Computational study of the energetics and defect clustering tendencies for Y- and La-doped UO₂

The energetics and defect ordering tendencies in solid solutions of fluorite-structured UO₂ with trivalent rare earth cations (M ³⁺ = Y, La) are investigated computationally using a combination of ionic pair potential and density functional theory based methods. Calculated enthalpies of formation with respect to constituent oxides show higher energetic stability for La solid solutions than for Y. Additionally, calculations performed for different atomic configurations show a preference for reduced (increased) oxygen vacancy coordination around La (Y) dopants. The current results are shown to be qualitatively consistent with related calculations and calorimetric measurements of heats of formation in other trivalent doped fluorite oxides, which show a tendency for increasing stability and increasing preference for higher oxygen coordination with increasing size of the trivalent impurity. The implications of these results are discussed in the context of the effect of trivalent impurities on oxygen ion mobilities in UO₂, which are relevant to the understanding of experimental observations concerning the effect of trivalent fission products on the oxidative corrosion rates of spent nuclear fuel.