Empirical potential calculations of UO₂ for a better understanding of its stringlike motion at high temperature

In addition to being the most used nuclear fuel, uranium dioxide is a superionic conductor like some battery materials, for which molecular dynamic calculations evidenced string-like collective motion that exists in glass forming materials. Its simple crystalline structure and the development of up to date dedicated modeling makes UO₂ a prototypical material for the study of anharmonic dynamic that lies underneath its specific properties. In this paper, the analysis of two specific phonons modes by empirical potential calculation led us to the identification of UO₂ high temperature phase transition as Fm-3m to Pa-3 first order transition. We propose that the dynamic coexistence of these two phases at high temperature could be an explanation for string-like motion. The prediction made by calculations of a phase transition in UO₂ questions its existence in other surperionic compounds having fluorite structure, and the manner by which it can be confirmed both theoretically and experimentally.