On the nature of the heterogeneous dynamics of ions in ionic conducting glasses

Molecular dynamics simulations were performed to study the complex and heterogeneous dynamics of ions in ionically conducting glasses. The dynamics of Li ions in lithium silicate glasses have been characterized by van-Hove functions, fractal dimension of random walks, multifractal analysis of density profile, and principal component analyses of time series. Hierarchy dynamics with local and successive jumps, which are followed by cooperative jumps involving more ions, are observed in the self-part of the van-Hove functions and in trajectories where they form a multifractal density profile. Fractal dimensions of the random walks can be used to characterize the motions of ions in both the short and long length-scale regions. Difference between them becomes clearer in the glassy state, and the corresponding power law dependences of the mean squared displacements can be well explained by these exponents. Phase-space plot using the de-noised data obtained by principal component analysis reveals that the trajectory extending from the short time to the long time dynamics in the plot is continuous and has long term memory. The motion examined by the principal component analysis of the time series shows the deterministic character of the single particle ion dynamics. Each ion moves among domains of collective motion. Recurrence plot and mutual information function shows long correlation of the dynamics. Therefore, the cause of the dynamic heterogeneity is rather deterministic in nature.