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At the atomic scale, uranium-plutonium mixed oxides (U,Pu)O_2 arecharacterized by cationic chemical disorder, which entails that U and Pucations are randomly distributed on the cation sublattice. In the present work,we study the impact of disorder on point-defect formation energies in (U,Pu)O_2using interatomic-potential and Density Functional Theory (DFT+U) calculations.We focus on bound Schottky defects (BSD) that are among the most stable defectsin these oxides. As a first step, we estimate the distance R_D around the BSDup to which the local chemical environment significantly affects theirformation energy. To this end, we propose an original procedure in which theformation energy is computed for several supercells at varying levels ofdisorder. We conclude that the first three cation shells around the BSD have anon-negligible influence on their formation energy (R_{D} = 7.0 \{AA}). Weapply then a systematic approach to compute the BSD formation energies for allthe possible cation configurations on the first and second nearest neighborshells around the BSD. We show that the formation energy can range in aninterval of 0.97 eV, depending on the relative amount of U and Pu neighboringcations. Based on these results, we propose an interaction model that describesthe effect of nominal and local composition on the BSD formation energy.Finally, the DFT+U benchmark calculations show a satisfactory agreement forconfigurations characterized by a U-rich local environment, and a largermismatch in the case of a Pu-rich one. In summary, this work provides valuableinsights on the properties of BSD defects in (U,Pu)O_2, and can represent avalid strategy to study point defect properties in disordered compounds.

Effect of cationic chemical disorder on defect formation energies in uranium-plutonium mixed oxides