Towards a theory for the mixed alkali effect in glasses

A general theoretical approach for the mixed alkali effect in glasses is discussed based on the idea that the covalent host network creates different structural energy landscapes for different types of mobile ions. By deriving a simplified model from this approach, it is shown how the experimentally observed changes in ion mobilities both upon mixing of two types of ions and upon changing the total concentration of mobile ions can be understood. Computer simulations of the model suggest that Coulomb forces have to be taken into account for explaining the differing behavior of activation energies in single modified and mixed ion glasses. It is further shown how the mixed alkali internal friction peak results from diffusional exchange processes of unlike ions in the glassy network. (C) 1999 Elsevier Science B.V. All rights reserved.