Electronic structures and phase transition characters of β-, P61-, P62-and δ-Si3N4 under extreme conditions: a density functional theory study

This paper describes the results of structural, electronic and elastic properties of silicon nitride(in its high-pressure P61 and P62 phases) through the first-principles calculation combined with an ultra-soft pseudopotential. The computed equilibrium lattice constants agree well with the experimental data and the theoretical results. The strongest chemical bond(N–Si bond) shows a covalent nature with a little weaker ionic character. P61-Si3N4 is more stable than P62-Si3N4 due mainly to the fact that the shorter N–Si bond in the P61 phase allows stronger electron hybridizations. We have also predicted the phase stability of Si3N4 using the quasi-harmonic approximation, in which the lattice vibration and phonon effect are both considered. The results show that the β→P61 phase transition is very likely to occur at 42.9 GPa and 300 K. The reason why the β→P61→δ phase transitions had never been observed is also discussed.