Abstract
We investigate the diffusion of sodium ions in an artificially frozen amorphous silica matrix via molecular dynamics simulations. The motion of the Si and O atoms is stopped at different temperatures Tfr and we study the activation energy Ea of the diffusing Na atoms. We observe an abrupt change in Ea around the glass transition temperature Tg: for Tfr < Tg high values of Ea are found whereas low values of Ea exist for Tfr > Tg. By studying the probability of the available sodium volume to percolate we show that due to the closing of the intertetrahedral SiOSi angle with increasing Tfr, this volume percolates for the high values of Tfr which explains thus the easy diffusion of the sodium ions.