PHASE EQUILIBRIA, STRUCTURE, AND PROPERTIES OF COMPLEX OXIDES IN THE Eu1–xSrxCo1–yFeyO3–δ SYSTEM AT 1373 K IN AIR
Legonkova V.S., Aksenova T.V., Cherepanov V.A.
Ural Federal University
620002, Ekaterinburg, Mira st., 19
The aim of this work is a systematic study of phase equilibria, determination of the homogeneity ranges, and investigation of the structural features of complex oxides in the quasi-quaternary system EuCoO3 – SrCoO3–δ – SrFeO3–δ – EuFeO3.
A series of samples with the composition Eu1–xSrxCo1–yFeyO3–δ with 0.1 ≤ x ≤ 0.9 and 0.1 ≤ y ≤ 0.9 was synthesized by a glycerol-nitrate technique, followed by final annealing at 1373 K in air and subsequent quenching to room temperature. The phase composition and crystal structure were studied by X‑ray diffraction and the Rietveld full-profile analysis.
An isobaric-isothermal section of the phase diagram of the quasi-quaternary system EuCoO3 – SrCoO3–δ – SrFeO3–δ – EuFeO3 at 1373 K in air has been constructed. It was found that depending on the strontium (x) and iron (y) content the solid solutions Eu1‑xSrxCo1–yFeyO3–δ crystallize in one of three perovskite structures: cubic (sp. gr. Pm–3m), tetragonal (sp. gr. I4/mmm), or orthorhombic (sp. gr. Pbnm). The cubic phase is stable for compositions enriched in strontium and iron (x = 0.9 and 0.1 ≤ y ≤ 0.9; x = 0.8 and 0.2 ≤ y ≤ 0.9; x = 0.7 and 0.5 ≤ y ≤ 0.9). The tetragonal superstructure 2ap×2ap×4ap, characterized by an ordered arrangement of Eu and Sr cations in the A-sublattice, observed for x = 0.8 and y = 0.1; for x = 0.7 and 0.1 ≤ y ≤ 0.4; for x = 0.6 and 0.1 ≤ y ≤ 0.3, while the orthorhombic structure is characteristic of europium-rich oxides with x = 0.1 and 0.1 ≤ y ≤ 0.9. The unit cell parameters exhibit a linear dependence on composition upon substitution of europium by strontium in the A-site and of cobalt by iron in the B-site obeying Vegard’s rule.
Thermogravimetric analysis of the oxides Eu1–xSrxCo1–yFeyO3–δ (0.6 ≤ x ≤ 0.9 and y = 0.3; x = 0.8 and 0.3 ≤ y ≤ 0.8) showed that all of them are oxygen-deficient. The oxygen content, determined by iodometric titration and reduction in a hydrogen stream, decreases with increasing temperature and increasing strontium and cobalt contents in the samples. Charge compensation upon substitution of Eu3+ by Sr2+ occurs through the formation of oxygen vacancies and an increase in the average oxidation state of 3d-metals.
The nonlinear temperature dependence of relative thermal expansion of Eu1‑xSrxCo0.7Fe0.3O3–δ ceramic samples (x = 0.6, 0.7) in the range of 300–1373 K in air is caused by thermal and chemical factors. The latter is associated with the release of oxygen from the lattice at T > 600 K and a change in the oxidation state of 3d-metals. The increase in LTEC with increasing strontium content correlates with the increase in the oxygen nonstoichiometry.
This work was supported financially by the Ministry of Science and Higher Education of the Russian Federation (grant No. FEUZ-2026-0011).