First-principles study on the electronic structure and phase transition of α-, β- and γ-Si3N4

The high-temperature lattice constants and elastic moduli of the silicon nitrides are calculated using the plane-wave pseudo-potential method combined with the vibrational Debye-like model. beta-Si3N4 is ductile at low temperature and brittle at high temperature. gamma-Si3N4 is found to be brittle and covalent in nature. We find a positive slope of the beta -> gamma phase boundary, hence, at higher temperatures it requires higher pressures to synthesize gamma-Si3N4. The alpha -> gamma phase boundary may be expressed as P = 16.29 - 1.835 x 10(-2)T + 9.33945 x 10(-5) T-2 - 2.16759 x 10(-7)T(3) + 2.91795 x 10(-10)T(4). We also obtain the electronic structures and energy bands of Si3N4 with and without pressure. The interaction between Si-s, p and N-s, p plays a dominant role in the stability of alpha-Si3N4. The alpha- and beta-Si3N4 have the Gamma(V)-Gamma(C) indirect band gaps (4.9 eV and 4.4 eV) while gamma-Si3N4 has a direct band gap(3.9 eV). The tops of the valence bands for alpha- and beta-Si3N4 are along the Gamma-M and Gamma-A direction, respectively. Our results are consistent with the experimental data and the theoretical results.