Ultra-hard hexagonal C12 with C3 cyclopropane-like moiety from first principles

Abstract

An original carbon allotrope, hexagonal C12, is proposed from crystal chemistry and quantum density functional theory DFT calculations of ground state and physical properties. The structure exhibits corner sharing distorted tetrahedra with the presence of C3 triangular cyclopropane-like moiety connecting planar carbon. Hex.C12 is found cohesive and stable both mechanically (elastic constants and their combinations) and dynamically (phonons band structures) and presents ultra-hardness with HVickers = 70 GPa. The temperature dependence of the heat capacity CV shows close magnitudes to experimental results of diamond. The electronic band structure reveals insulating behavior with 4 eV band gap, smaller than diamond's.