Optical properties of β-C3N4and its pressure dependence

The electronic and the optical properties of β-${\mathrm{C}}_3$ ${\mathrm{N}}_4$ are studied by means of first-principles local-density calculations. The structural properties obtained via total-energy calculations are in good agreement with the results of Liu and Cohen [Phys. Rev. B 41, 10727 (1990)]. The frequency-dependent optical properties are calculated after an approximate self-energy correction scheme is applied to the energy spectrum. It is found that the band gap in β-${\mathrm{C}}_3$ ${\mathrm{N}}_4$ increases linearly with pressure. The absorption spectrum shows two major peaks at 7.9 and 13.9 eV with a static dielectric constant of 3.2 predicted.