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, La 0.6 Sr 0.4 Ti x Fe 1− x O 3− δ (LSTF, x =0, 0.1, 0.2, and 0.3) were measured in both air and a H 2 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 H 2 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, Δ L / L , of 1.089% in the H 2 atmosphere at 400 °C, that of LSTF ( x =0.3) was reduced to 0.548%. This difference of Δ L / 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 H 2 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. [ABSTRACT FROM AUTHOR]