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

Abstract

A thermodynamical model of the long–standing problem of the mixed mobile ion effect (MMIE) or the mixed alkali effect, respectively, in glasses is presented. The basic idea of the model is an internal dynamic interaction between 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 properties 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 dc-conductivity in dependence on the composition ratio, f, of the different cations introduced into a covalent network, i.e. the occurrence of the MMIE. The main result is a relation between the dc conductivity and temperature dependent relaxation time that is controlled by f. Furthermore, the conductivity properties of ion exchanged glasses can be predicted quantitatively if the exchange of ions take place either above or below the glass transition temperature.