First-principles lattice dynamics calculations of the phase boundary between β-Si3N4 and γ-Si3N4 at elevated temperatures and pressures

The phase boundary between beta-Si3N4 and gamma-Si3N4 is investigated at high pressure and high temperature using first-principles lattice dynamics calculations within the quasi-harmonic approximation. We find a positive slope of the phase boundary, hence, at higher temperatures it requires higher pressures to synthesize the high-pressure polymorph of silicon nitride. It turns Out that the thermal expansion of the spinel-type gamma-phase is larger than that of the phenacite-type beta-phase. On the other side, pressure affects more the volume of beta-Si3N4 than of gamma-Si3N4, reflected in the higher bulk modulus of gamma-Si3N4 up to about 40 GPa. The origin of the different temperature behavior of these phases, consequently, is rooted in a larger volume dependence of the zero point energy in gamma-Si3N4 in comparison to beta-Si3N4. (C) 2008 Wiley Periodicals, Inc.