Thermally Jammed States in Superionic Conductors

Superionic solids exhibit a significant increase in ionic conductivity within a certain temperature range. Studies of superionic conductors have shown the presence of dynamical heterogeneity (DH) among the mobile ions in these systems, in which groups of ions exhibit different dynamical behavior than those in nearby regions, a feature also observed in supercooled liquids. Using atomistic simulations in conjunction with the two-phase thermodynamic (2PT) method that models a system as a superposition of hard spheres (gas) and a harmonic oscillators (solid), we quantify the thermal packing fraction φ of the oxygen ions of UO₂, which exhibits superionic properties at temperatures above 2000 K. Our analysis shows that at temperatures near 2000 K, the O ions are in a jammed state with φ close to that of a random close-packed configuration (~0.64). Interestingly, at the λ-transition temperature of ~2600 K, φ for O ions attains a value corresponding to a cubic lattice (~0.52), which is consistent with the sub-lattice structure of the oxygen ions. Our work provides an alternate direction for investigating the disordering process in superionic conductors and solid materials that have significant thermal disorder.