INFLUENCE OF ORDER-DISORDER TRANSITIONS ON OXYGEN PERMEABILITY THROUGH SELECTED NONSTOICHIOMETRIC PEROVSKITE-TYPE OXIDES

New results on the oxygen permeability of perovskite-type oxides SrCo0.8B'0.2O3-delta (with B' = Cr, Fe, Co and Cu) and La0.6Sr0.4CoO3-delta are presented. The occurrence of order-disorder transitions at elevated temperatures (790-940-degrees-C) in these phases has been confirmed by DSC measurements and, in some cases, by X-ray powder diffraction of samples either slowly cooled or quenched from high temperature after annealing in different atmospheres. The oxygen permeability found upon exposing opposite sides of sealed disc specimens to a stream of air and of helium, respectively, increases sharply (between 5-6 orders of magnitude up to 0.3-3 x 10(-7) mol cm-2 s-1) at the onset of the transition from a low-temperature vacancy-ordered state to defect perovskite, except for SrCo0.8Fe0.2O3-delta. In the latter case only a slight anomaly is found in the Arrhenius plot of the oxygen permeability at approximately 790-degrees-C. The comparatively high oxygen flux through SrCo0.8Fe0.2O3-delta observed at intermediate temperatures is interpreted in terms of a two-phase mixture of a vacancy-ordered state and disordered perovskite, while above approximately 790-degrees-C the sample is single-phase of defect perovskite structure.