Rhombohedral superhard structure of BC2N

Ab initio evolutionary algorithm was employed to resolve the crystal structure of the observed superhard BC2N. We uncovered two polymorphs with rhombohedral (2 f.u./cell) and orthorhombic (2 f.u./cell) symmetries, with which the experimental x-ray diffraction pattern is well reproduced. Analysis of the total energy results and the simulated energy-loss near-edge spectroscopy suggests that the rhombohedral structure is the best candidate for the superhard BC2N. We further demonstrated that earlier proposed high density and low density forms are likely from this single rhombohedral phase.