Motivated by a recently predicted structure of diamond-like BC2 with a high claimed hardness of 56 GPa (J. Phys. Chem. C 2010, 114, 22688–22690), we focus on whether this tetragonal BC2 (t-BC2) is superhard or not in spite of such an ultrahigh theoretical hardness. The mechanical properties of t-BC2 were thus further extended by using the first principles in the framework of density functional theory. Our results suggest that the Young’s and shear moduli of t-BC2 exhibit a high degree of anisotropy. For the weakest shear direction, t-BC2 undergoes an electronic instability and structural collapse upon a shear strain of about 0.11, with its theoretically ideal strength of only 36.2 GPa. Specifically, the plastic deformation under shear strain along the (110)[001] direction can be attributed to the breaking of d1 B–C bonds.
