科研之友
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摘要
In this work, we report a quite different conclusion from Tian et al. [Phys. Rev. B 78 (2008) 235431]. It is proved that beta-C3N2 is the only phase under high pressure, and alpha-C3N2 does not exist. beta-C3N2 is a covalent crystal composed of strong C-C and C-N covalent bonds. Band gap of beta-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 (C-v) is proportional to T-3; and at intermediate temperatures, G is governed by the details of vibrations of the atoms; finally, G reaches to beta-C3N2's Dulong-Pettit limit (about 120 J/mol K). Though thermal expansion coefficient (a) increases with temperature, a is less than 1 x 10(-5) K-1. Elastic constants rise with pressure, but shear moduli is quite steady which increases just a little with pressures.
