Effect of Fe content on atomic and electronic structure of complex oxides Sr (Ti,Fe)O3-δ

We present a study of the electronic and atomic structure of two series of SrTi1 - xFe5O3 - (delta) (STFO) powders with different Fe content produced by two different methods, spray pyrolysis or modified Pechini synthesis, by means of soft X-ray absorption spectroscopy. Partial substitution of Ti by Fe atoms in SrTiO3 were found to cause asymmetric distortion of TiO6 octahedrons, which increases with increasing Fe content that may violate the cubic symmetry of STFO. The presence mainly of Fe3+ states in octahedral environment at small concentration of Fe atoms along with essentially smaller content of Fe4+ states in octahedral environment where the latter contribution increases with increasing Fe content was traced. It is found that the modified Pechini method allows to synthesize more stable structures but a tendency of the SrOx formation in the structure prepared by this technique was marked. The spray pyrolysis method gives the structure free of SrO precipitates but the presence of Fe3+ states in tetrahedral environment with Fe content higher than 50% and even a certain amount of Fe2+ ions in an octahedral environment at concentrations higher than 75% in the STFO prepared by this method was established. The O1s (K)-absorption spectra point to increase in oxygen vacancy concentration with increasing Fe content. The lowest degree of structure distortions with enough high oxygen vacancy concentration was traced in STFO (x = 0.25 to x = 0.35) produced by modified Pechini synthesis, which makes it mostly appropriate for technical applications, e.g., as gas sensors, oxygen separation membranes or as fuel cell material.