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

Molecular dynamics study of coagulation in silica-nanocolloid-water-NaCl systems based on the atomistic model

In the present work, large scale molecular dynamics (MD) simulations of nanocolloidal silica in aqueous NaCl solutions were performed using a fully atomistic model to study the microscopic structures and dynamics of the systems that lead to aggregation or gelation. Our attention is focused on the self-organizations that occur in the structures of the colloidal silica and water for various concentrations of NaCl. As the salt concentration increased, coagulation developed through the direct bonding of SiO4 units. The trend was explained by the systematic changes in the pair correlation functions related to the barrier height in the potential of mean force [J. G. Kirkwood, J. Chem. Phys., 1935, 3, 300]. Network structures of silica were visualised, and their fractal dimensions were examined by computing the running coordination numbers of Si-Si pairs and also by the analysis of two dimensional images. The calculated dimension by the former method was comparable to the experimental observations for the aggregation of silica colloids, and at longer length scales, super-aggregation was evident in the gelation process. The result from the 2D images is found to be insensitive to the differences in the structure. Clear changes in both the structure and mobility of the water were observed as the NaCl concentration increased, suggesting the importance of the solvent structures to these processes, although such a feature is lacking in the conventional models and most simulations of colloids.

Simplified interpretation of transport in disordered inorganic ion conductors from vacancy dynamics

Ion transport in structurally disordered inorganic ion conductors can be interpreted as cation jumps between sites provided by the network. Because of the small number of vacant sites and strong intercationic Coulomb interaction, their dynamics is very complex. Based on molecular dynamics simulations we recast the ion dynamics via a sophisticated mapping procedure into the corresponding vacancy dynamics. Remarkably, in this framework, the transport can be interpreted to a very good approximation as a noninteracting single-particle processes. In particular, the macroscopic conductivity can be directly obtained from the local vacancy hopping rates.

Pair-hopping characteristic of lithium diffusive motion in Li-doped beta-phase manganese phthalocyanine

We report a first-principles molecular dynamics study on Li-doped beta-phase manganese phthalocyanine (MnPc). Four electronegative sites next to pyrrole-bridging nitrogen atoms in single MnPc were characterized by analyzing electrostatic potentials. In one-dimensional stacked MnPc, six binding sites were unambiguously located, among which two were newly identified by both static and dynamic simulations. Molecular dynamics simulations were conducted to explore the underlying mechanism governing the diffusive motion of Li dopants. The continuous diffusion of Li atoms among these six binding sites of adjacent MnPc molecules has a distinct cooperative pair-hopping character. The activation energy is significantly lower for pair-hopping than for a single jump.

The mixed alkali effect in ionically conducting glasses revisited: A study by molecular dynamics simulation

When more than two kinds of mobile ions are mixed in ionic conducting glasses and crystals, there is a non-linear decrease of the transport coefficients of either type of ion. This phenomenon is known as the mixed mobile ion effect or Mixed Alkali Effect (MAE), and remains an unsolved problem. We use molecular dynamics simulation to study the complex ion dynamics in ionically conducting glasses including the MAE. In the mixed alkali lithium-potassium silicate glasses and related systems, a distinct part of the van Hove functions reveals that jumps from one kind of site to another are suppressed. Although, consensus for the existence of preferential jump paths for each kind of mobile ions seems to have been reached amongst researchers, the role of network formers and the number of unoccupied ion sites remain controversial in explaining the MAE. In principle, these factors when incorporated into a theory can generate the MAE, but in reality they are not essential for a viable explanation of the ion dynamics and the MAE. Instead, dynamical heterogeneity and "cooperativity blockage" originating from ion-ion interaction and correlation are fundamental for the observed ion dynamics and the MAE. Suppression of long range motion with increased back-correlated motions is shown to be a cause of the large decrease of the diffusivity especially in dilute foreign alkali regions. Support for our conclusion also comes from the fact that these features of ion dynamics are common to other ionic conductors, which have no glassy networks, and yet they all exhibit the MAE.

Multifractal analysis of dynamic potential surface of ion-conducting materials

A multifractal analysis using singularity spectra [T.C. Halsey et al., Phys. Rev. A 33, 1141 (1986)] provides a general tool to study the temporal-spatial properties of particles in complex disordered materials such as ions in ionically conducting glasses and melts. Obtained by molecular-dynamics simulations, the accumulated positions of the particles dynamically form a structural pattern called the dynamical potential surface. In this work, the complex dynamical potential surfaces of Li ions in the lithium silicates were visualized and characterized by the multifractal analysis. The fractal dimensions and strength of the singularity related to the spatial intermittency of the dynamics are examined, and the relationship between dynamics and the singularity spectra is discussed.

Coupling of ion and network dynamics in lithium silicate glasses: a computer study

We present a detailed analysis of the ion hopping dynamics and the related nearby oxygen dynamics in a lithium metasilicate glass via molecular dynamics simulation. For this purpose we have developed numerical techniques to identify ion hops and to sample and average dynamic information of the particles involved. This leads to an instructive insight into the microscopic interplay of ions and network. It turns out that the cooperative dynamics of lithium and oxygen can be characterized as a sliding door mechanism. It is rationalized why the local network fluctuations are of utmost importance for the lithium dynamics.

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.

“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.

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.

Trap models and slow dynamics in supercooled liquids

The predictions of a class of phenomenological trap models of supercooled liquids are tested via computer simulation of a model glass-forming liquid. It is found that a model with a Gaussian distribution of trap energies provides a good description of the landscape dynamics, even at temperatures above T(c), the critical temperature of mode-coupling theory. A scenario is discussed whereby deep traps are composed of collections of inherent structures above T(c) and single inherent structures below T(c). Deviations from the simple Gaussian trap picture are quantified and discussed.

Molecular dynamics study of cage decay, near constant loss, and crossover to cooperative ion hopping in lithium metasilicate

Molecular dynamics (MD) simulations of lithium metasilicate (Li2SiO3) in the glassy and supercooled liquid states have been performed to illustrate the decay with time of the cages that confine individual Li+ ions before they hop out to diffuse cooperatively with each other. The self-part of the van Hove function of Li+ ions, G(s)(r,t), is used as an indicator of the cage decay. At 700 K, in the early time regime t<t(x1), when the cage decays very slowly, the mean square displacement <r(2)> of Li+ ions also increases very slowly with time approximately as t(0.1) and has weak temperature dependence. Such <r(2)> can be identified with the near constant loss (NCL) observed in the dielectric response of ionic conductors. At longer times, when the cage decays more rapidly as indicated by the increasing buildup of the intensity of G(s)(r,t) at the distance between Li+ ion sites, <r(2)> broadly crosses over from the NCL regime to another power law t(beta) with beta approximately 0.64 and eventually it becomes t(1.0), corresponding to long-range diffusion. Both t(beta) and t(1.0) terms have strong temperature dependence and they are the analogs of the ac conductivity [sigma(omega) proportional, variant omega(1-beta)] and dc conductivity of hopping ions. The MD results in conjunction with the coupling model support the following proposed interpretation for conductivity relaxation of ionic conductors: (1) the NCL originates from very slow initial decay of the cage with time caused by few independent hops of the ions because t(x1)<<tau(o), where tau(o) is the independent hop relaxation time; (2) the broad crossover from the NCL to the cooperative ion hopping conductivity sigma(omega) proportional, variant omega(1-beta) occurs when the cage decays more rapidly starting at t(x1); (3) sigma(omega) proportional, variant omega(1-beta) is fully established at a time t(x2) comparable to tau(o) when the cage has decayed to such an extent that thereafter all ions participate in the slowed dynamics of cooperative jump motion; and (4) finally, at long times sigma(omega) becomes frequency independent, i.e., the dc conductivity. MD simulations show the non-Gaussian parameter peaks at approximately t(x2) and the motion of the Li+ ions is dynamically heterogeneous. Roughly divided into two categories of slow (A) and fast (B) moving ions, their mean square displacements <r(2)(A)> and <r(2)(B)> are about the same for t<t(x2), but <r(2)(B)> of the fast ions increases much more rapidly for t>t(x2). The self-part of the van Hove function of Li+ reveals that first jumps for some Li+ ions, which are apparently independent free jumps, have taken place before t(x2). While after t(x2) the angle between the first jump and the next is affected by the other ions, again indicating cooperative jump motion. The dynamic properties are analogous to those found in supercooled colloidal particle suspension by confocal microscopy.

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.