Long-timescale molecular dynamics simulations lasting 150 to 250ps were performed on alkali-metal disilicate glasses at 1400K with the general formula M2Si2O5 (M = Na, K, or an equi-atomic mixture of the two). The simulations were long enough to analyse the diffusive behaviour of the alkali-metal ions in detail. The calculated diffusion coefficients exhibited the mixed alkali-metal effect when systems having different alkali-metal contents were treated at the same pressure, but not when they were treated at the same volume (and therefore at higher pressure). There was no evidence for curvature in the time-dependent mean-square displacements of the alkali-metal ions over the 10 to 60ps range, which suggested the absence of long-term memory effects. The van Hove correlation functions confirmed the hopping mechanism for diffusion, though there was only partial evidence for the existence of a selection effect in the alkali-metal mobility in the mixed glass.

Molecular Dynamics Simulation of Alkali-Metal Diffusion in Alkali-Metal Disilicate Glasses. Smith, W., Forester, T.R., Greaves, G.N., Hayter, S., Gillan, M.J.: Journal of Materials Chemistry, 1997, 7[2], 331-6