These compounds were synthesized by solid-state reaction, in the case of K additions, and by ion exchange in the case of Na, Li and H additions. Their crystal structures were analyzed by using a Rietveld method. In all of the compounds, the main framework was built up of a double layer of perovskite units (Nb2LaO7), and an alkali ion or proton was located at the sites between the layers. Ionic conduction, due to the interlayer ions, was observed above 200C in all of the compounds. The conductivity at room temperature was very low and was less than 10-8S/cm for all of the materials. At high temperatures, a large polarization was observed in all samples upon applying a direct current. This confirmed that ionic conduction due to the interlayer alkali ions predominated at high temperatures. The activation energies which were calculated for temperatures above 300C were 24, 29, 28, and 55kJ/mol for the K-, Na-, Li- and H-containing materials, respectively. The ionic conductivities of the present materials were slightly smaller than those of fast ionic conductors. It was noted that the activation energies for the Na- and Li-containing compounds were almost the same, and were slightly greater than that for the K-containing material. In these materials, the alkali ion sites in the interlayer were occupied to the extent of 50%. In the Na- and Li-containing compounds, the ionic motion was restricted to the interlayer plane, where the shortest distances between the adjacent conducting ions were 2.75 and 2.74Å, respectively. Ionic motion in the K-containing material was more favorable in the a-axis direction, with a shortest direction of 2.05Å; probably giving rise to the lower activation energy here, as compared with the Na- and Li-containing forms. On the other hand, the H-containing compound exhibited 2 types of conduction process. The conduction in the low-temperature region was suggested to be due to a Grotthus mechanism which involved trace amounts of H3O+ molecules that remained in the sample. As the proton site in the interlayer was 100% occupied, proton conduction at high temperatures was expected to occur via a hopping mechanism with a large activation energy.

M.Sato, J.Abo, T.Jin, M.Ohta: Journal of Alloys and Compounds, 1993, 192[1-2], 81-3