Surperhard monoclinic BC6N allotropes: First-principles investigations**Project supported by the National Natural Science Foundation of China (Grant Nos. 21671168 and 21875205), the Hebei Natural Science Foundation, China (Grant No. B2015203096), and the Qinhuangdao Science and Technology Support Program, China (Grant No. 201703A014).

Abstract

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)[01 ], 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.