An Orthorhombic Phase of Superhard o-BC4N**Supported by the National Natural Science Foundation of China under Grant Nos 21671168 and 21875205, the Hebei Natural Science Foundation under Grant No B2015203096, and the Qinhuangdao Science and Technology Support Program under Grant No 201703A014.

Abstract

A potential superhard o-BC4N with Imm2 space group is identified by ab initio evolutionary methodology using CALYPSO code. The structural, electronic and mechanical properties of o-BC4N are investigated. The elastic calculations indicate that o-BC4N is mechanically stable. The phonon dispersions imply that this phase is dynamically stable under ambient conditions. The structure of o-BC4N is more energetically favorable than g-BC4N above the pressure of 25.1 GPa. Here o-BC4N is a semiconductor with an indirect band gap of about 3.95 eV, and the structure is highly incompressible with a bulk modulus of 396.3 GPa and shear modulus of 456.0 GPa. The mechanical failure mode of o-BC4N is dominated by the shear type. The calculated peak stress of 58.5 GPa in the (100)[001] shear direction sets an upper bound for its ideal strength. The Vickers hardness of o-BC4N reaches 78.7 GPa, which is greater than that of t-BC4N and bc-BC4N proposed recently, confirming that o-BC4N is a potential superhard material.