Dynamical significance in alkali metasilicate glasses

In this work, we elucidate the effect of the less mobile ions on the dynamics of the more mobile ions by molecular dynamics simulations of lithium ions motion in lithium metasilicate glass by freezing some randomly chosen lithium ions (5%, 10% and 25%) at their initial locations at 700 K. A remarkable slowing down of the dynamics of the majority mobile Li ions was observed both in the self-part of the density–density correlation function, F_s(k,t), and in the diffusion coefficients. On the other hand, there is no significant change in the structure. These results show many similarities to the mixed alkali effect (MAE) with mixing of the small content of foreign alkali (10% and 25% of K₂O), where large reduction of the dynamics was also observed in both experiments and MD simulations. This immobilization of faster ions causes the large MAE as already discussed in relation to the mechanism of the cooperative ion jump motions. Although of lesser importance, the ion dynamics are influenced by the matrix of oxygen atoms, because the jump motions of Li ions are assisted by the localized motions of oxygen atoms.     

Dynamics of caged ions in glassy ionic conductors

At sufficiently high frequency and low temperature, the dielectric responses of glassy, crystalline, and molten ionic conductors all invariably exhibit nearly constant loss. This ubiquitous characteristic occurs in the short-time regime when the ions are still caged, indicating that it could be a determining factor of the mobility of the ions in conduction at longer times. An improved understanding of its origin should benefit the research of ion conducting materials for portable energy source as well as the resolution of the fundamental problem of the dynamics of ions. We perform molecular dynamics simulations of glassy lithium metasilicate (Li2SiO3) and find that the length scales of the caged Li+ ions motions are distributed according to a Levy distribution that has a long tail. These results suggest that the nearly constant loss originates from "dynamic anharmonicity" experienced by the moving but caged Li+ ions and provided by the surrounding matrix atoms executing correlated movements. The results pave the way for rigorous treatments of caged ion dynamics by nonlinear Hamiltonian dynamics.     

Dynamical structure of superionic conductors

Ionic motion is analysed of a model system for α-AgI which has ionic potentials consisting of only a Coulombic and a soft-core repulsive terms. A tetrahedron (abbreviated as TH) in the anion bcc sublattice is investigated from a dynamical point of view. The residence time of a cation in a TH is rigorously evaluated. Dynamical correlation between anions and cations is examined by the observation of dynamical behaviors of anions forming a TH and a cation. A mechanism of cation diffusion will be suggested.     

Soft-core model for ionic crystals normal ionic salts and superionic conductors

Soft-core potentials are assumed for the pair interactions between ions, and with the Lennard-Jones and Devonshire cell theory, the conditions for the occurrence of sublattice melting as a transition from the normal crystalline to the super-ionic phase are obtained.     

Time series analysis of ion dynamics in glassy ionic conductors obtained by a molecular dynamics simulation

We present several characteristics of ionic motion in glassy ionic conductors brought out by time series analysis of molecular dynamics (MD) simulation data. Time series analysis of data obtained by MD simulation can provide crucial information to describe, understand and predict the dynamics in many systems. The data have been treated by the singular spectrum analysis (SSA), which is a method to extract information from noisy short time series and thus provide insight into the unknown or partially unknown dynamics of the underlying system that generated the time series. Phase-space plot reconstructed using the principal components of SSA exhibited complex but clear structures, suggesting the deterministic nature of the dynamics.     

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 (Lévy 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. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characteristics of slow and fast ion dynamics in a lithium metasilicate glass

Molecular dynamics simulations of lithium metasilicate (Li(2)SiO(3)) glass have been performed. The motion of lithium ions is divided into slow (A) and fast (B) categories in the glassy state. The waiting time distribution of the jump motion of each component shows power law behavior with different exponents. Slow dynamics are caused by localized jump motions and the long waiting time. On the other hand, the fast dynamics of the lithium ions in Li(2)SiO(3) are characterized as Lévy flight caused by cooperative jumps. Short intervals of jump events also occur in the fast dynamics in the short time region. Both the temporal and spatial terms contribute to the dynamics acceleration and the heterogeneity caused by these two kinds of dynamics is illustrated. The slow dynamics characteristics of the "glass transition" and in the "mixed alkali effect" are discussed.     

"Cooperativity blockage" in the mixed alkali effect as revealed by molecular-dynamics simulations of alkali metasilicate glass

The relaxation dynamics of a complex interacting system can be drastically changed when mixing with another component having different dynamics. In this work, we elucidate the effect of the less mobile guest ions on the dynamics of the more mobile host ions in mixed alkali glasses by molecular-dynamics (MD) simulations. One MD simulation was carried out on lithium metasilicate glass with the guest ions created by freezing some randomly chosen lithium ions at their initial locations at 700 K. A remarkable slowing down of the dynamics of the majority mobile Li ions was observed both in the self-part of the density-density correlation function, Fs(k,t), and in the mean-squared displacements. On the other hand, there is no significant change in the structure. The motion of the Li ions in the unadulterated Li metasilicate glass is dynamically heterogeneous. In the present work, the fast and slow ions were divided into two groups. The number of fast ions, which shows faster dynamics (Levy flight) facilitated by cooperative jumps, decreases considerably when small amount of Li ions are frozen. Consequently there is a large overall reduction of the mobility of the Li ions. The result is also in accordance with the experimental finding in mixed alkali silicate glasses that the most dramatic reduction of ionic conductivity occurs in the dilute foreign alkali limit. Similar suppression of the cooperative jumps is observed in the MD simulation data of mixed alkali system, LiKSiO3. Naturally, the effect found here is appropriately described as "cooperativity blockage." Slowing down of the motion of Li ions also was observed when a small number of oxygen atoms chosen at random were frozen. The effect is smaller than the case of freezing some the Li ions, but it is not negligible. The cooperativity blockage is also implemented by confining the Li metasilicate glass inside two parallel walls formed by freezing Li ions in the same metasilicate glass. Molecular-dynamics simulations were performed on the dynamics of the Li ions in the confined glass. Slowing down of the dynamics is largest near the wall and decreases monotonically with distance away from the wall.