Study of high-pressure and high-temperature behaviors and α-to-β phase transition of forsterite by first-principles and quasi-harmonic Debye model

The high-pressure and/or high-temperature behavior and α/β phase transition of forsterite (Mg₂SiO₄) were investigated by first principle calculations: local density approximation (LDA), generalized gradient approximation (GGA) and the quasi-harmonic Debye model in consideration of the temperature effect. At zero pressure and zero temperature, the calculated values of lattice parameters and elastic constants are well concordant with experimental data. The calculated values of cell volume at different pressures and temperatures are compatible with experiments. The results calculated by LDA are more concordant with the experimental results than those by GGA for describing Mg₂SiO₄ crystals. The boundaries of the P-T plots for α/β phase transition are calculated to be 0.00779 GPa/K by LDA and 0.00772 GPa/K by GGA, suggesting the upper and lower limits of the phase boundary for α/β forsterite. At temperature of less than 1000 K, the calculated results agree reasonably with experimental ones. Thus, the present results indicate that combination of first-principles and quasi-harmonic Debye model is an efficient approach to simulate the behavior of minerals at high pressure and/or high temperature.