Structural characterization and ion conductivity of MCa2NaNb4O13 (M = Rb, Na) with four units of perovskite layer

Abstract

Layered perovskite, RbCa2NaNb4O13, with four units of perovskite layer was synthesized by solid state reaction. A sodium ion-exchanged compound, NaCa2Na Nb4O13·1.7H2O, was also synthesized by ion-exchange reaction of a rubidium compound with NaNO3 molten salt. The anhydrous sodium ion-exchanged compound was stable in the temperature range from 350 to 600 °C in an ambient atmosphere. The crystal structures of the three materials were determined using the Rietveld method by powder XRD data. The most reliable solutions were obtained for the structural model assuming that the site for a large cation within the perovskite layer, corresponding to the A site for an ideal perovskite, is occupied by the mixed species with the atomic ratio of Ca:Na = 2/3:1/3. The structures of these compounds are substantially the same as those of MLaNb2O7 (M = Rb, Na) except for the thickness of the perovskite layer. A fairly high ion conductivity attributed to the interlayer cation was observed at high temperatures for the sodium compound.