3.6 Thermodynamic stability
Thermodynamic description of various potentials defines the possible stability of these materials against high temperature and pressure. The use of quasi-harmonic Debye model (QHM) [54] formulates the various thermodynamic properties like Specific heat at constant volume (Cv), Grüneisen parameter (γ) and thermal expansion coefficient (α) in the temperature/pressure range of (0-800) K/ (0-25) GPa. However, remaining under (QHM) model we have potrayed the Specific performance of a material at constant volume (CV) which is one of the prime factor of the material relating dynamics of the material. Here, the graphical variation in Fig. 8 (a, b) , shows that the materials possess greater capability of heat transport upon T3 relation upto room temperature. The high temperature limit suggests that change is accordance with the Delong-Petit law with a constant value followed at high temperatures [55].
Fig. 8 (a, b) : Variation of specific heat (Cv) of RE2SnFeO6 (RE=Ca,Ba) with pressure and temperature.
Next, from the knowledge of the Grüneisen parameter (γ) labels the anharmonicity and detailed description about the phonon frequency modes. Fig. 8 (c, d) shows a softly increasing exponential trend at the lower temperatures but remains almost constant at higher temperatures. However the impact of pressure on (γ) has negligible effect on it. The recorded value of Grüneisen parameter at temperature 300K and pressure 0 GPa are 2.20 and 2.25 respectively.
Fig. 8 (c, d) :Variation of Grüneisen parameter (γ) of RE2SnFeO6 (RE=Ca,Ba) with pressure and temperature.
Thermal expansion (α) so for as theoretical as well as experimental point of view predicts the thermodynamic equation of state. The graphical representation w.r.t temperature and pressure variation for layered alloys is shown in Fig. (e, f) . The fast increasing trend of (α) at lower temperatures, while high temperature variation tends towards constant value.
Fig. 8 (e, f) : Variation of thermal expansion (α) of RE2SnFeO6 (Re=Ca,Ba) with pressure and temperature.