Superhard semiconducting C3N2 compounds predicted via first-principles calculations

In this paper we introduce the two compounds alpha-C3N2 and beta-C3N2 with cubic symmetry, which are derived from a simple cubic carbon (C-20) reported recently. Our first-principles calculations show that both the solids are highly incompressible with high bulk modulus (380 and 343 GPa), large shear modulus G (365 and 368 GPa), great elastic constant C-44 (327 and 329 GPa), and high Vickers hardness H-V (both are 86 GPa), respectively. Accordingly we come to a conclusion that both the C3N2 phases are potentially superhard semiconductor materials. From the electronic partial density of states, it is found that the superhard character of the two phases is mainly attributed to the strong covalent bond between C and N. The alpha-C3N2 is dynamically stable at pressures below 1.2 GPa but unstable above 1.2 GPa because an optical branch softens to zero at the Gamma point. The alpha-C3N2 will transform into beta-C3N2 at about 1.2 GPa which is energetically more stable. Both phonon-dispersion and elastic constant calculations at zero and high pressures show that this beta-C3N2 remains mechanically and dynamically stable in a pressure range from 0 to at least 20 GPa.