Novel Ultrahard Extended Hexagonal C10, C14 and C18 Allotropes with Mixed sp2/sp3 Hybridizations: Crystal Chemistry and Ab Initio Investigations

Based on 4H, 6H and 8H diamond polytypes, novel extended lattice allotropes C10, C14 C18 characterized by mixed sp3/sp2 carbon hybridizations were devised based crystal chemistry rationale first-principles calculations of the ground state structures energy derived properties: mechanical, dynamic (phonons), electronic band structure. The found increasingly cohesive along series, with values approaching those polytypes. Regarding mechanical properties, C14, ultrahard Vickers hardness slightly below that diamond. All them are dynamically stable, positive phonon frequencies reaching maxima higher than in due to stretching modes C=C=C linear units. expectedly reveal insulating character all three polytypes conductive hybrid allotropes. From analysis bands crossing Fermi level, a nesting surface was identified, allowing us predict potential superconductive properties.