Investigations of meta-stable and post-spinel silicon nitrides

Since the discovery of post-spinel Si₃N₄, its fundamental physical properties are highly required. In this paper, theoretical calculations are performed to investigate the structural and elastic properties of the β-, γ-, wII- and post-spinel Si₃N₄ polymorphs. The calculated ground-state properties compare well with available experiments. The phase transformations of the β-, γ-, wII- and post-spinel phases are investigated by the famous plane-wave pseudo-potential density functional theory. From the elastic constants obtained, we find that β-, γ- and wII-Si₃N₄ are stable at 0 GPa and the post-spinel phase is unstable/stable at 0 GPa/160 GPa. When the high-temperature β→γ transformation is bypassed due to kinetic reasons, β-Si₃N₄ is predicted to undergo a first-order phase transition to a new phase (wII-Si₃N₄). It is found that the transition pressures of β→wII and γ→post-spinel transitions are 20.8 GPa and 152.5 GPa, respectively. The phase boundary of the γ→post-spinel transition can be described as P=152.3631−6.39×10⁻³T+2.01062×10⁻⁵T²−1.93962×10⁻⁹T³. Through the quasi-harmonic approximation, the dependences of heat capacity, entropy, thermal expansion coefficient and the Debye temperature on temperature, are also successfully predicted.