XRD and in-situ XAFS investigation on high-temperature thermal expansion of La0.6Sr0.4TixFe1−xO3−δ (0≤x≤0.3)

High temperature stability of perovskite oxide is important for the application of cathode materials in solid oxide fuel cells (SOFCs) and oxygen-permeable conductors. The thermal expansion behavior of perovskite oxides with the formula, La0.6Sr0.4TixFe1−xO3−δ (LSTF, x=0, 0.1, 0.2, and 0.3) were measured in both air and a H2 atmospheres and were investigated using thermogravimetry (TG), powder X-ray diffractometry, and X-ray absorption spectroscopy. The additional expansion caused by reduction of transition metals in the H2 atmosphere was observed over 200 °C and the rate of increase in expansion became slow above 400 °C for all compositions. While LSF (x=0) showed large thermal expansion ratio, ΔL/L, of 1.089% in the H2 atmosphere at 400 °C, that of LSTF (x=0.3) was reduced to 0.548%. This difference of ΔL/L corresponded to the decrease of the oxygen anion non-stoichiometry, δ, estimated by TG analysis, from 0.20 to 0.07 with x=0, and 0.3, respectively. The Fe cation valence of LSTF was also studied by in-situ X-ray absorption near edge structure in both air and a H2 atmosphere over the range of 25 to 800 °C, and compared to the Fe cation valances estimated from the values of δ. The valence states Fe and Ti in LSTF and the differences in the reducing kinetics for Ti substitution are discussed.