Solid State Ionics 70/71 (1994) 323-327 North-Holland zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA SOLID zyxwvutsrqponmlkjihgfe STATE zyxwvutsrqponml IONUS Electrical and mechanical mixed alkali effect in a lithium/sodium borate glass at GHz frequencies D. Wilmer, T. Kantimm, 0. Lamberty, K. Funke ’ Institut ftir Physikalische Chemie der Westfdlischen Wilhelms-Universitiit Miinster, Schlossplatz 4. 48149 Miinster, Germany M.D. Ingram Department of Chemistry, University ofAberdeen. Meston Walk, Old Aberdeen, Scotland, UK and A. Bunde I. Institutfu’r Theoretische Physik, Universitiit Hamburg, Jungiusstr. 9, 20355 Hamburg, Germany We present results of electrical and mechanical studies of the mixed alkali effect in glass, at frequencies up to 60 GHz. The measurements have been performed on glassy 0.3 [xNazO. (1 -x)Li20] ,0.7Bz01, where x has been varied from 0.0 to 1 .O. 1. Introduction The present paper reports results of high fre- quency electrical and mechanical studies of the mixed alkali glassy ion conductor 0.3 [ xNaz 0. ( l- x)Li,O] .0.7Bz03. Mixed alkali glasses are of gen- eral interest since their transport properties strongly deviate from linear functions of x, a pattern of be- haviour known as the mixed alkali effect [l-3]. In a previous study of 0.3 [ xNa20. ( l- x)LiZO] .0.7Bz03 glasses, measurements of the fre- quency dependent electric conductivity up to 500 kHz have shown a pronounced minimum in the “dc” conductivity at xw 0.6 and a variety of dispersive ef- fects [ 41. The present results refer to both electrical and mechanical high-frequency responses and ex- tend up to 60 GHz. Various models have been proposed to explain the mixed alkali effect; the early ones [ 2 ] were classified into two kinds. Continuing this approach, one can still divide most current theories into two groups: (1) The first kind emphasizes structural differ- ’ To whom all correspondence should be addressed. ences between single and mixed alkali glasses. Here, the alkali ion distribution or size [ 51 are made re- sponsible for the changes in the electric properties. (2) For the second type, differences in bonding and coordination environment of the alkali ions, sometimes accompanied by interaction between dis- similar alkali ions and percolative effects are used to account for the lowering of the cationic mobility in mixed alkali glasses [ 6- 111. None of these theories fully accounts for all fea- tures of the mixed alkali effect. Recently, Bunde, In- gram and Maass have proposed a new model [ 12- 15 ] which emphasizes for the first time the dynamic aspects of glass structure [ 161. Key features include a site memory effect which creates vacancies appro- priate to each kind of mobile ion, and a mismatch energy which emerges whenever an ion enters (or at- tempts to enter) a different kind of site. The con- tinuously evolving structure of glass is manifested by structural relaxations well below zyxwvutsrqponmlkjihgfedcbaZY TB (see e.g. the in- ternal friction peaks reported by Shelby and Day [ 17 ] ), and also explains how different kinds of site can retain their identities and coexist in glasses con- taining more than one mobile ion [ 18 1. The model 0167-2738/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved.