A superhard orthorhombic carbon allotrope: Cmca-C16

A novel three-dimensional sp3 hybrid carbon allotrope, namely Cmca-C16, is predicted through structure searching within density functional theory (DFT) calculations. The structural, electronic, relative formation enthalpy and mechanical properties including bulk, shear, Young's modulus, ideal strength and Vickers' hardness are systematically investigated. The obtained results demonstrated that Cmca-C16 exhibiting excellent mechanical and dynamic stability. The calculations of ideal strength indicate that the failure mode occurs along the [100] direction, with the lowest peak stress of 80.8 GPa. The theoretical hardness of Cmca-C16 is calculated to be 88.6 GPa, which is comparable to that Lonsdaleite (90.5 GPa) and C14-carbon (89.7 GPa), but higher than most other known carbon phases.