A cooperative model for the mixed mobile ion effect in covalent glasses

A thermodynamical model of the long-standing problem of the mixed mobile io n effect (MMIE) or the mixed alkali effect, respectively, in glasses is pre sented. The basic idea of the model is an internal dynamic interaction betw een the cations and the dynamics of the cooperative regions in glasses. Two types of cation traps due to structural disorder in the glassy network are assumed, in which the deeper traps mainly determine the glass relaxation p roperties and the cations with a reduced binding energy are responsible for the movement of ions. The modification of the usual diffusion coefficient of cations has been calculated under these considerations. That leads to a Variation of de-conductivity in dependence on the composition ratio, f, of the different cations introduced into a covalent network, i.e. the occurren ce of the MMIE. The main result is a relation between the de conductivity a nd temperature dependent relaxation time that is controlled by f. Furthermo re, the conductivity properties of ion exchanged glasses can be predicted q uantitatively if the exchange of ions take place either above or below the glass transition temperature. (C) 1998 Elsevier Science B.V. All rights res erved.