Investigations of high-pressure and high-temperature behaviors of the newly-discovered willemite-Ⅱ and post-phenacite silicon nitrides

Using the first-principles method of the plane-wave pseudo-potential, the structural properties of the newly-discovered willemite-Ⅱ Si₃N₄ (wⅡ phase) and post-phenacite Si₃N₄ (δ phase) are investigated. The α phase is predicted to undergo a first-order α→wⅡ phase transition at 18.6 GPa and 300 K. Within the quasi-harmonic approximation (QHA), the α→wⅡ phase boundary is also obtained. When the well-known β→γ transition is suppressed by some kinetic reasons, the β→δ phase transformation could be observed in the phase diagram. Besides, the temperature dependences of the cell volume,thermal expansion coefficient, bulk modulus, specific heat, entropy and Debye temperature of the involved phases are determined from the non-equilibrium free energies. The thermal expansion coefficients of wⅡ-Si₃N₄ show no negative values in a pressure range of 0-30 GPa, which implies that the wⅡ-Si₃N₄ is mechanically stable. More importantly, the δ-Si₃N₄ is found to be a negative thermal expansion material. Further experimental investigations may be required to determine the physical properties of wⅡ- and δ-Si₃N₄ with higher reliability.