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

TL;DR

This study uses molecular dynamics simulations to analyze lithium ion movement in LiPO3 glass, revealing that non-exponential relaxation is due to correlated jumps and heterogeneities, with fast ions migrating along pathways.

Contribution

It provides a detailed analysis of lithium ion dynamics in glass, highlighting the roles of heterogeneities and correlated jumps, and compares behaviors in glass and melt states.

Findings

Dynamical heterogeneities increase upon cooling.

Fast ions exhibit correlated back-and-forth jumps.

Fast ion migration occurs along preferential pathways.

Abstract

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.