Synthesis of Ba_1−xLn_xFeO_3−δ and BaFe_1−xLn_xO_3−δ (Ln: lanthanoid or Y) with cubic perovskite structures and disordered oxide ion vacancies: Effect of ionic radius on substitution site and crystal structure

Abstract

Various types of samples with nominal compositions of Ba_1−xLn_xFeO_3−δ and BaFe_1−xLn_xO_3−δ (Ln: lanthanoid or Y) were prepared and their phases were analyzed. The types of Ln and their substitutional amounts were clarified to obtain the cubic perovskite phase with a random arrangement of oxide ion vacancies, which have attracted interest as new hole- and oxide-ion-mixed conductors. Ln was classified into three groups based on its ionic radius: Large La was substituted for only the Ba site, whereas small Ln such as Y, Ho, Er, Tm, and Yb were substituted for only the Fe site. Ln with intermediate ionic radii, such as Nd, Sm, Eu, Gd, and Dy, could be substituted for both Ba and Fe sites. Results indicated that the differences in ionic radius between 12-coordinated Ln and Ba as well as between 6-coordinated Ln and Fe were responsible for the determination of substitutional sites. The ratio of the ionic radius of Ln³⁺ to that of O²⁻ was also proposed to be a factor in the determination of substitutional sites, which is in agreement with the rigid sphere model. A cubic perovskite phase with a random arrangement of oxide ion vacancies was obtained for Ba_1−xLn_xFeO_3−δ and BaFe_1−xLn_xO_3−δ with a tolerance factor between 0.996 and 1.035 and between 1.008 and 1.035, respectively.