Novel ultra-hard hexacarbon allotropes from first principles

Novel ultra-hard hexacarbon C6 allotropes are proposed based on crystal chemistry rationale and geometry optimization onto ground state structures. Similar to diamond, the orthorhombic, tetragonal and trigonal C6 are cohesive networks of C4 tetrahedra illustrated by charge density projections exhibiting sp3-like carbon hybridization. All three allotropes are identified as mechanically (elastic constants) and dynamically (phonons) stable. The electronic band structures are characteristic of insulators with large band gaps of 4 to 5 eV, like diamond. From three different models evaluating Vickers hardness HV, all new carbon allotropes are identified as ultra-hard.