Cathodoluminescence of CeO₂ doped with Gd₂O₃ in a high voltage electron microscope

Ceria (CeO2) doped with Gd2O3 (Ce1-xGdxO2-x/2) is considered a surrogate of a burnable poison-doped nuclear fuel. Oxygen vacancies (V-O) are induced in Ce(1-x)Gd(x)O(2-x/2 )by the substitution of Gd3+ ions into the Ce4+ sites, and the oxygen deficiency is mainly controlled by the Gd2O3 concentration. Oxygen vacancies can also be generated by elastic collisions with high-energy electrons for recoil energies of oxygen atoms above the threshold displacement energy (E-d,E-O). Oxygen vacancy formation is a key factor to determine the radiation tolerance since it enhances the recombination of point defects. The behavior of point defects in ceramics is known to depend on their charge states, although there is only limited research on this topic. In this study, in situ cathodoluminescence (CL) spectroscopy is applied to studying the charge states of defects induced in the oxygen-deficient cerium dioxides, Ce1-xGdxO2-x/2, by using a high-voltage electron microscope for electron energies from 400 to 1250 keV. The CL emission bands of Ce3+ - VO, Ce3+, and F+ centers are obtained for Ce1-xGdxO2-x/2 as well as pure ceria. An energy shift and quenching of CL emission in Ce1-xGdxO2-x/2 are observed. Those effects are discussed as a consequence of the generation of extrinsic oxygen vacancies induced by Gd2O3 doping (V-O,V-ext) and the influence of the respective electronic configurations of the Ce3+ ions and V-O,V-ext. A schematic picture of energy levels of defects in the bandgap of Ce1-xGdxO2-x/2 is suggested.