ELECTRONIC BAND STRUCTURES OF AND IONIC-CONDUCTION IN BETA-ALUMINA-TYPE SUPERIONIC CONDUCTORS

The electronic band structures of beta-alumina-type superionic conduct ors (SIC) are proposed and discussed using experimental results for th e reflection and absorption spectra in the ultraviolet and vacuum-ultr aviolet regions and photoemission spectra. The measurements were perfo rmed using beam Lines with synchrotron radiation. The valence bands of all beta-alumina-type SIC are constructed from the 2p orbital of oxyg en (O 2p). In the case of alkali beta ''-aluminas, the lowest conducti on band is due to the s orbitals of alkali ions. It is concluded from this result that all conduction ions are not perfectly free, but const ruct bonds, giving rise to an ordered structure existing in a limited region. On the other hand, in the case of alkali beta-aluminas, this i s due to the 3s orbital of aluminum. The low lying exciton bands due t o the orbitals of Ag ions and due to the orbitals of Tl ions are obser ved in Ag beta- and Ag beta ''-aluminas, and in Tl beta- and Tl beta ' '-aluminas, respectively. In these cases, there are also ordered regio ns in the conduction plane or slab. It is therefore suggested that an ordered structure of the conduction plane or slab should be considered in order to understand the ionic conduction in beta-alumina-type SIG. (C) 1998 Elsevier Science B.V. All rights reserved.