Ab initio study of phase stability, thermodynamic and elastic properties of C3N2 derived from cubic C20

In this work, we report a quite different conclusion from Tian et al. [Phys. Rev. B 78 (2008) 235431]. It is proved that β-C3N2 is the only phase under high pressure, and α-C3N2 does not exist. β-C3N2 is a covalent crystal composed of strong CC and CN covalent bonds. Band gap of β-C3N2 increases with pressure. The width of antibonding state, shown in partial density of states (PDOS), keeps about 5 eV with rising pressures, which brings stable CN or CC covalent bonds. At sufficiently low temperatures, heat capacity (Cv) is proportional to T3; and at intermediate temperatures, Cv is governed by the details of vibrations of the atoms; finally, Cv reaches to β-C3N2's Dulong-Pettit limit (about 120 J/mol K). Though thermal expansion coefficient (α) increases with temperature, α is less than 1×10−5 K−1. Elastic constants rise with pressure, but shear moduli is quite steady which increases just a little with pressures.