Monoclinic mC28 carbon: A sp2-sp3 hybridized carbon allotrope with superhard and metallic properties

• A novel carbon structure with mixed sp 2 and sp 3 bonds (named as mC 28 carbon) was theoretically proposed. • The mC 28 carbon is dynamically stable and has a lower enthalpy than graphite under ~ 29 GPa, which implied it could be a metastable phase of carbon. • It is also calculated to be a metal combined with high hardness (the calculated Vickers hardness value reached 85 GPa ) New carbon allotropes with mixed sp , sp 2 and sp 3 bonds are now the focus of theoretical and experimental researches due to their abnormal properties. We here theoretically proposed a novel three-dimensional carbon structure with mixed sp 2 and sp 3 bonds (named as mC 28 carbon) via particle swarm optimization method. mC 28 carbon is built by the ‘diamond-like’ sp 3 networks connected by sp 2 bonded carbon atoms. It has a lower enthalpy than graphite after ∼29 GPa and is dynamically stable at zero pressure. More interesting, mC 28 carbon is metallic, which can be attributed to the connected sp 2 bonding carbon atoms. They provide a channel for their extra delocalized valence electron that siting at p z orbital. mC 28 carbon is also a superhard material with theoretical Vickers hardness of 85 GPa.