Density-functional study of the thermodynamic properties and the pressure-temperature phase diagram of Mg₂SiO

First-principles ab initio calculations were used to explore the structural and vibrational behavior of Mg 2 SiO 4 under compression. We show that at 10GPa and 12.2GPa, Mg 2 SiO 4 experiences a pressure-induced phase shift from Pnma to Imma and Imma to F3dm structure, respectively. From DFPT computation, we found that the forsterite structure (space group Pnma) is thermodynamically unstable due to frequencies less than zero. As a result, we used the free energy-based renormalisation approach, which allows DFT to calculate vibrational free energy even for materials having negative curvature of the potential energy surface in relation to atomic displacement. Furthermore, Kieffer's model depicted the vibrational density of states and thermodynamic features of pure substances in pressure-temperature space.