Optical reflectivity of ion-irradiated cerium dioxide sinters

Reflection spectra of cerium dioxide sintered samples were measured in the UV-visible range after irradiation with various heavy ions (2.4-MeV Cu, 5-MeV W, 10-MeV W, 36-MeV W, 100-MeV Kr, and 200-MeV Xe). Differential reflectance spectra of irradiated samples after subtraction of the reference sample spectrum are fit with six broad Gaussian bands centered at about 1.2, 2.2, 2.8, 4.0, 4.8, and 6.2 eV. The growth curves of most bands show a saturation behavior vs ion fluence. Reflection spectra are consistent with the UV-visible absorption spectra of electron-irradiated ceria single crystals for photon energies lower than 3.2 eV, corresponding to the optical gap. The spectra are tentatively analyzed on the basis of charge transfer bands and the 4f-5d transitions related to the Ce³⁺ ions in the distorted environment formed by irradiation. More insight into defect formation is given by the reflection spectra rather than the absorption ones due to the limitation in the absorption of the optical gap. These data are also discussed with reference to the radiation damage processes by electronic excitations and nuclear collisions in ceria.