Comparison of ion sites and diffusion paths in glasses obtained by molecular dynamics simulations and bond valence analysis

Based on molecular dynamics simulations of a lithium metasilicate glass we study the potential of bond valence sum calculations to identify sites and diffusion pathways of mobile Li ions in a glassy silicate network. We find that the bond valence method is not well suitable to locate the sites, but allows one to estimate the number of sites. Spatial regions of the glass determined as accessible for the Li ions by the bond valence method can capture up to 90% of the diffusion path. These regions however entail a significant fraction that does not belong to the diffusion path. Because of this low specificity, care must be taken to determine the diffusive motion of particles in amorphous systems based on the bond valence method. The best identification of the diffusion path is achieved by using a modified valence mismatch in the BV analysis that takes into account that a Li ion favors equal partial valences to the neighboring oxygen ions. Using this modified valence mismatch it is possible to replace hard geometric constraints formerly applied in the BV method. Further investigations are necessary to better understand the relation between the complex structure of the host network and the ionic diffusion paths.Comment: 16 pages, 10 figure

Backward correlations and dynamic heterogeneities: a computer study of ion dynamics

We analyse the correlated back and forth dynamics and dynamic heterogeneities, i.e. the presence of fast and slow ions, for a lithium metasilicate system via computer simulations. For this purpose we define, in analogy to previous work in the field of glass transition, appropriate three-time correlation functions. They contain information about the dynamics during two successive time intervals. First we apply them to simple model systems in order to clarify their information content. Afterwards we use this formalism to analyse the lithium trajectories. A strong back-dragging effect is observed, which also fulfills the time-temperature superposition principle. Furthermore, it turns out that the back-dragging effect is long-ranged and exceeds the nearest neighbor position. In contrast, the strength of the dynamic heterogeneities does not fulfill the time-temperature superposition principle. The lower the temperature, the stronger the mobility difference between fast and slow ions. The results are then compared with the simple model systems considered here as well as with some lattice models of ion dynamics.Comment: 12 pages, 10 figure

Loosening up of the Structure in a Mixed Alkali Glass

金沢大学理学部It is known that both loosening of glass structure and reduction of ionic diffusion coefficient occur in the mixed alkali system. We have performed molecular dynamics simulations of the mixed alkali system and have observed both characteristics in different time regions. A large decrease in the ionic diffusion coefficient is due to the interception of the jump where the jump path keeps its characteristics. The contribution of accelerated dynamics due to cooperative jumps decreases in such paths. On the other hand, in longer time regions, the motion of the framework has been found to accompany the small number of jump events of alkali metal ions to unlike ion sites. @S1063-651X~98!03210-3

Ｍonte Carlo Simulation of the mixed alkali effect with cooperative jumps

金沢大学理学部In our previous works on molecular dynamics ~MD! simulations of lithium metasilicate (Li2SiO3), it has been shown that the long time behavior of the lithium ions in Li2SiO3 has been characterized by the component showing the enhanced diffusion ~Le´vy flight! due to cooperative jumps. It has also been confirmed that the contribution of such component decreases by interception of the paths in the mixed alkali silicate (LiKSiO3). Namely, cooperative jumps of like ions are much decreased in number owing to the interception of the path for unlike alkali-metal ions. In the present work, we have performed a Monte Carlo simulation using a cubic lattice in order to establish the role of the cooperative jumps in the transport properties in a mixed alkali glass. Fixed particles ~blockage! were introduced instead of the interception of the jump paths for unlike alkali-metal ions. Two types of cooperative motions ~a pull type and a push type! were taken into account. Low-dimensionality of the jump path caused by blockage resulted in a decrease of a diffusion coefficient of the particles. The effect of blockage is enhanced when the cooperative motions were introduced

Fracton Excitation and Levy Flight Dynamics in Alkali Silicate Glasses

金沢大学理学部We have examined the relaxation behavior of alkali metal ions in lithium metasilicate glasses by means of molecular dynamics simulation. We have observed a change of slope of the mean squared displacement at ;300 ps. In shorter time regions, localized motion of lithium ions within neighboring sites is observed, which is caused by the small fracton dimension ~fracton excitation!. On the other hand, an accelerated motion of particles due to cooperative jumps is found, which characterizes the diffusion and conduction mechanisms of the alkali metal ions in longer time regions. The dynamics of accelerated motion is discussed in relation to Le´vy flight dynamics. @S0163-1829~97!03510-8

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

金沢大学理学部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 Li1 ions before they hop out to diffuse cooperatively with each other. The self-part of the van Hove function of Li1 ions, Gs(r,t), is used as an indicator of the cage decay. At 700 K, in the early time regime t,tx1 , when the cage decays very slowly, the mean square displacement ^r2& of Li1 ions also increases very slowly with time approximately as t0.1 and has weak temperature dependence. Such ^r2& 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 Gs(r,t) at the distance between Li1 ion sites, ^r2& broadly crosses over from the NCL regime to another power law tb with b\u270.64 and eventually it becomes t1.0, corresponding to long-range diffusion. Both tb and t1.0 terms have strong temperature dependence and they are the analogs of the ac conductivity @s(v)}v12b # 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 tx1!t o , where t o is the independent hop relaxation time; ~2! the broad crossover from the NCL to the cooperative ion hopping conductivity s(v)}v12b occurs when the cage decays more rapidly starting at tx1 ; ~3! s(v)}v12b is fully established at a time tx2 comparable to t 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 s~v! becomes frequency independent, i.e., the dc conductivity. MD simulations show the non-Gaussian parameter peaks at approximately tx2 and the motion of the Li1 ions is dynamically heterogeneous. Roughly divided into two categories of slow ~A! and fast ~B! moving ions, their mean square displacements ^rA 2 & and ^rB 2 & are about the same for t,tx2 , but ^rB 2 & of the fast ions increases much more rapidly for t.tx2 . The self-part of the van Hove function of Li1 reveals that first jumps for some Li1 ions, which are apparently independent free jumps, have taken place before tx2 . While after tx2 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

Some Finite Size Effects in Simulations of Glass Dynamics

We present the results of a molecular dynamics computer simulation in which we investigate the dynamics of silica. By considering different system sizes, we show that in simulations of the dynamics of this strong glass former surprisingly large finite size effects are present. In particular we demonstrate that the relaxation times of the incoherent intermediate scattering function and the time dependence of the mean squared displacement are affected by such finite size effects. By compressing the system to high densities, we transform it to a fragile glass former and find that for that system these types of finite size effects are much weaker.Comment: 12 pages of RevTex, 4 postscript figures available from W. Ko

Complex lithium ion dynamics in simulated LiPO3 glass studied by means of multi-time correlation functions

Molecular dynamics simulations are performed to study the lithium jumps in LiPO3 glass. In particular, we calculate higher-order correlation functions that probe the positions of single lithium ions at several times. Three-time correlation functions show that the non-exponential relaxation of the lithium ions results from both correlated back-and-forth jumps and the existence of dynamical heterogeneities, i.e., the presence of a broad distribution of jump rates. A quantitative analysis yields that the contribution of the dynamical heterogeneities to the non-exponential depopulation of the lithium sites increases upon cooling. Further, correlated back-and-forth jumps between neighboring sites are observed for the fast ions of the distribution, but not for the slow ions and, hence, the back-jump probability depends on the dynamical state. Four-time correlation functions indicate that an exchange between fast and slow ions takes place on the timescale of the jumps themselves, i.e., the dynamical heterogeneities are short-lived. Hence, sites featuring fast and slow lithium dynamics, respectively, are intimately mixed. In addition, a backward correlation beyond the first neighbor shell for highly mobile ions and the presence of long-range dynamical heterogeneities suggest that fast ion migration occurs along preferential pathways in the glassy matrix. In the melt, we find no evidence for correlated back-and-forth motions and dynamical heterogeneities on the length scale of the next-neighbor distance.Comment: 12 pages, 13 figure

Fragility, Stokes-Einstein violation, and correlated local excitations in a coarse-grained model of an ionic liquid

Dynamics of a coarse-grained model for the room-temperature ionic liquid, 1-ethyl-3-methylimidazolium hexafluorophosphate, couched in the united-atom site representation are studied via molecular dynamics simulations. The dynamically heterogeneous behavior of the model resembles that of fragile supercooled liquids. At or close to room temperature, the model ionic liquid exhibits slow dynamics, characterized by nonexponential structural relaxation and subdiffusive behavior. The structural relaxation time, closely related to the viscosity, shows a super-Arrhenius behavior. Local excitations, defined as displacement of an ion exceeding a threshold distance, are found to be mainly responsible for structural relaxation in the alternating structure of cations and anions. As the temperature is lowered, excitations become progressively more correlated. This results in the decoupling of exchange and persistence times, reflecting a violation of the Stokes-Einstein relation.Comment: Published on the Phys. Chem. Chem. Phys. websit