First-principles prediction of a semilayered carbon allotrope

A new carbon allotrope composed of carbon hexagons and cubes is proposed, which is named α-graphite. It is semilayered with a single interlayer bond in the unit-cell region, possessing a formation energy about 1.16 eV/atom higher than 2H-graphite. First-principles calculations indicate that this allotrope is dynamically and mechanically stable. The Young’s modulus of α-graphite is about 521 GPa and Vickers hardness is about 36 GPa. It behaves as a semiconductor with an energy gap of 0.31 eV. At a compressive strain about 5%, α-graphite undergoes a phase transition between the semiconductor and the metal. The monolayer of α-graphite is called α-graphene, which is also meta-stable. It is metallic with a flat band near the Fermi level. Its stiffness is 300 N/m, close to that (330 N/m) of graphene. Silicon can also be meta-stable in the two-dimensional case, which is metallic, but it becomes unstable in the three-dimensional case. Other group-IVA elements are not stable in the α phase, no matter in two- or three-dimensions.