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Crystalline materials at elevated temperatures and pressures can exhibitproperties more reminiscent of simple liquids than ideal crystalline materials.Superionic crystalline materials having a liquid-like conductivity {\sigma} areparticularly interesting for battery, fuel cell, and other energy applications,and we study UO2 as a prototypical superionic material since this material iswidely studied given its commercial importance as reactor fuel. Using moleculardynamics, we first investigate basic thermodynamic and structural properties.We then quantify structural relaxation, dynamic heterogeneity, and average ionsmobility. We find that the non-Arrhenius diffusion and structural relaxationtime of this prototypical superionic material can be quantitatively describedin terms of a generalized activated transport model ('string model') in whichthe activation energy varies in direct proportion to the average string length.Our transport data can also be described equally well by an Adam-Gibbs model inwhich the excess entropy density of the crystalline material is estimated fromspecific heat and thermal expansion data, consistent with the average scale ofstring-like collective motion scaling inversely with the excess entropy of thecrystal. Strong differences in the temperature dependence of the interfacialmobility from non-ionic materials are observed, and we suggest that thisdifference is due to the relatively high cohesive interaction of ionicmaterials. In summary, the study of superionic UO2 provides insight into therole of cooperative motion in enhancing ion mobility in ionic materials andoffers design principles for the development of new superionic materials foruse in diverse energy applications.

Superionic UO2: A Model Anharmonic Crystalline Material