Molecular dynamics study of dynamical heterogeneity in ion conducting glasses

Molecular dynamics (MD) simulations of lithium metasilicate (Li2SiO3) glass have been performed. Dynamic heterogeneity of lithium ions has been examined in detail over 4 ns at 700 K. Particles showing displacements less than the distance at the first minimum of g(r)(Li-Li) during a given time T(= 920 ps) were defined as type A. Particles showing a displacement greater than the distance of the first minimum of g(r)(Li-Li) during T were defined as type B. The type A particles show slow dynamics in accordance with a long tail of waiting time distribution of jump motion and localized jumps within neighboring sites (fractons), while the type B particles show fast dynamics related to the cooperative jumps with strong forward correlation probability (Levy flights). The mutual changes of two kinds of dynamics with a relatively long time scale have been observed. The 'mixed alkali effect' in the LiKSiO3 system can be explained by the mutual interception of jump paths. The paths of lithium and potassium are nearly independent in a relatively short time scale while the mixing of the jump paths occurs in a long time scale. The mixed alkali system also shows a kind of heterogeneity. The heterogeneity can be realized only when the 'memory' of the characteristics of the dynamics is longer than the relaxation time for the mixing. Observation of the heterogeneity also depends on the time (or spatial) resolution.