Diamond-XII: a new type of exotic cubic carbon allotrope

Abstract

Based on the tetragonal diamond segments (CH₄, C₅H₁₂, C₁₄H₂₄, C₃₀H₄₀, C₅₅H₆₀ and C₉₁H₈₄), a new type of cubic carbon crystalline structure (Diamond-XII) with 12-fold distorted diamond segments was proposed and investigated by first-principles calculations. The phonon band structures and elastic properties show that all these Diamond-XII structures are both mechanically and dynamically stable. They are more stable than the previously proposed T-carbon, BC-8 and C3-205 phases and are confirmed to be transparent superhard insulators. Especially, the lattice constants for Diamond-XII-4 and Diamond-XII-5 of 12.83 Å and 15.68 Å are very close to that (14.7 Å) of the superhard transparent cubic carbon phase discovered in the Popigai crater [A. El Goresy et al., C. R. Geoscience, 2003, 335, 889–898]. Their simulated XRD results can explain many of the XRD peaks from the experiment. The matches in lattice, hardness, XRD and electronic properties provide us with an inaugural and fundamental understanding of the super-size cubic carbon for the first time.