Formulation of the thermal volume consistent with Swensonʹs concept of thermal pressure

In order to represent the pressure dependent thermodynamic functions for phase diagram calculations, the thermal volume model extends the Swenson concept of thermal pressure which is independent of volume and varies linearly with temperature. The expressions for the thermodynamic potentials: enthalpy, Gibbs energy and their derivatives such as molar volume, thermal expansion and heat capacity are then deduced for non-magnetic solids. The phases considered are simple oxides and silicates of the Earthʹs lower mantle under high pressure and high temperature. As a practical application, the thermodynamic functions of periclase are calculated and compared with available experimental data. A good agreement is achieved.