Matrix-controlled channel diffusion of sodium in amorphous silica

To find the origin of the diffusion channels observed in sodium silicate glasses, we have performed classical molecular dynamics simulations of Na2O-4SiO2 during which the mass of the Si and O atoms has been multiplied by a tuning coefficient. We observe that the channels disappear and that the diffusive motion of the sodium atoms vanishes if this coefficient is larger than a threshold value. Above this threshold the vibrational states of the matrix are not compatible with those of the sodium ions. We thus interpret the decrease of the diffusion by the absence of resonance conditions. (letter to the editor)