Theoretical Study on the High-Temperature P(6)over-bar and P(6)over-bar′ Phases of Si3N4: A Tool to Aid in Ceramics Development

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 P (6) over bar and P (6) over bar' phases of Si3N4. beta-Si3N4 is set as a benchmark system since accurate experiments are available. The calculated lattice constants and elastic constants of beta-Si3N4 are in good agreement with the experimental data. The crystal anisotropy, mechanical stability, and brittle behavior of P (6) over bar-. and P (6) over bar'-Si3N4 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 beta -> P (6) over bar'->delta transitions are also analysed. The beta phase is predicted to undergo a phase transition to the P (6) over bar' phase at 40.0 GPa and 300 K. Upon further compression, the P (6) over bar'->delta 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.