SYNTHESIS AND PHASE ANALYSIS OF FLUORINE-DOPED PEROVSKITES BASED ON BARIUM-CALCIUM NIOBATES

Abstract

The paper takes into account the fact that today, as a material for the electrolyte of solid oxide fuel cells, the compounds with a perovskite-like structure characterized by oxygen-ion conductivity in the dry air and proton conductivity in the air atmosphere are actively studied. The possibility of proton defects in these compounds is due to the presence of oxygen vacancies. The presence of vacant positions in the oxygen sublattice promotes reversible dissociative absorption of water from the gas phase and the appearance of proton conductivity. An example of a phase characterized by a perovskite structure and having an oxygen deficiency in the anion sublattice is barium-calcium niobate Ba 2 CaNbO 5,5 described in the literature as an oxygen-ion and proton conductor. Isovalent and heterovalent doping of cationic sublattices is the one of methods of modification of this type structure and optimization of physicochemical properties of complex oxide materials. However, the development of anionic doping can be an alternative for the obtaining of new materials with improved properties. It was established that the anionic substitution F − →O 2− leads to increase in oxygen and proton conductivities under small fluorine concentration (a mixed anion effect). The present research is an extension of the study of the effect of heterovalent anionic doping on oxygen-ion and proton transport. In this paper, the fluorine-doped complex oxides based on barium-calcium niobate Ba 2-0,5x CaNbO 5 , 5-x F x were obtained, X-ray analysis was performed. The phases were made by the anionic doping of oxygen sublattice of niobate barium-calcium Ba 2 CaNbO 5,5 by fluorine ions. The synthesis was carried out by a solidphase method, the maximum annealing temperature was 1300 о С. All the samples (0 < х ≤ 0,4) were single-phase and characterized by the cubic structure of a double perovskite. It has been established that the introduction of ions with a smaller radius (F - ) leads to a decrease of the lattice parameter of the fluorine-substituted compositions compared with the undoped barium-calcium niobate Ba 2 CaNbO 5,5 .