Oxygen-18 Tracer Measurements of Anion Diffusion in Uranium Dioxide Thin Films

Oxygen 18 was used as a tracer to quantify anion diffusion in thin-film UO₂ using secondary ion mass spectroscopy to measure one-dimensional depth profiles. Both thermal and heavy ion bombardment (1.8 MeV Kr⁺) treatments were employed over a temperature range from 295 K to 623 K. Textured and single-crystal thin-film samples were grown using reactive-gas magnetron sputtering at ambient temperature. Both microstructures resulted in similar thermal activation energies, E_a= 0.2 eV. This activation energy is significantly lower than the known value for intrinsic anion vacancy-self diffusion in stoichiometric UO_2.00 (E_a= 2.5 eV). We attribute this to an interstitialcy mechanism in our hyper-stoichiometric films. The activation energy for irradiated textured films was approximately half that of thermal diffusion, consistent with the chemical rate theory. The opposite was true for the single-crystal microstructure (irradiated E_a= 0.36 eV). This may be due to radiation-induced changes in the microstructure. The mixing parameter was quantified on the anion sublattice as well, ξ = 2.1 ± 0.2 Å⁵eV⁻¹.