Low-temperature Mossbauer studies of substituted La$_{0.67}$Sr$_{0.33}$FeO$_{3-\gamma}$ lanthanum ferrite

The valence states of Fe atoms and the formation of oxygen vacancies in substituted La$_{0.67}$Sr$_{0.33}$FeO$_{3-\gamma}$ orthoferrite have been studied in detail by low-temperature Mossbauer spectroscopy under oxygen removal. It has been shown that the averaged valence state of Fe atoms is not revealed with a decreasing measurement temperature. This makes it possible to reveal Fe$^{4+}$ ions. The Analysis of the obtained data allows us to conclude that the presence of several Zeeman sextets associated with Fe$^{3+}$ ions is related to the appearance of oxygen vacancies and Fe$^{4+}$ ions in the nearest ionic surrounding of Fe ions. Using the Mossbauer data, the number of oxygen vacancies and oxygen ions has been determined for all the studied samples depending on the vacuum annealing temperature.