PECULIARITIES OF PHASE EQUILIBRIA IN OXIDE SYSTEMS BASED ON RARE EARTH, ALKALINE EARTH AND 3d (Fe, Co) METALS

Cherepanov, V.A.; Aksenova, T.V.; Bastron, I.A.; Davydova, M.V.; Gavrilova, L.Ya.; Legon’kova, V.A.; Vlasova, M.A.; Volkova, N.E.

PECULIARITIES OF PHASE EQUILIBRIA IN OXIDE SYSTEMS BASED ON RARE EARTH, ALKALINE EARTH AND 3d (Fe, Co) METALS

Cherepanov V.A., Aksenova T.V., Bastron I.A., Davydova M.V.,
Gavrilova L.Ya., Legon’kova V.A., Vlasova M.A., Volkova N.E.

Ural Federal University

620002, Ekaterinburg, Mira st., 19

The results of a systematic study of phase equilibria in the oxide systems ½ Ln₂O₃ – MO – TO_x (Ln – rare earth elements; M – Ca, Sr, Ba; T = Fe, Co) at 1100°C in air are presented in the form of phase diagrams. It is shown that for M = Ca, Sr, the general appearance of the phase diagrams of systems containing iron and cobalt is similar, while for M = Ba, they differ significantly. The similarity of the diagrams of the first group implies the formation of solid solutions with the perovskite structure (Ln_1-xM_x)TO_3-δ and related representatives of the Ruddlesden-Popper type (Ln_1-xM_x)_n+1T_nO_3n+1-δ homologous series for practically all systems. The similarity of the diagrams, however, does not mean they are completely identical. As a rule, the homogeneity ranges of solid solutions in Sr-containing systems are much wider than in similar Ca-containing systems, and the set of Ruddlesden-Popper phases is larger in Fe-containing systems than in similar Co-containing systems.

A characteristic feature of Ba-containing systems, which distinguishes them from Ca- and Sr-containing analogues, is the formation of layered structures due to the increasing difference in the sizes of rare earth element and barium ions.

However, in Co-containing systems, the formation of a double perovskite phase LnBaCo₂O_6-δ is observed in air, whereas in similar Fe-containing analogous the formation of a triple-layered LnBa₂Fe₃O_8+w (Ln = Dy, Ho, Y) or even a more complex quintuple perovskite structure Ln_2-εBa_3+εFe₅O_13+w (Ln =Sm, Eu) is revealed. Fe-based double perovskites LnBaFe₂O_5+w can also be obtained, but under much more severe reducing conditions (Po₂ ≈ 10^-15 atm). It should be noted that the above-mentioned single-phase triple-layered 123-type ferrites exhibit significant nonstoichiometry in the metallic sublattices in air conditions, for example, Y_1.05Ba_1.92Fe_3.03O_8+w or Dy_1.05Ba_1.95Fe₃O_8+w.

Another specific feature of phase relations in Ba-containing systems is the possibility of partial replacement of iron by a rare earth element in barium ferrite with the formation of cubic perovskite solid solutions BaFe_1-yLn_yO_3-δ even for relatively large Pr. A similar substitution was also found in barium cobaltite BaCo_1-yLn_yO_3-δ, but for smaller rare earth elements (Ln = Sm, Y).

This work was carried out with the financial support of the Ministry of Science and Higher Education of the Russian Federation (project № FEUZ-2026-0011)