Chemical Diffusion and Oxygen Surface Transfer of La[sub 1−x]Sr[sub x]CoO[sub 3−δ] Studied with Electrical Conductivity Relaxation

Abstract

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 <i>p</i>O₂ 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 L<i>c</i>, 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, L<i>c</i> is found to vary between 50 and 150 µm.