INFLUENCE OF HALOGEN NATURE ON PROTON TRANSPORT IN PEROVSKITE-RELATED COMPOUNDS Ba₂CaNbO_5,5 AND Ba₂In₂O₅

The complex oxides with a perovskite-like structure stable in a wide range of partial pressures of oxygen and temperatures have successfully proven themselves as a promising material suitable for use as an electrolytic material of solid oxide fuel cells. Optimization of their transport properties is achieved by various kinds of substitutions. Today the cationic doping is the most studied method. A new promising method is anionic doping, which allows discovering fundamentally new ways to modify the structure and properties of compounds. In order to understand the consequences of the anion dopant influence on transport properties and to reveal general patterns of proton transfer, the paper studies the fluorine and chlorine-substituted solid solutions based on perovskite Ba₄Ca₂Nb₂O₁₁ and brownmillerite Ba₂In₂O₅. The paper presents the results of investigation of thermal and electrical properties, and discusses the influence of the halogen dopant nature on the degree of hydration and partial (О ^2- , Н ⁺ ) conductivity, as well as the mobility of protons. The Fand Clsubstituted phases based on oxygen-deficient compounds were synthesized by solid state method: the double oxide Ba₄Ca₂Nb₂O₁₁ and the brownmillerite Ba₂In₂O₅. The halogen doped phases of Ba₄Ca₂Nb₂O_10.95Х_0.1 and Ba₂In₂O_4.95 Х₀.₁ (X = F, Cl) were found to be capable of reversible water uptake and the formation of proton defects ОН_О^* . The analysis of the oxygen ion conductivity has been performed. The introduction of the donor dopant, both F_О^* and Cl_О^* , was found to lead to an increase in the oxygen conductivity in both the double perovskite and brownmillerite due to the appearance of the effects of additional electrostatic repulsion of the donor dopant with oxygen vacancies V_o^**. The study of proton transfer showed a sympathetic change in the proton and oxygen ionic conductivities in doped Ba₄Ca₂Nb₂O₁₁ which allows one to speak of the influence of the dynamics of the oxygen sublattice on proton transport: F- and Cl-doped phases with greater oxygen mobility are characterized by large proton conductivities in comparison with the matrix composition. For doped brownmillerite Ba₂In₂O₅, the introduction of F ions led to an increase in the proton mobility, and the presence of Cl-dopant led to a decrease due to a change in the ionicity of the metal halogen bonding and the associated increase in the degree of covalence of the neighboring metal oxygen bondings. The method of anionic doping described in the paper demonstrates a new strategy for increasing oxygen-ion and proton conductivities in perovskites and perovskite-like compounds.

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