A superhard orthorhombic carbon with all six-membered-ring in sp3 bonding networks

We identify by first-principles calculations a new diamond-like carbon phase with a 16-atom orthorhombic primitive cell in Pbcn (D-2h(14)) symmetry. This new carbon allotrope consists of all sp(3) six-membered rings like as the reported BC8, BC12 and R16 carbon, while energetically more stable than these carbon phases. Its dynamical stability has been confirmed by phonon mode analysis and molecular dynamics simulations. The calculated bulk modulus (435 GPa) and Vickers hardness (93 GPa) are comparable to that of diamond, showing as a superhard carbon material. Electronic band calculations reveal that it is an insulator with an indirect band gap of 4.35 eV. Simulated X-ray diffraction spectrum presents a complicated pattern, showing an amorphous form of diamond, reflecting its structural complexity stemming from its multiple bond lengths and bond angles, but matches well with the diffractions peaks found in the diamond-rich coatings on stainless steel substrate. These findings lay a foundation for further study of this new diamond-like carbon allotrope and its outstanding properties. (C) 2019 Elsevier B.V. All rights reserved.