Mobile ions site identification through the isoconfigurational ensemble to reveal the ion dynamics diversity in a glass

Abstract

When applying the Molecular Dynamics (MD) formalism to the short-time dynamics on a paradigmatic lithium metasilicate glass system, it reveals the major features of the mobility. Experimental evidence of the physical phenomenon known as the Nearly Constant Loss (NCL) can be attributed to the independent motion of a small proportion of uncorrelated lithium ions which can leave their cages. Dynamical and structural results in this work when applying the isoconfigurational ensemble method (IEM) to the lithium metasilicate MD simulation, reveal the existence of two different lithium environment, lithium ions with high and low propensity to move. The dynamics of each kind of lithium shows that those ions hosted in high-propensity sites displace continuously in time and are responsible for contributing to the slow Mean Square Displacement (MSD) increase in time due to a highly interconnected path. On the other side, the low-propensity lithium ions remain confined to their cages for a longer time until they leave them and jump to the next site. Low-propensity lithium ions only contribute to the MSD when they reach the pre-diffusive time, after 40 ps, even after the maximum dynamical heterogeneity time at 700 K. The results reported in this research evidence that the NCL microscopic origin is due to those highly-dynamically- connected system regions which do not confine the lithium ion, i.e. the so called high-propensity lithium ions in this work.