Thermodynamic quantities and defect equilibrium in the perovskite-type oxide solid solution La 1− xSr xFeO 3−δ

Abstract

In order to clarify the relationship between the thermodynamic quantities and the defect equilibrium of the solid solution La 1− x Sr x FeO 3−δ, analysis and statistical thermodynamic calculations were made using the authors' previously reported nonstoichiometry data. From the δ-log P O 2  T relationships, using the Gibbs-Helmholtz equation, the partial molar enthalpy ( h 0 − h° 0) and the partial molar entropy ( s 0 − s° 0) of oxygen in La 1− x Sr x FeO 3−δ were determined as functions of x and δ. Here, the standard states of h 0 and s 0, h° 0, and s° 0 are in equilibrium with 1 atm oxygen gas. For δ < x 2 and δ > x 2 , ( h 0 − h° 0) is essentially constant and ( s 0 − s° 0) increases with δ. Around δ = x 2 , drastic decreases in ( h 0 − h° 0) and ( s 0 − s° 0) are observed. Statistical thermodynamic calculations were made for the oxygen chemical potential, μ 0, the partial molar enthalpy, h 0, and the partial molar entropy, s 0, assuming random distribution of the defects, V ·· O, Fe′ Fe, and Fe · Fe, on each lattice site. Numerical calculations for ( h 0 − h° 0) and ( s 0 − s° 0) were made using the defect concentrations, the equilibrium constant, K i , for the reaction of 2Fe x Fe = Fe′ Fe + Fe · Fe, and K ox for the reaction of 1 2 O 2(g) + V ·· O + 2Fe x Fe = 2 Fe · Fe + O x O . The calculated h 0 − h° 0) and s 0 − s° 0) vs δ relationships based on the statistical thermodynamic model agreed quite well with those determined from the experimental data using the Gibbs-Helmholtz equation.