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

Abstract

Using the first-principles method of the plane-wave pseudo-potential, the structural properties of the newly-discovered willemite-II Si 3 N 4 (wII phase) and post-phenacite Si 3 N 4 (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-Si 3 N 4 show no negative values in a pressure range of 0-30 GPa, which implies that the wII-Si 3 N 4 is mechanically stable. More importantly, the δ -Si 3 N 4 is found to be a negative thermal expansion material. Further experimental investigations may be required to determine the physical properties of wII-and δ -Si 3 N 4 with higher reliability.