Near-infrared study of radiation damage in cerium dioxide

Radiation damage is studied in electron-irradiated cerium dioxide (CeO ₂ ) single crystals. Near Infra-red (NIR) spectra are recorded at room temperature between 2800 and 11,000 cm ⁻¹ (i.e. ∼0.9–3.57 µm in wavelength and ∼0.35–1.36 eV in photon energy). Measurements were carried out in the transmission mode for the single crystals and diffuse reflectivity mode for sintered CeO ₂ and (Ce, Gd)O _2-x reference un-irradiated samples. The ceria single crystals were irradiated with 1.4-MeV and 2.5-MeV electrons for high fluences up to 3 × 10 ¹⁶ cm ⁻² . Five broad absorption bands centered at 3600, 4100, 4600, 6000, and 7100 cm ⁻¹ (i. e. ∼0.44, 0.51, 0.57, 0.74, and 0.88 eV) are deduced from fits for the 2.5-MeV electron irradiation, whereas no absorption band is recorded for the 1.4-MeV electron energy. Those bands are consistent with the broad 0.9-eV band (7260 cm ⁻¹ , 1.37 µm) in the UV–visible range associated with the change in color from light blue to deep green for the 2.5-MeV energy. Optical absorption arises from point-defect formation by elastic collisions for electron irradiation above the threshold energy near 1.4 MeV corresponding to the onset of cerium atom displacement. No defect bleaching occurs after isothermal annealing at 700 °C. No such bands are recorded for the virgin (Ce, Gd)O _2-x samples for 5, 10, and 15 mol% Gd ₂O ₃ with an increasing amount of oxygen vacancies. The origin of absorption bands in the NIR range is discussed in relation to the available experimental data and ab-initio calculations of point defects in CeO ₂ and other dioxides with the cubic fluorite-like structure. Absorption bands are tentatively assigned to electronic transitions involving cerium vacancy levels in the band gap.