Radiation damage in ion-irradiated CeO₂ and (Ce, Gd)O₂ sinters: Effect of the Gd content

The radiation damage induced in cerium dioxide CeO₂ and cerium-gadolinium mixed oxides (Ce, Gd)O_2-x was studied by micro-Raman spectroscopy and diffuse UV-visible reflectance spectroscopy. Sintered samples of undoped CeO₂ and (Ce, Gd)O_2-x for 1 mol% and 5 mol% of Gd₂O₃ were irradiated at room temperature with 2.5-MeV Ar²⁺ and 12-MeV Ar⁴⁺ ions up to high fluences (i.e. 5 × 10¹⁴ cm⁻² and 2.1 × 10¹⁴ cm⁻² respectively). The Raman spectra show that no amorphization of those oxides occurs whatsoever, regardless of the Gd content. No clear change of the damage cross section with the Gd³⁺ content is deduced from the decay of F_2g peak intensity versus fluence. Moreover, the reflectivity spectra show a clear decrease of the band-gap energy and increase of the Urbach energy arising from point defect accumulation. Similar effects are found for 1 mol% and 5 mol% Gd₂O₃, whereas a larger effect on band tailing is seen for undoped ceria. The respective roles of nuclear collisions and electronic excitations are discussed for both ion energies knowing that the two kinds of measurements do not probe the same sample depth. The substitution of Gd³⁺ ions for Ce⁴⁺ ions is likely reducing the damage and the formation of band tails related to the formation of Ce³⁺ ions by electronic excitation processes.