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.