Oxygen nonstoichiometry and thermo-chemical stability of La0.6Sr0.4CoO3−δ

The oxygen nonstoichiometry of La0.6Sr0.4CoO3−δ has been the topic of various reports in the literature, but has been exclusively measured at high oxygen partial pressures, pO2, and/or elevated temperatures. For applications of La0.6Sr0.4CoO3−δ, such as solid oxide fuel cell cathodes or oxygen permeation membranes, knowledge of the oxygen nonstoichiometry and thermo-chemical stability over a wide range of pO2 is crucial, as localized low pO2 could trigger failure of the material and device. By employing coulometric titration combined with thermogravimetry, the oxygen nonstoichiometry of La0.6Sr0.4CoO3−δ was measured at high and intermediate pO2 until the material decomposed (at log(pO2/bar)≈−4.5 at 1073 K). For a gradually reduced sample, an offset in oxygen content suggests that La0.6Sr0.4CoO3−δ forms a "super-reduced" solid solution before decomposing. When the sample underwent alternate reduction–oxidation, a hysteresis-like pO2 dependence of the oxygen content in the decomposition pO2 range was attributed to the reversible formation of ABO3 and A2BO4 phases. Reduction enthalpy and entropy were determined for the single-phase region and confirmed interpolated values from the literature. - Graphical abstract: Oxygen nonstoichiometry (shown as 3−δ) of La0.6Sr0.4CoO3−δ as a function of pO2 at 773–1173 K. The experimental data were obtained by thermogravimetric analysis (TG) and coulometric titration (measured either by a simple reduction (CT1) or a "two-step-forward one-step-back" reduction–oxidation (CT2) procedure). D1 and D2 denote the decomposition pO2. The solid lines are the fit to the thermogravimetry and CT1 data. The dashed lines represent the non-equilibrium region where the sample shows a super-reduced state. Highlights: ► Oxygen nonstoichiometry of La0.6Sr0.4CoO3−δ at intermediate temperatures and p(O2). ► Experimental confirmation of previously interpolated reduction enthalpy. ► Decomposition p(O2) assessed by coulometric titration. ► Hysteresis-like p(O2) dependence of oxygen content at decomposition p(O2).