New Metallic Allotropes of Planar and Tubular Carbon

We propose a new family of layered ${\mathrm{sp}}^2$-like carbon crystals, incorporating five-, six-, and seven-membered rings in 2D Bravais lattices. These periodic sheets can be rolled so as to generate nanotubes of different diameter and chirality. We demonstrate that these sheets and tubes are metastable and more favorable than ${\mathrm{C}}_{60}$, and it is also shown that their mechanical properties are similar to those of graphene. Density of states calculations of all structures revealed an intrinsic metallic behavior, independent of orientation, tube diameter, and chirality.