Molecular dynamics simulations were used to study Li dynamics in a model of a LiPO3 glass, at temperatures below the glass transition. An analysis of the ionic trajectories showed that Li diffusion resulted from jumps between sites. Their positions and properties were essentially unmodified on the time-scale of the Li ionic relaxation. This permitted the detailed identification and characterization of sites. The results indicated that the number of Li sites was only slightly higher than the number of Li ions, so that the fraction of vacant sites was very limited at every instant. Mapping the ionic trajectories onto sequences of jumps between sites provided a direct indication of Li jump dynamics. For each site, the mean residence time of an ion was determined, together with the probability that a jump from this site to another site was followed by a direct backward jump. The broad distribution showed that different sites featured very diverse Li dynamics. High values of the probability gave direct evidence of correlated backward-and-forwards jumps. A strong decrease in probability, with increasing residence time, indicated that the backward jump probability depended upon the dynamic state of an ion. It was found that correlated back-and-forth jumps were important at short times in the relaxation process; but not on the time-scale of the Li relaxation, where the hopping motion resembled a random walk. A further study was made of how the local glass structure and the local energy landscape affected Li jump dynamics. Substantial effects due to the energy landscape were observed, which were difficult to reproduce using single-particle approaches. The results implied that Li migration was governed by a competition of the ions for a small fraction of vacant sites in a disordered energy landscape. A statistical analysis showed that a vacancy mechanism dominated re-population of the Li sites.

Identification of Lithium Sites in a Model of LiPO3 Glass - Effects of the Local Structure and Energy Landscape on Ionic Jump Dynamics. M.Vogel: Physical Review B, 2004, 70[9], 094302 (10pp)