Chemical diffusion and oxygen surface transfer of La_1-xSr_xCoO_3-δ studied with electrical conductivity relaxation

The chemical diffusion coefficient and oxygen-transfer coefficients of selected compositions in the series La_1-xSr_xCoO_3-δ were studied using the conductivity relaxation technique. Measurements were performed in the temperature range 600-850°C and oxygen partial pressure 10^-4 to 1 bar. Chemical diffusivity and oxygen surface transfer in the La_1-xSr_xCoO_3-δ perovskites appear to be highly correlated. The general trend displayed is that both parameters decrease with decreasing pO2 below about 10^-2 bar at all temperatures. This is attributed to ordering of induced vacancies at low oxygen partial pressures. The observation that the correlation between both parameters extends even to the lowest pO₂ values in this study suggests a key role of the concentration of mobile oxygen vacancies, rather than of the extent of oxygen nonstoichiometry, in determining the rates of both processes. The characteristic thickness Lc, which equals the ratio of the chemical diffusion coefficient to the surface transfer coefficient, shows only a weak dependence on oxygen partial pressure and temperature. For different compositions La_1-xSr_xCoO_3-δ, Lc is found to vary between 50 and 150 µm.