Crystal chemistry rationale and DFT investigations of novel hard tetragonal C6 built from tetrahedral C(sp3) lattice embedding allene-like sp2 linear tricarbon.

From crystal chemistry rationale novel tetragonal carbon allotrope: C6 featuring allene-like C(sp2) tricarbon unit >C=C=C< embedded in diamond-like structure C(sp3) lattice is proposed. Quantum mechanics DFT calculations onto ground state let identify C6 as cohesive with localized charge around tetrahedral C-sp3 and smeared charge density along linear allene-like tricarbon unit characteristic of C-sp2 hybridization. Novel C6 is found mechanically stable from elastic properties with large Vickers hardness: HV= 58 GPa. Dynamically stable with positive phonon acoustic and optic frequencies, C6 is characterized with particularly high optical phonon frequency =60 THz assigned to antisymmetric stretching mode (C----C-C), found to characterize I.R. active mode in C3H4 allene isolated molecule. The electronic band structure reveals semi-conducting electronic properties.