A high-temperature 57Fe Mössbauer study of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ

Abstract An in-situ 57Fe Mossbauer study of mixed ionic electronic conducting (Ba0.5Sr0.5)(Co0.857Fe0.08Fe0.12)O3−δ (BSCF5582) has been made at temperatures up to 1000 °C. In the paramagnetic high-temperature phase (T ≥ 315 °C), quadrupolar interactions show that local symmetry at iron sites is lower than cubic. At 700 °C, 850 °C, and 1000 °C, spectra have been measured as a function of oxygen partial pressure, PO2 (1·10−5 ≲ PO2/bar ≤1). The high-temperature Mossbauer center shifts and their independence of oxygen partial pressure are discussed in respect to the average oxidation state and average local coordination of iron. The latter information has been derived from a thermodynamic analysis of the defect chemistry of BSCF5582 accounting for different types of oxygen vacancies as well as for multiple oxidation states of the transition metal cations. The isothermal decrease of high-temperature signal intensity observed with decreasing PO2 is attributed to changes in the vibrational properties of the highly oxygen-deficient material which have been analyzed in the framework of the Gruneisen model.