Molecular dynamics simulations of structural changes in mixed alkali (Li-K) silicate glasses

The structure of Li-silicate, K-silicate and mixed (Li-K) silicate glasses, of composition 25R(2)O . 75SiO(2), has been investigated by molecular dyna mics simulations. The environment of each type of alkali ion differs from t hat of the other type, regardless of whether in the single or mixed alkali glass, though there are structural changes. There exists a site mismatch en ergy, which can act as a barrier for diffusion of alkali ions. Alkali ions preferentially jump into sites that were previously occupied by the same ty pe of alkali ions. We attribute the mixed alkali effect to site differences and a decrease in the available number of neighboring sites when the alkal i ions are mixed. The alkali ions diffuse by various ways in the silicate g lasses. Among them, vacancy and exchange mechanisms are directly observed i n this study and they seem to have a resemblance to diffusion in a crystall ine phase. (C) 1999 Elsevier Science B.V. All rights reserved.