Fractal conduction pathways governing ionic transport in a glass

Abstract: We present a systematic characterization of the fractal conduction pathways governing ionic transport in a non-crystalline solid below the glass-transition temperature. Using classical molecular dynamics simulations of lithium metasilicate, we combine mobility-resolved dynamical analysis with a real-space description of the regions explored by lithium ions. Ensemble-averaged velocity autocorrelation functions rapidly decorrelate and do not resolve the pronounced dynamic heterogeneity of the system, whereas single-ion analysis reveals short-lived episodes of nearly collinear motion. By mapping active-site clusters over increasing time windows, we show that ion-conducting pathways are quasi one-dimensional at short times and evolve into larger, branched structures characterized by a robust fractal dimension $d_f\simeq1.7$. This geometry persists while the silicate backbone remains structurally arrested, whereas near the glass-transition temperature the loss of structural memory leads to the reappearance of small clusters. These results provide a real-space structural interpretation of ionic transport in non-crystalline solids and support fractal pathway models of high-frequency ionic response.