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

Abstract

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.