A complete identification of lithium sites in a model of LiPO$_3$ glass: effects of the local structure and energy landscape on ionic jump dynamics

Abstract

We perform molecular dynamics simulations to study lithium dynamics in a model of LiPO$_3$ glass at temperatures below the glass transition. A straightforward analysis of the ionic trajectories shows that lithium diffusion results from jumps between sites that are basically unmodified on the time scale of the lithium ionic relaxation. This allows us a detailed identification and characterization of the sites. The results indicate that the number of lithium sites is only slightly bigger than the number of lithium ions so that the fraction of vacant sites is very limited at every instant. Mapping the ionic trajectories onto series of jumps between the sites provides direct access to lithium jump dynamics. For each site, we determine the mean residence time $\tau_s$ and the probability $p_s^b$ that a jump from this site to another site is followed by a direct backjump. While a broad distribution $G(\lg \tau_s)$ shows that different sites feature diverse lithium dynamics, high values of $p_s^b$ give direct evidence for back-and-forth jumps. We further study how the local glass structure and the local energy landscape affect lithium jump dynamics. We observe substantial effects due to the energy landscape, which are difficult to capture within single-particle approaches.