Density functional theory calculations have been carried out to study the stability and migration of charged oxygen interstitials in ThO_2 and CeO_2. The calculations demonstrate that the oxygen interstitial is likely to lose electrons under p-type conditions and gain electrons under n-type conditions. Neutral (O_(split)~0 and singly positive (O_(split)~+ O-O(110) split interstitials, and doubly negative octahedral (O_(octa)~(2-)) oxygen interstitial are found to be the lowest-energy configurations within a certain Fermi energy range. In both oxides, the O_(split)~0 is the most mobile, and the migration energies of the split oxygen interstitials in ThO_2 are lower than in CeO_2, indicating higher oxygen interstitial mobility in ThO_2 than in CeO_2.
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