Original Ultra-hard Orthorhombic Carbon allotropes C8 with lonsdaleite-type lon topology: Crystallographic and DFT investigations

Abstract: The lon topology inherent to lonsdaleite C4 hexagonal diamond is shown to characterize three original carbon allotropes C8 in the orthorhombic system with irregular orientations of the C4 tetrahedra. The octacarbon stoichiometries were devised from crystal structure engineering and identified close to lonsdaleite from density functional theory (DFT) based calculations of ground state structures and energy derived physical properties. Characterized with high densities, the three allotropes are identified with ultra hard with hardness magnitudes close and superior to lonsdaleite. Dynamically, all three allotropes were found stable with positive frequencies revealed from their phonons band structures with specific heat CV = f(T) calculated curves close to diamond experimental values from literature.