Superionic transition in uranium dioxide: Insights from molecular dynamics and lattice dynamics simulations

This study employs molecular dynamics (MD) simulations and lattice dynamics (LD) calculations to investigate the superionic transition in ${\mathrm{UO}}_{2}$, as well as the characteristics of the superionic phase. The superionic transition is found to be a second-order phase transition associated with an inflection point in enthalpy and lattice parameter at 2600 K. This resembles displacive phase transitions and is associated with an oxygen vibration mode becoming imaginary. The superionic state shows a combination of different local environments and dynamic features similar to glass-forming liquids. The analysis of its dynamics reveals seemingly contradictory characteristics: properties follow those of the lower-temperature crystalline phase in some respects, while exhibiting liquidlike behavior in others. The study highlights the complexity of the superionic phase, including heterogeneous mobility and diffusion mechanisms involving stringlike anion clusters.