Surperhard monoclinic BC₆N allotropes: First-principles investigations

Via structural searching methodology and first-principles calculations, we predicted two new BC₆N allotropes, a C-centered monoclinic BC₆N (Cm-BC₆N) and a primitive-centered monoclinic BC₆N (Pm-BC₆N). The lattice vibrations, elastic properties, ideal strength, theoretical hardness, and electronic structure of the predicted BC₆N were investigated systematically. Our results reveal that Cm-BC₆N is more favorable energetically than 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 are indirect semiconductors with band gaps of 2.66 eV and 0.36 eV, respectively. Cm-BC₆N exhibits the excellent ideal shear strength of 53.9 GPa in (011)011, much greater than that of Pm-BC₆N (25.0 GPa in (010)101 shear direction), and Cm-BC₆N shows a much lower anisotropy in shear strength than Pm-BC₆N. The Vickers hardness of Cm-BC₆N is estimated to be above 80 GPa, which is more outstanding than those of t-BC₆N and r-BC₆N.