Accommodation of excess oxygen in fluorite dioxides

• Peroxide ions are the dominant defect accommodating excess oxygen in ThO2 and CeO2.
• Mechanism for peroxide migration through fluorite systems discussed in detail.
• Migration mechanism for peroxide in ThO2/CeO2 different to interstitial in UO2.
• Migration energy barrier similar, independent of defect accommodation mechanism.
• Frenkel defects dominated by charged interstitials (due to Coulombic effects).

Accommodation of excess oxygen in CeO2, ThO2 and UO2 has been investigated using ab-initio modelling. Calculations indicate that hyperstoichiometry is preferentially accommodated by the formation of peroxide species in CeO2 and ThO2 but not in UO2, where oxygen interstitial defects are dominant. Migration of the excess oxygen defects was also studied; the peroxide ion in CeO2 and ThO2 is transported via a different mechanism, due to the formation of peroxide molecules, to the oxygen interstitial in UO2. Frenkel pair defects were investigated to understand if the interstitial component could assume a peroxide like configuration in the vicinity of the vacancy. While it was already expected that this would not be the case for UO2 since peroxide was not stable, it was also not found to be the case for CeO2 and ThO2 with the peroxide disassociating into a lattice species and a separate interstitial ion.