Prediction of a pure-carbon planar covalent metal

Carbon is well-known as an insulator, a semimetal, a molecular solid, and a one-dimensional semiconductor or low-density-of-states metal. We propose a pure-carbon planar structure composed of pentagons and heptagons that is metallic with a density of states at the Fermi level of {~}0.1 state per eV per atom. This structure, planar carbon pentaheptite, is metastable with a total energy per carbon atom comparable to that of C₆₀. The structure can be rolled into tubes in a manner similar to graphite. Possible synthetic pathways are discussed.