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

• Published in: Physics of metals and metallography Vol. 124; no. 2; pp. 153 - 160
• Main Authors: Sedykh, V. D., Rusakov, V. S., Gubaidulina, T. V., Rybchenko, O. G., Kulakov, V. I.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01.02.2023; Springer Nature B.V
• Subjects: Chemistry and Materials Science; Electrical and Magnetic Properties; Ferric ions; Heat treatment; Iron; Isomers; Lanthanum; Low temperature; Magnetic fields; Materials Science; Metallic Materials; Mossbauer spectroscopy; Oxygen; Room temperature; Spectra; Spectrum analysis; Substitutes; Vacuum annealing; Valence; valence states; substituted lanthanum ferrites; oxygen vacancies
• ISSN: 0031-918X; 1555-6190
• DOI: 10.1134/S0031918X22601950

In this paper, we studied substituted lanthanum orthoferrite La 0.67 Sr 0.33 FeO 3 – γ by Mössbauer spectroscopy using X-ray diffraction data. A series of vacuum annealings was performed in the temperature range of t ann = 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 state between Fe 3+ and Fe 4+ . Upon vacuum annealing, as the temperature t ann increased, the average hyperfine magnetic field on the 57 Fe 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 Fe 4+ . Mössbauer measurements 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 Fe 4+ ions, and the rest belong to Fe 3+ . The presence in the spectra of several sextets related to Fe 3+ ions is due to the appearance of oxygen vacancies (breaking of the Fe 3+ –O 2– –Fe exchange bond) and Fe 4+ ions (weakening of the Fe 3+ –O 2– –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 Fe 4+ ions per formula unit depending on vacuum annealing temperature t ann were determined for all samples. It was shown that at t ann above 450°C, the process of oxygen leaving the lattice ends and only Fe 3+ ions are detected.