Radiation damage of Ceria and Ceria-gadolinia mixed oxides: effect of the Gd content and Ion stopping power

We have studied the effects of 27 MeV Xe ion irradiation on sintered samples of ceria (CeO ₂) and ceria-gadolinia (Ce, Gd)O _2-x mixed oxides for Gd contents of 5 and 10 Gd at%. The resulting damage for the fluence of 5 × 10 ¹⁴ cm ^-2 was characterized by means of micro-Raman spectroscopy and diffuse reflectance spectroscopy at room temperature . Spectra of reference (Ce, Gd)O _2-x samples up to 25 mol% Gd ₂O ₃ (50 Gd at%.) were also recorded. A clear broadening of Raman peaks and growth of extra broad bands are found for gadolinia contents above 15 mol% Gd ₂O ₃ (30 Gd at%). There is a large increase of the lattice disorder in the virgin samples due to the increase of concentration of oxygen vacancies (V _O ^..) compensating the Gd ³⁺ ions substituting for the Ce ⁴⁺ ions. For the 27 MeV Xe ion irradiated samples, a similar evolution is found irrespective of the Gd content up to 10 Gd at% with the growth of a broad band at 600 cm ^-1 assigned to extra oxygen-vacancy defect formation. The evolution of band gap and Urbach edge of unirradiated and irradiated samples is deduced from diffuse reflectance spectra for the various Gd contents. Other diffuse reflectance data for irradiations with 5-MeV Ar, 12 MeV Ar, 2.5 MeV Cu, 5 MeV W, 10 MeV W, 36 MeV W, 100 MeV Kr, and 200 MeV Xe ions are compared to the new 27 MeV Xe ion data. There is an exponential decay of the band-gap and exponential increase of Urbach energy up to saturation versus fluence. The damage cross sections deduced from these plots are increasing up to saturation as a function of the electronic stopping power, in the same way as the cross section for the growth of an absorption band at 2.8 eV related to Ce ³⁺ ion formation. A simple model of the evolution of damage cross sections versus electronic stopping power is proposed where the asymptotic values of these cross sections are interpreted as a limit of the Ce ³⁺ ion concentration owing to Ce ³⁺- V _O defect cluster formation. The addition of Gd ₂O ₃ reduces the band tailing related to the increase of Urbach energy linked to Ce ³⁺ ion formation by electronic excitations.