Theoretical Study on the High-Temperature P6̄ and P6̄′ Phases of Si3N4: A Tool to Aid in Ceramics Development

Abstract

Atomistic modeling based on the density functional theory combined with the quasi-harmonic approximation is used to investigate the lattice parameters and elastic moduli of the P6 and P6 phases of Si 3 N 4. β-Si 3 N 4 is set as a benchmark system since accurate experiments are available. The calculated lattice constants and elastic constants of β-Si 3 N 4 are in good agreement with the experimental data. The crystal anisotropy, mechanical stability, and brittle behavior of P6-and P6-Si 3 N 4 are also discussed in the pressure range of 30−55 GPa. The results show that these two polymorphs are metallic compounds. The brittleness and elastic anisotropy increase with applied pressure increasing. Besides, the phase boundaries of the β→P6 →δ transitions are also analysed. The β phase is predicted to undergo a phase transition to the P6 phase at 40.0 GPa and 300 K. Upon further compression, the P6 →δ transition can be observed at 53.2 GPa. The thermal and pressure effects on the heat capacity, cell volume and bulk modulus are also determined. Some interesting features are found at high temperatures.