Characterization of La_0.8Sr_0.2FeO_{3- δ} and La_0.7Sr_0.2FeO_{3- δ} as a function of temperature by x-ray absorption spectroscopy

A temperature-dependent x-ray absorption spectroscopy (XAS) study was performed to investigate the changes in electronic and atomic structure of La-deficient La_0.7Sr_0.2FeO_3−δ (L7S2FO3) and stoichiometric La_0.8Sr_0.2FeO_3−δ (L8S2FO3). La_0.8Sr_0.2FeO_3−δ is a promising cathode material for intermediate operating temperature (700–800°C) solid oxide fuel cells. Performance improvements have been shown by increasing the La- or A-site deficiency in this material but a clear understanding of the mechanisms responsible for this improvement are still needed. Here we report an x-ray absorption spectroscopy (XAS) study as a function of temperature to investigate electronic and atomic structure changes of La-deficient La_0.7Sr_0.2FeO_3−δ (L7S2FO3) and stoichiometric La_0.8Sr_0.2FeO_3−δ (L8S2FO3). In particular we have measured the temperature-dependent changes in oxidation state, bond distance, Fe coordination number, and oxygen vacancies for both compounds. L7S2FO3 contains 10% A-site vacancies compared to stoichiometric L8S2FO3, which has a fully occupied A site and thus some form of charge compensation is necessary in the former to maintain charge neutrality. X-ray absorption near edge spectroscopy shows the presence of Fe³⁺ and Fe⁴⁺ in both L7S2FO3 and L8S2FO3 (mixed valence) as established by comparison with model compounds. Studies from room temperature to 850°C show that Fe³⁺ dominates over Fe⁴⁺ in both materials with increasing dominance as the temperature is increased. Furthermore, the temperature-dependent study revealed the La-deficiency in L7S2FO3 leads to a higher concentration of both electron holes (i.e., more Fe⁴⁺ created) and oxygen vacancies, compared to the stoichiometric L8S2FO3. Analysis of the extended x-ray absorption fine structure shows that the Fe-O bond increases with the increase in temperature for both the systems.