An electron paramagnetic resonance study of impurities in ceria, CeO2

We report on electron paramagnetic resonance (EPR) spectroscopy of polycrys talline ceria, CeO2-x, at temperatures between 80 and 950 K. At elevated te mperatures, samples were equilibrated at various oxygen partial pressures i n order to change the stoichiometry of the material and to influence the va lence states of EPR-acrive impurities. Some of the EPR signals in nominally pure material could be attributed to Gd3+ and Mn2+ impurities. Manganese o ccupies sites of cubic and of lower symmetry and can be oxidized to the EPR -silent Mn3+. Gd3+ in sires of cubic symmetry, exhibit an axial appearance in the EPR powder spectrum. though an isotropic g-factor is reported from s ingle crystal measurements. For this reason, the signal due to Gd3+ is some times mistaken for another anisotropic signal with g(perpendicular to) = 1. 964 and g(parallel to) = 1.943 which occurs in many ceria samples. The latt er is frequently attributed to Ce3+. On the basis of our experiments on oxy gen deficient ceria. we dismiss some arguments leading to this assignment a nd attribute this signal to Cr3+. (C) 2001 Elsevier Science B.V. All rights reserved.