Crystal structure determination and ionic conductivity of layered perovskite compounds NaLnTiO(4) (Ln=rare earth)

The layered perovskite compounds NaLnTiO(4) (Ln = La, Pr, Nd, Sm, Eu, Gd, Y and Lu) were synthesized by the solid state reaction. The crystal structure of these compounds were determined by the Rietveld analysis. Single phases of the compounds (except for Ln = Lu) could be prepared only under very restricted preparation conditions. The composition of NaLuTiO4 was not a single phase under the conditions employed in this study. Compounds NaLnTiO, have a tetragonal symmetry for Ln = La-Nd, while an orthorhombic symmetry is observed for Ln = Sm-Lu. The stabilization of NaLnTiO, is discussed on the basis of the relative sizes of the rare earth and the alkali metal ions. The lowering of the symmetry is considered to be introduced by the mismatch between TiO2 and LnO(2) layers. Ionic conductivities attributed to the interlayer sodium ions were observed at high temperatures. The magnitude of ionic conductivity of NaLaTiO4, with a single perovskite layer, was much higher than that of Na2La2Ti3O10, with a triple perovskite layer. This high ionic conductivity is due to the weak interaction between the perovskite layer and interlayer sodium ions.