Investigations on Defect Equilibrium, Thermodynamic Quantities, and Transport Properties of La0.5Sr0.5FeO3-δ

In this work, relationship between defect equilibrium and thermodynamic quantities is investigated to elucidate mass and charge transport properties of La 0.5 Sr 0.5 FeO 3-δ (LSF55). The oxygen nonstoichiometry (δ) was measured as a function of oxygen partial pressure in 10 −19 ≤ ≤ 0.21 and 750 ≤ (T/°C) ≤ 900 range. The relationships indicated an electronic n-p transition point at δ = 0.25 which moved to a higher value with increasing temperature. The relative partial molar enthalpy ( ) and entropy ( ) of oxygen indicated that across the electronic stoichiometric point stabilized around −86.9 ± 3.6 kJ/mol in p-type and −402.2 ± 12.6 kJ/mol in n-type regimes, whereas values were kept changing because of the contribution from configuration entropy ( S O ( Conf ) ). From the DC 4-probe conductivity and electrical conductivity relaxation (ECR) measurement, the oxygen-ion conductivity at 900°C was 0.29 S.cm −1 with activation energy of 0.72 ± 0.04 eV and the oxygen self-diffusivity ( D O ) for LSF55 with δ = 0.114 during oxidation increased from (8.27 ± 0.05) × 10 −8 cm 2 ⋅s −1 at 800°C to (2.55 ± 0.01) × 10 −7 cm 2 ⋅s −1 at 900°C with an activation energy of 1.22 ± 0.14 eV. Dilatometry measurements indicated an isothermal chemical expansion as function of , which was explained on the basis of the relative change in mean ionic radius of transition metal cation Fe against δ. This model showed that transition metal cation existed as a mixture of high-spin and low-spin states and made a transition from low-spin to high-spin state with increasing δ.