Surperhard monoclinic BC 6 N allotropes: First-principles investigations

Via structural searching methodology and first-principles calculations, we predicted two new BC6N allotropes of Cm-BC6N and Pm-BC6N. The lattice vibrations, elastic properties, ideal strength and theoretical hardness, electronic structure of the predicted BC6N are investigated systematically. Our results reveal that Cm-BC6N is more favorable energetically than that of graphite-like g-BC6N above 20.6 GPa, which is lower than the transition pressures of r-BC6N, t-BC6N and Pm-BC6N. Both Cm-BC6N and Pm-BC6N have indirect semiconductors with band gap of 2.66eV and 0.36 eV, respectively. Cm-BC6N exhibits the excellent ideal shear strength of 53.9 GPa in (011)[011], much greater than Pm-BC6N (25.0 GPa in (010)[101] shear direction), and also Cm-BC6N shows much lower anisotropies in shear strength than Pm-BC6N. The Vickers hardness of Cm-BC6N is estimated above 80 GPa, which is more outstanding than those t-BC6N and r-BC6N.