Dynamical fluctuations in ion conducting glasses: slow and fast components in lithium metasilicate

Molecular dynamics 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. 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 (fracton), while type B particles show fast dynamics related to cooperative jumps and a strong forward correlated motion (Lévy flight). Mutual changes of two kinds of dynamics with the relatively long time scale have been observed. The changes cause an extremely large fluctuation of the mean squared displacements as well as the squared displacement of each particle, which depends on the time window of observation. Localized jump motion (fracton) cannot contribute to the long-time-translational diffusion but it can contribute to the rotational diffusion. On the other hand, forward correlated jump motion mainly contributes to long-time-translational diffusion and not to the short-time-rotational diffusion, although this can be a slower part of the rotational diffusion. These results have been compared with those of simple glass-forming liquids exhibiting the dynamic heterogeneity near T(g). The translation-rotation paradox can be explained by the characteristics of slow and fast dynamics.