V. D. Sedykh, V. S. Rusakov, O. G. Rybchenko, A. M. Gapochka, M. E. Matsnev, V. I. Kulakov, A. I. Ivanov, A. A. Toporkova, “The crystal structure features of Ca-substituted ferrites La$_{1-x}$Ca$_x$FeO$_{3-\gamma}$”, Fizika Tverdogo Tela, 2025, Volume 67, Issue 9,Pages 1765

Lattice dynamics, thermal properties

The crystal structure features of Ca-substituted ferrites La$_{1-x}$Ca$_x$FeO$_{3-\gamma}$

V. D. Sedykh^a, V. S. Rusakov^b, O. G. Rybchenko^a, A. M. Gapochka^b, M. E. Matsnev^b, V. I. Kulakov^a, A. I. Ivanov^a, A. A. Toporkova^a

^a Osipyan Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow region
^b Lomonosov Moscow State University

Abstract: The crystal structure features of the Ca-substituted lanthanum ferrite La$_{1-x}$Ca$_x$FeO$_{3-\gamma}$ have been studied as the Ca concentration dependence ($x$ = 0.0, 0.3, 0.5, 0.7, 1.0) using X-ray diffraction analysis and Mössbauer spectroscopy. It has been investigated both the synthesized samples and the samples annealed at 650$^\circ$C in vacuum, when Fe$^{4+}$ ions are completely transferred to Fe$^{3+}$. When the Ca concentration is $x <$ 0.5, the Pbnm orthorhombic phase is formed. At higher Ca concentrations ($x >$ 0.5), the vacancy-ordered Grenier (LaCa$_2$Fe$_3$O$_8$) and Brounmillerite (Ca$_2$Fe$_2$O$_5$) phases are formed. The portion of each valence Fe state, the number of oxygen vacancies, and oxygen content were determined using the low-temperature Mössbauer data for all the samples studied. The synthesized sample with $x$ = 0.5 has a maximum number of Fe$^{4+}$ ions. The Fe ions are in a trivalent state for the initial and last numbers of the concentration series, $x$ = 0 (LaFeO$_3$) and $x$ = 1 (Ca$_2$Fe$_2$O$_5$). In the synthesized samples, the relation of contributions of the different Fe$^{3+}$ ion structural states to the Mössbauer spectrum has been shown to change towards the oxygen environments characteristic of the vacancy-ordered phases when increasing Ca concentration. The vacuum annealing (when there are no Fe$^{4+}$ ions) results in the formation of lower defective local environment of Fe ions.

Keywords: orthoferrites, Fe valence states, oxygen vacancies, Mössbauer spectroscopy, X-ray diffraction.

Received: 25.05.2025
Revised: 28.08.2025
Accepted: 29.08.2025

DOI: 10.61011/FTT.2025.09.61631.139-25