The Interrelation between Iron Valence and Oxygen Vacancies in Substituted Orthoferrite La0.67Sr0.33FeO3– during Heat Treatment

Abstract

In this paper, we studied substituted lanthanum orthoferrite La0.67Sr0.33FeO3– γ by Mössbauer spectroscopy using X-ray diffraction data. A series of vacuum annealings was performed in the temperature range of tann = 200–650°C, after which no significant changes in the structure of the samples were detected. The Mössbauer measurements at room temperature show that the Fe ions are in an average valence statebetween Fe3+ and Fe4+. Upon vacuum annealing, as the temperature tann increased, the average hyperfine magnetic field on the 57Fe nuclei and the isomer shift of the spectrum increased, which is associated with an increase in the number of vacancies and, accordingly, a decrease in the amount of Fe4+. Mössbauer measure-ments at 85 K showed that the average valence state of iron does not manifest itself. The hyperfine parameters of the low-temperature Mössbauer subspectra obtained from the model interpretation indicate that one of them belongs to Fe4+ ions, and the rest belong to Fe3+. The presence in the spectra of several sextets related to Fe3+ ions is due to the appearance of oxygen vacancies (breaking of the Fe3+–O2––Fe exchange bond) and Fe4+ ions (weakening of the Fe3+–O2––Fe exchange bond) in the nearest ionic neighborhood of Fe atoms. Both factors cause a decrease in the hyperfine magnetic field and a change in the isomer shift of the spectrum. As a result of model interpretation of the Mössbauer spectra, the numbers of oxygen vacancies and Fe4+ ions per formula unit depending on vacuum annealing temperature tann were determined for all samples. It wasshown that at tann above 450°C, the process of oxygen leaving the lattice ends and only Fe3+ ions are detected.