New Metallic Carbon Crystal

Recently, mathematical analysis clarified that $s{p}^2$ hybridized carbon should have a three-dimensional crystal structure ($K_4$) which can be regarded as a twin of the $s{p}^3$ diamond crystal. In this study, various physical properties of the $K_4$ carbon crystal, especially for the electronic properties, were evaluated by first principles calculations. Although the $K_4$ crystal is in a metastable state, a possible pressure induced structural phase transition from graphite to $K_4$ was suggested. Twisted $\pi$ states across the Fermi level result in metallic properties in a new carbon crystal.

Figure 1

Binding energy vs volume curves of diamond (blue), graphite (grey and black), and $K_4$ (red) crystal structures composed of carbon atoms based on (a) LDA, and (b) GGA developed by Ceparley and Alder (CA) [24], and Perdew and Wang (PW91) [25]; The curves are fits of the Murnaghan’s equation of state [27] to the calculated points. Tangential line between the curves of $K_4$-graphite crystals; XRD patterns [in the inset of (a)].

Figure 2

(a) Total and partial density of states (DOS). (b) Band structures for the valence electrons based on LDA calculations. The first Brillouin zone with selected pass for the electronic band structure [in the inset of (a)]. (c) The partial valence-electron density. The number of partial valence-electron densities for each group in a primitive unit cell [right side of (b)]. (d) Isosurfaces of the total valence-electron density.