The influence of Ca2+ ion concentration in La1 – xCaxFeO3 – γ (x = 0, 0.3, 0.5, 0.7, 1.0) orthoferrites on their crystal structure, oxygen vacancies, and Fe valence states has been studied by X-ray diffraction and Mössbauer spectroscopy at room temperature. The samples were synthesized by sol-gel method in air at 1200°C. Substitution of Ca2+ ions for La3+ ions leads to the appearance of mixed-valence states of Fe. After synthesis, the samples were annealed in vacuum at 650°C for 4 h, resulting in complete reduction of Fe4+ to Fe3+ ions. It should be noted that the samples with extreme x values (x = 0, 1) contain only Fe3+ ions. At x = 0 (LaFeO3), there are no oxygen vacancies in the sample, while at x = 1 (CaFeO2.5), their number is maximum. With an increase in the Ca2+ content, the number of oxygen vacancies and the Fe3+/Fe4+ valence ratio change. At x > 0.5, vacancy-ordered Grenier (LaCa2Fe3O8) and brownmillerite (Ca2Fe2O5) phases are formed. The obtained results demonstrate the effectiveness of Mössbauer spectroscopy for studying the structural features, local iron states, and the distribution of oxygen vacancies in Ca-substituted orthoferrites.
Sedykh et al. (Mon,) studied this question.