Next-near-neighbour interactions with Al in Li+- and Rb+-exchanged Na+ beta-aluminas, detected by synchrotron X-ray absorption spectroscopy

Synchrotron X-ray absorption near-edge structure (XANES) spectroscopy studi es have been carried out on the electronic and crystal structure environmen ts around the Al atom in Na+ beta-alumina and in two beta-aluminas with Na exchanged by Li+ and Rb+. The aim is to define the type of interaction, if any existing between the Al located in the 'spinel block' and the fast-con ducting cations in the 'conduction plane'. Na+ beta"-alumina has also been studied for comparison. All beta-alumina spectra differ from that of alpha- alumina (corundum) by showing additional features due to the presence of te trahedral Al. Moreover, they all show a much greater degree of local disord er. There are definite. but small, interactions between tetrahedral Al (and , possibly, also octahedral Al) in the 'spinel block' and the Na+ and Rb+ c ations in the 'conduction plane'. Na+ and Rb+ B-aluminas have similar Al K- edge XANES features, but with intensities that change in relation to the we ight of the 'conduction plane' atom. Despite differences in composition and structure, Na+ beta"-alumina shows the same behaviour, thus confirming the substantial similarity of the Al local environments. Li+-exchanged beta-al umina has an Al K-edge XANES spectrum that apparently differs from all othe rs, but actually conveys the same basic information. Indeed, interaction be tween Al and Li is much greater than in any other beta-alumina because Limoves laterally off the 'conduction plane' to become close to a facing tetr ahedral Al, and strongly interacts with it. Thus, this study also draws att ention to the fact that beta-aluminas react differently to alkali exchange. (C) 2000 International Union of Crystallography Printed in Great Britain - all rights reserved.