Structural Transformations and Valence States of Fe in Substituted Strontium Ferrite Sr2LaFe3O9-δ

The structural features and valence states of Fe in substituted strontium ferrite Sr2LaFe3O9-delta after a series of annealing events in vacuum at different temperatures from 400 degrees C to 650 degrees C are studied by X-ray diffraction (XRD) and Mossbauer spectroscopy. According to X-ray data, the two samples with extreme (in terms of oxygen content) compositions (Sr2LaFe3O9 and Sr2LaFe3O8) are single phase and have a rhombohedral and orthorhombic structure, respectively. When a vacancy appears, the structural state changes, multiphase states are formed, and an intermediate orthorhombic phase close to the structure of non-substituted Sr4Fe4O11 arises. The ratio of phase contents in the mixture changes as the oxygen content decreases. When an oxygen-vacancy concentration reaches one vacancy per three perovskite unit cells, the final orthorhombic phase Sr2LaFe3O8 is formed. According to the obtained Mossbauer data, Fe ions in the sample Sr2LaFe3O9 with a rhombohedral structure have two valence states: Fe4+ with an octahedral symmetric oxygen environment and an averaged-valence state Fe3.5+. The sample Sr2LaFe3O8 with an orthorhombic structure, according to Mossbauer data, is magnetic; the iron ion has the Fe3+ valence state with two oxygen environments, octahedral and tetrahedral, as in the brownmillerite phase of unsubstituted Sr2Fe2O5. Analysis of the complete data set of Fe valence states, their redistribution as the oxygen concentration decreases, and transitions from the paramagnetic state to the magnetic-ordering state allows us to correlate the information on the local environment of Fe cations with structural data.