EFFECT OF INDIUM SUBSTITUTION ON THE STRUCTURE OF SrFeO_3-δ AND ITS TRANSPORT PROPERTIES

Egorova A.V.^(1,2), Legonkova V.S.^(1,2), Zhukov V.P.⁽²⁾, Merkulov O.V.⁽²⁾

⁽¹⁾ Ural Federal University

620002, Ekaterinburg, Mira st., 19

⁽²⁾ Institute of Solid State Chemistry UB RAS

620990, Ekaterinburg, Pervomaiskaya St., 91

Materials based on SrFeO_3-δ are promising candidates for cathodes in solid oxide fuel cells and oxygen-permeable membranes. However, undoped SrFeO_3-δ exhibits a high thermal expansion coefficient and is prone to phase transformations in a reducing atmosphere, which limits its practical application. The introduction of dopants into the B‑sublattice enables control over structural stability, oxygen nonstoichiometry, and transport properties. This approach is realized in the present work by substituting Fe^3+/4+ with In³⁺ and studying its effect on these characteristics.

Samples with the formula SrFe_1-xIn_xO_3-δ (x = 0.00–0.35) were synthesized using a sol-gel method with citric acid, followed by annealing at 1100 °C and sintering at 1190 °C. The phase composition of the samples was studied by X-ray diffraction with Rietveld refinement. The homogeneity range is within 0.0 ≤ x ≤ 0.30. The samples with an indium content of x = 0.05–0.30 crystallize in a cubic perovskite cell (space group Pm-3m), whereas the base SrFeO_3-δ composition possesses an orthorhombic structure (space group Cmmm). Cubic symmetry can exist for the undoped composition, however, this modification is unstable under standard conditions. Thus, the introduction of indium stabilized the cubic structure of SrFeO_3-δ system. The unit cell parameter increases monotonically with increasing indium content in the samples, which is attributed both to the size effect from substituting Fe^3+/4+ ions with larger In³⁺ ions and to the accompanying decrease in the average oxidation state of iron.

Oxygen nonstoichiometry was determined using iodometric titration. Doping with In³⁺ leads to an increase in the concentration of oxygen vacancies at room temperature. Thermogravimetric measurements revealed that the release of oxygen from the crystal lattice begins at 300 °C, with samples containing high indium content exhibiting a lower oxygen loss upon heating. Thermal expansion was measured using high-temperature dilatometry. The temperature dependences of relative elongation are nonlinear.

Electrical conductivity measurements were carried out using the four-probe DC method in a controlled atmosphere (pO₂ = 0.5–10^-20 atm) in the temperature range 750–950 °C. The samples are characterized by mixed ionic-electronic conductivity throughout the entire temperature range studied. The maximum ionic conductivity value is observed for the composition x = 0.05. Upon further increase in indium content, the ionic conductivity decreases, which may be attributed to ordering of the crystal structure and partial blocking of the migration pathways for O^2- ions.

This work was carried out in accordance with the state assignment for the Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, topic No. 124061300025-8.