An Orthorhombic Phase of Superhard o-BC_4N

A potential superhard o-BC_4N with Imm2 space group is identified by ab initio evolutionary methodology using CALYPSO code. The structural, electronic and mechanical properties of o-BC_4N are investigated. The elastic calculations indicate that o-BC_4N is mechanically stable. The phonon dispersions imply that this phase is dynamically stable under ambient conditions. The structure of o-BC_4N is more energetically favorable than g-BC_4N above the pressure of 25.1 GPa. Here o-BC_4N 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-BC_4N 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-BC_4N reaches 78.7 GPa, which is greater than that of t-BC_4N and bc-BC_4N proposed recently, confirming that o-BC_4N is a potential superhard material.