Classical molecular dynamics simulations were used to study the motion of Na atoms in amorphous Na2O - 4SiO2 over a wide range of temperatures below and above the glass transition temperature Tg. It was found that the region of space most visited by the Na atoms forms a network of connected pockets that were termed channels. These channels were strongly correlated with the locations of the non-bridging oxygen atoms of the matrix and give rise to a pre-peak in the structure factor. In order to understand the physical origin of the channels a study was made of the characteristics of the Na atoms inside and outside the channels. It was shown that neither the potential energy nor the local environment made it possible to distinguish between in and out atoms. Nevertheless, the results indicated that the channels corresponded to regions of higher Na density as compared to the rest of the system. Finally, as expected, it was shown that the mean square displacement of the Na atoms inside the channels was significantly larger than that of the Na atoms outside.

Numerical Investigation of Ionic Transport in Glasses - the Example of Sodium in Amorphous Silica. E.Sunyer, P.Jund, R.Jullien: Journal of Physics - Condensed Matter, 2003, 15[16], S1659-71