First-principles investigation on thermodynamic phase stability of jadeite under high temperature and high pressure

In this paper, the functions of enthalpy, entropy, free energy and heat capacity with temperature from 0 to 1800K and pressure from normal (101.325 KPa) to 80Gpa are obtained by the calculation of CASTEP module based on density functional theory (DFT) and compared with the corresponding values from manual thermodynamics data. Besides, the thermodynamic parameters such as enthalpy changes, entropy of various phases at different pressures and temperatures for the reaction of 2NaAlSi2O6 → NaA1Si3O8 + NaAlSiO4 are calculated and the Gibbs free energy of the reaction is gotten. Furthermore, the Gibbs free energy as a function of both temperature and pressure is presented with the three-dimensional graph by surface fitting. The calculation results show that the calculated values of the thermodynamic parameters are consistent with the data in the manual and the error values of that are less than or equal to 1%. In addition, there is a transition between endothermic and exothermic at 450 K under normal pressure. Compared with high temperature and low pressure or high temperature and high pressure, ΔrG is closer to zero at low temperature and high pressure (60 Gpa, near 400 k), at the same time, the phase stability is better.