Aliovalent Cation Substitution in UO₂: Electronic and Local Structures of U_1-yLa_yO_2±x Solid Solutions

For nuclear fuel related applications, the oxygen stoichiometry of mixed oxides U_1-yM_yO_2±x is an essential property as it affects fuel properties and may endanger the safe operation of nuclear reactors. A careful review of the open literature indicates that this parameter is difficult to assess properly and that the nature of the defects, i.e., oxygen vacancies or U^V, in aliovalent cation-doped UO₂ is still subject to controversy. To confirm the formation of U^V, we have investigated the room-temperature stable U_1-yLa_yO_2±x phase using several experimental methods (e.g., XRD, XANES, and NMR) confirmed by theoretical calculations. This paper presents the experimental proof of U^V and its effect we identified in both electronic and local structure. We observe that U^V is formed in quasi-equimolar proportion as La^III in U_1-yLa_yO_2±x (y = 0.06, 0.11, and 0.22) solid solutions. The fluorite structure is maintained despite the cationic substitution, but the local structure is affected as variations of the interatomic distances are found. Therefore, we provide here the definitive proof that the substitution of U^IV with La^III is not accommodated by the creation of O vacancies as has often been assumed. The UO₂ fluorite structure compensates the incorporation of an aliovalent cation by the formation of U^V in quasi-equimolar proportions.