Defect chemistry and phase equilibria of (La_1-xCa _x)FeO_3-δ thermodynamic modeling

Thermodynamics of defects in the (La_xCa_1-x)FeO _3-δ perovskite is modeled by means of the CALPHAD approach. In this phase, the A-sites are occupied by La⁺³ and Ca⁺², and Fe in the B-site is known to exist in +2, +3, and +4 oxidation states depending on the oxygen vacancy concentration. Therefore, the ionic sublattice model: (La⁺³, Ca⁺²)(Fe⁺², Fe⁺³, Fe ⁺⁴)(O^-2, Va)₃ is used to describe the phase, and the model parameters are evaluated from experimental oxygen nonstoichiometry and phase equilibria data. With the Fe⁺² and Fe⁺⁴ treated as the major species in the B-site, the calculated phase diagrams are in good agreement with the experimentally reported phase equilibria data. The concentration of various defects in (La_xCa_1-x)FeO _3-δ as a function of oxygen partial pressure and temperature are calculated at different concentrations of Ca. At high oxygen partial pressures, Fe⁺⁴ is predicted to be dominant while Fe⁺² is dominant at low oxygen partial pressures.