Novel Ultrahard C6 with Mixed sp3-sp2 Hybridizations and Semiconducting Behavior: Crystal Chemistry and First Principles Investigations

Novel tetragonal carbon allotrope C6 made of allene-like C(sp2) tricarbon unit >C=C=C< embedded in diamond-like C(sp3) lattice is proposed from crystal chemistry backed with first principles (DFT) pointing to localized charge density around tetrahedral C(sp3) and smeared charge along linear allene-like tricarbon unit characteristic of C(sp2) hybridization. From phonons band structures C6 is dynamically stable characterized with particularly high optical phonon frequency w = 60 THz assigned to antisymmetric C-C-C stretching mode, also identified as I.R. active in isolated C3H4 allene molecule. Novel C6 is found mechanically stable with large Vickers hardness. The electronic band structure reveals semi-conducting electronic properties. Hybrid electronic configurations C(sp3)-C(sp2) identified in nanodiamonds benefit from a growing interest in different fields of materials science, whence the relevance of novel C6 allotrope as model for such investigations.