Numerical investigation of ionic transport in glasses: the example of sodium in amorphous silica

We use classical molecular dynamics simulations to study the motion of sodium atoms in amorphous Na2O-4SiO(2) over a wide range of temperatures below and above the glass transition temperature T-g. We find that the region of space most visited by the sodium atoms forms a network of connected pockets that we call channels. These channels are strongly correlated with the locations of the non-bridging oxygens of the matrix and give rise to a pre-peak in the structure factor. In order to understand the physical origin of the channels we have studied the characteristics of the sodium atoms INside and OUTside the channels. We show that neither the potential energy nor the local environment permit us to distinguish between IN and OUT atoms. Nevertheless, our results indicate that the channels correspond to regions of higher sodium density compared to the rest of the system. Finally, as expected, we show that the mean square displacement of the sodium atoms inside the channels is significantly larger than that of the sodium atoms outside.