First-principles study with charge effects of the incorporation of iodine in UO2

The energetics of iodine atoms in uranium dioxide are calculated using Density Functional Theory. The so-called DFT + U method is applied with the U-ramping technique to circumvent the multiple minima problem that may arise in such calculations. The interstitial position as well as mono, di- and tri-vacancy sites are considered for the insertion of iodine. The possibility for these sites to be charged is explicitly taken into account. One observes that iodine tends to act like an acceptor thus favouring lower charge states for the sites filled with iodine compared to empty ones. The incorporation energies of the various sites exhibit a linear dependence with the position of the Fermi level in the gap of UO2. Solution energies as a function of stoichiometry deviations in UO2+x are deduced from a dilute defect model for the concentrations of available sites. The most favourable insertion sites for iodine are the di- and tri-vacancy sites for understoichiometric and overstoichiometric oxide respectively. For all stoichiometries, iodine is found to have positive solution energy, i.e. to be insoluble in UO2. A high concentration of iodine is shown to have a strong effect on the concentrations of self-defects and the position of the Fermi level.