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(1)],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.