Disordered off-center direction of Ti⁴⁺ in pseudo-cubic type BaTiO₃ prepared by mixed hydroxide process

Recently, perovskite-type BaTiO₃ showing sharp X-ray diffraction peak comparable to that synthesized by a conventional solid-state reaction was obtained by mixing Ba(OH)₂ and TiO₂ hydrous gel and by leaving the mixture at 323 K or less. However, the obtained BaTiO₃ was cubic crystal system, which was different from that synthesized by a conventional solid-state reaction. In this study, the structural analysis of cubic-BaTiO₃ obtained by this technique was conducted, and the factors to form cubic structure were investigated. Since no second harmonic generation signal was observed, the structure of BaTiO₃ was essentially different from that of traditional tetragonal-BaTiO₃. Rietveld analysis with the initial structure model of ideal cubic perovskite structure exhibited that Ti⁴⁺ had exceptionally large isotropic atomic displacement parameter. A reasonable isotropic atomic displacement parameter was obtained by considering the static site disorder in the initial structural model, that is, Ti⁴⁺ locates at off-center position in TiO₆ octahedron by displacing in a disordered direction, resulting in pseudo-cubic symmetry.