Synthesis of new perovskite-type oxyfluorides, BaInO2F and comparison of the structure among perovskite-type oxyfluorides

摘要

Most perovskite-type oxyfluorides have a cubic structure even though the tolerance factors of these compounds are not unity. In this study, new perovskite-type oxyfluorides, BaInO2F, with a tolerance factor less than unity was synthesized and its crystal structure was compared with other perovskite-type oxyfluorides. In addition, its local structure was investigated using the atomic pair distribution function (PDF) technique. BaInO2F was synthesized by a low-temperature fluorination method, and its structure was cubic with an abnormally large displacement factor of anions. By comparing the structures of various perovskite-type oxyfluorides, we found that the ion species showing abnormally large displacement factor varied with the tolerance factors. In the case of BaFeO2F, the abnormally large displacement factor of the B-site ion, Fe3+, originated from the displacement of the Fe3+ ion from the center of a BX6 octahedron along <110> direction. According to the PDF analysis, the local structure of BaInO2F was not cubic but a triclinic system with tilted and distorted BX6 octahedra. On the basis of these results, we considred that the structure of oxyfluorides with a tolerance factor less than unity was relaxed by the tilt of the octahedra, similar to perovskite-type oxides. The long-range order of the anion displacements, however, was suppressed by the structural disorder arising from the random distribution of oxide and fluoride ions, therefore the average structure observed by diffraction methods was predicted to be a cubic structure.