Fe ion valence states and oxygen vacancies in the La0.5Sr0.5FeO3-γ ferrite under vacuum annealing

Abstract

The changes in the substituted La0.5Sr0.5FeO3−γ orthoferrite under vacuum annealing in the temperature range of 200–650 °C have been studied by X-ray diffraction analysis, as well as Mössbauer and Raman spectroscopy. Annealing of the as–prepared ferrite with the rhombohedral structure (R3‾c) resulted in its transition to the cubic one (Pm3‾m) at 650 °C. In the as–prepared ferrite being paramagnetic at room temperature, Fe ions were detected in an averaged–valence state between Fe3+ and Fe4+, which was not revealed with a decrease in the temperature down to 85 K. Gradual oxygen loss and an increase in the number of oxygen vacancies took place with an increase in the vacuum annealing temperature. Only Fe3+ ions were present in the ferrite at a vacuum annealing temperature above 500 °C. Several Zeeman sextets in the Mössbauer spectra associated with Fe3+ ions were resulted from the presence of oxygen vacancies and Fe4+ ions in the local environment of Fe3+ ions. The variations in the ratio of the valence states of Fe ions obtained from Mössbauer data depending on a vacuum annealing temperature allowed determining the content of oxygen in all the investigated samples. The contribution of Fe3+ ions that did not have Fe4+ ions and oxygen vacancies in their local environment was shown to increase with a vacuum annealing temperature from 12% (for the as–prepared sample) to 60% (for the sample annealed at 650 °C). On a whole, the process taking place under vacuum annealing can be characterized as a variation of the local environment of Fe3+ ions towards a decrease in its distortion. It was found that the width of the peaks of the Raman spectra decreased and their amplitude increased with an increase in the vacuum annealing temperature, which also demonstrated the improvement of the structural perfection of the samples.