Phonon-Influenced Transport Characteristics of Superionic ThO₂

Fluorites, which belong to the class of superionic materials, exhibit a second order phase transition at a critical temperature below the melting point. Thoria (ThO₂), which is a promising oxide nuclear fuel, portrays superionic behavior at high temperatures, particularly in accident conditions. Phonon properties such as dispersion and lifetimes are crucial to understanding the thermal response of nuclear fuel under such conditions. However, the interplay between the phonon properties and anion transport in the superionic state remains as a fundamental knowledge gap. In this work, molecular dynamics simulations are performed with ThO₂ at high temperatures using the BPNN-SCAN potential [<i>Sci. Rep.</i> 12(1) (2022)]. First, the temperatures corresponding to the onset of the superionic state and the second order phase transition are determined. Then, the oxygen ion transport, which occurs through correlated hopping in the superionic state, is quantified. Lastly, the excited phonon modes and interactions are correlated to anion hopping and glass-like dynamics in the superionic state. Our work highlights how phonon dynamics influences anion transport in ThO₂ at high temperatures.