Ab initio models for polycrystalline diamond constructed from coldcompressed disordered graphite

Ageneral scheme is proposed to construct systematically a family of superhard sp3carbon phases of cold-compressed graphite by combining hexagonal to cubic diamond (named as X-Carbon). Based on calculations employing density functional theory (DFT), we find that our currently proposed Xcarbon can occur by compressing disordered graphite, and the X-carbon is more stable in energy than the previously proposed M, Z, W, bct-C4, P allotropes. Thus, the X-carbon is predicted to be the transition of cold-compressed graphite. The results show that the simulated x-ray diffraction pattern, Vickers hardness and bulk modulus of X-Carbon match well with the experimental data (Mao et al Science 302, 425 (2003)). These new phases are transparent superhard materials with a large hardness and wide electronic band gaps comparable to cubic diamond.