Anharmonic thermodynamic properties of cubic CaSiO₃ perovskite from phonon quasiparticles

Cubic CaSiO₃ perovskite (cCaPv) is the third most abundant phase in the Earth’s lower mantle (7 vol%). The widely used quasiharmonic approximation combined with harmonic phonon spectra cannot be used to calculate the free energy of cCaPv. The latter is strongly anharmonic and presents imaginary frequencies in harmonic phonon calculation. Here we present an ab initio study of the thermodynamic properties of cCaPv over the pressure and temperature range of the lower mantle. We compute the anharmonic phonon dispersion throughout the Brillouin zone by using the phonon quasiparticle approach. This method characterizes the intrinsic temperature-dependence of phonon frequencies and, in principle, captures full anharmonicity. Such temperature-dependent phonon dispersion is used to calculate ab initio free energy in the thermodynamic limit (N → ∞) within the framework of the phonon gas model. Accurate free energy calculations enable us to investigate the thermodynamic properties of cCaPv, e.g., thermal expansivity, Grüneisen parameter, bulk modulus, and heat capacity, where anharmonic effects are demonstrated. The present methodology is important for exploring thermodynamic and thermoelastic properties and phase boundaries in strongly anharmonic systems at high pressures and temperatures.