Thermodynamic stability of the UO₂ surfaces: Interplay between over-stoichiometry and polarity compensation

The thermodynamic stability of UO₂ surfaces is investigated using ab initio calculations. We employ the GGA+U framework to properly model the strong electronic correlations of the uranium 5f electrons. Among the seven terminations of the (100), (110), and (111) orientations studied in this paper, we predict that the stoichiometric O-(111) is the most stable one under oxygen-poor or -intermediary environments. At odds with other fluorite surfaces, the over stoichiometric and polar O₂-(100) and O₂-(111) terminations become the most stable in oxygen-rich environments. For the latter, strong modifications of the electronic structure appear within the upper layers, in order to fulfill the polarity compensation criterion. Some U-5f states are emptied, leading to higher oxidation 5+ and 6+ states for uranium in the outermost layers, but leaving the surface insulating. This unexpected polarity compensation mechanism is not observed for other charge transfer compounds (such as PuO₂) and can be related to the f -f Mott-Hubbard band gap of the UO₂ material. By considering the most stable stoichiometric and over stoichiometric terminations, the Castell's ratio can be fulfilled, explaining the Wulff shape of nano-voids in UO₂ crystals. (authors)