DEFECT MODELS FOR THE OXYGEN POTENTIALS OF GADOLINIUM-DOPED AND EUROPIUM-DOPED URANIA

A defect model for the thermochemistry of the mixed rare-earth uranium oxides has been developed. The parameters of the model were determined by fitting the theory to experimental oxygen potential data. The basic defects and associated equilibria found previously for pure urania were used in the mixed oxide defect model. In addition, the model prohibits oxygen interstitial occupancy of six to ten sites neighboring a trivalent dopant ion and assumes vacancy-dopant clusters in an excess of oxygen vacancies. Successful modeling of the oxygen potential as a function of temperature and oxide composition was achieved by assuming gadolinium to be present as isolated ions in the lattice, but europium modeling required complexes consisting of six Eu3+ and one anion vacancy.