An electron paramagnetic resonance study of impurities in ceria, CeO2

We report on electron paramagnetic resonance (EPR) spectroscopy of polycrystalline ceria, CeO2−x, at temperatures between 80 and 950 K. At elevated temperatures, samples were equilibrated at various oxygen partial pressures in order to change the stoichiometry of the material and to influence the valence states of EPR-active impurities. Some of the EPR signals in nominally pure material could be attributed to Gd3+ and Mn2+ impurities. Manganese occupies sites of cubic and of lower symmetry and can be oxidized to the EPR-silent Mn3+. Gd3+ in sites of cubic symmetry, exhibit an axial appearance in the EPR powder spectrum, though an isotropic g-factor is reported from single crystal measurements. For this reason, the signal due to Gd3+ is sometimes mistaken for another anisotropic signal with g⊥=1.964 and g‖=1.943 which occurs in many ceria samples. The latter is frequently attributed to Ce3+. On the basis of our experiments on oxygen deficient ceria, we dismiss some arguments leading to this assignment and attribute this signal to Cr3+.