Orthorhombic C14 carbon: A novel superhard sp3 carbon allotrope

We establish a novel carbon phase in purely sp(3)-bonded networks with a 14-atom orthorhombic unit cell, termed C14 carbon, by swarm structural searches. This novel carbon phase is dynamically and mechanically stable under ambient conditions. The C14 carbon exhibits a high bulk modulus of 407 GPa and Vickers hardness of 84.3 GPa, which are comparble to those of diamond. Simulated X-ray diffraction pattern is in satisfactory agreement with the previously unresolved diffraction peaks found in detonation soot samples. Interestingly, electronic band structure calculations reveal that the C14 carbon is a semi-conductor with a direct band gap of around 4.60 eV. These results establish a novel carbon phase with outstanding mechanical and electronic properties and offer insights for further exploring other carbon allotropes. (C) 2019 Elsevier Ltd. All rights reserved.