Novel ultrahard sp2/sp3 hybrid carbon allotrope from crystal chemistry and first principles: body-centered tetragonal C6 ('neoglitter')

A novel ultrahard carbon allotrope, body-centered tetragonal C6 (space group I-4m2) presenting mixed sp2/sp3 carbon hybridizations is proposed by crystal chemistry approach and studied for the ground state structure and stability (both dynamic and mechanical) using density functional theory calculations. Given that C4 tetrahedra in-plane stacking with corner-sharing and connected out-of-plane with C–C trigonal carbon, a close relationship with so-called 'glitter', hypothetical dense carbon network devised ~30 years ago, is established, so we named the new allotrope 'neoglitter'. 'Neoglitter' is characterized by large bulk and shear moduli and very high hardness, and its metallic-like electronic structure is assigned mainly to the itinerant role of trigonal carbon π-electrons.