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 elevat ed temperatures (790-940-degrees-C) in these phases has been confirmed by DSC measurements and, in some cases, by X-ray powder diffraction o f samples either slowly cooled or quenched from high temperature after annealing in different atmospheres. The oxygen permeability found upo n 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 trans ition from a low-temperature vacancy-ordered state to defect perovskit e, except for SrCo0.8Fe0.2O3-delta. In the latter case only a slight a nomaly is found in the Arrhenius plot of the oxygen permeability at ap proximately 790-degrees-C. The comparatively high oxygen flux through SrCo0.8Fe0.2O3-delta observed at intermediate temperatures is interpre ted in terms of a two-phase mixture of a vacancy-ordered state and dis ordered perovskite, while above approximately 790-degrees-C the sample is single-phase of defect perovskite structure.