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 dynamics simulations. The environment of each type of alkali ion differs from that of the other type, regardless of whether in the single or mixed alkali glass, though there are structural changes. There exists a site mismatch energy, which can act as a barrier for diffusion of alkali ions. Alkali ions preferentially jump into sites that were previously occupied by the same type of alkali ions. We attribute the mixed alkali effect to site differences and a decrease in the available number of neighboring sites when the alkali ions are mixed. The alkali ions diffuse by various ways in the silicate glasses. Among them, vacancy and exchange mechanisms are directly observed in this study and they seem to have a resemblance to diffusion in a crystalline phase. (C) 1999 Elsevier Science B.V. All rights reserved.