Oxygen deficiency, vacancy clustering and ionic transport in (La,Sr)CoO3 − δ

The equilibrium p(O2)–T–δ diagrams of perovskite-type La1 − xSrxCoO3 − δ (x = 0.3–0.7), collected at 873–1223 K in the oxygen partial pressure range 10− 5–1 atm by coulometric titration and thermogravimetric analysis, were analyzed in order to appraise the effects of the point-defect interactions. The nonstoichiometry variations were adequately described combining the rigid-band approach for delocalized holes and the pair-cluster formation reaction involving oxygen vacancies and Co2+ cations, whilst coulombic repulsion between the positively charged vacancies can be neglected. The resultant relationships between the oxygen chemical potential and mobile vacancy concentration were used for numerical regression analysis of the steady-state oxygen permeation through dense La1 − xSrxCoO3 − δ membranes, affected by the surface exchange kinetics when Sr2+ content is higher than 40–50%. The calculated ionic conductivity is strongly influenced by the defect association processes, and decreases with decreasing concentration of the mobile vacancies as clustering starts to prevail on reduction. The Mössbauer spectroscopy studies of La1 − xSrxCoO3 − δ, doped with 1 mol% 57Fe isotope and moderately reduced at p(O2) ≈ 10−5 atm, show no long-range vacancy ordering at x ≤ 0.5.