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 La0.5Sr0.5FeO3-delta (LSF55). The oxygen nonstoichiometry (delta) was measured as a function of oxygen partial pressure in 10(-19) <= (P-O2/atm)<= 0.21 and 750 <= (T/degrees C) <= 900 range. The delta - P-O2 - T relationships indicated an electronic n-p transition point at delta = 0.25 which moved to a higher P-O2 value with increasing temperature. The relative partial molar enthalpy (Delta(H) over bar (O)) and entropy (Delta(S) over bar (O)) of oxygen indicated that across the electronic stoichiometric point Delta(H) over bar (O) stabilized around -86.9 +/- 3.6 kJ/mol in p-type and -402.2 +/- 12.6 kJ/mol in n-type regimes, whereas Delta(S) over bar (O) values were kept changing because of the contribution from configuration entropy (S-O(Con f)). From the DC 4-probe conductivity and electrical conductivity relaxation (ECR) measurement, the oxygen-ion conductivity at 900 degrees 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 delta = 0.114 during oxidation increased from (8.27 +/- 0.05) x 10(-8) cm(2) . s(-1) at 800 degrees C to (2.55 +/- 0.01) x 10(-7) cm(2) . s(-1) at 900 degrees C with an activation energy of 1.22 +/- 0.14 eV. Dilatometry measurements indicated an isothermal chemical expansion as function of P-O2, which was explained on the basis of the relative change in mean ionic radius of transition metal cation Fe against delta. 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 delta.