of B2O: An isoelectronic compound of diamond

Using ab initio crystal structure prediction, we have predicted several polytypic structures for B2O under high pressure. The structural parameters, total energies, electronic properties and hardness of the obtained structures have been studied using pseudopotential density functional theory within the local density approximation. We find that a monoclinic C2/m phase is the most energetically favorable structure; however, its X-ray diffraction pattern could not fit into the experimental data [J. Mater. Sci. Lett. 6 (1987) 683]. Instead, X-ray diffraction patterns of the energetically less stable tetragonal P42mc and designed P-4m2 structures are in overall agreement with the experimental data. The simulated hardness of P42mc and P-4m2 are 40.5 and 44.8 GPa, respectively, in accordance with the experimental values of 33.5 ~ 40.5 GPa.

Research Highlights
► We have investigated the candidate structures of the synthesized superhard B2O.
► A monoclinic C2/m B2O with hardness of 66.7 GPa is most energetically favorable.
► From the simulated XRD data, P42mc and P-4m2 are the most likely structures.
► The simulated hardness of P42mc and P-4m2 are in accordance with experimental data.
► The large positive energies could explain why B2O is not easily synthesized later.