Molecular dynamics simulation of alkali-metal diffusion in alkali-metal disilicate glasses

Long-timescale molecular dynamics simulations of duration 150-250 ps have been performed on alkali-metal disilicate glasses at 1400 K with the general formula M(2)Si(2)O(5) (M=Na, K, or an equiatomic mixture of the two). The simulations are long enough to analyse the diffusive behaviour of the alkali-metal ions in detail. The calculated diffusion coefficients show the mixed alkali-metal effect when systems having different alkali-metal contents are treated at the-same pressure, but not when they are treated at the same volume (and therefore at higher pressure). There is no evidence for curvature in the time-dependent mean-square displacements of the alkali-metal ions over the 10-60 ps range, which suggests the absence of long-time memory effects. The van Hove correlation functions confirm the hopping mechanism for diffusion, though there is only partial evidence for the existence of a selection effect in the alkali-metal mobility in the mixed glass.