Structural characterization of Tb- and Pr-doped ceria

The crystallographic structure of Tb- and Pr-doped ceria is investigated through X-ray and neutron powder diffraction, combining pair distribution function and UV-vis spectroscopy. Compared to trivalent dopants, Tb and Pr show peculiar optical and crystallographic properties: whilst Tb have a mixed +3/+4 valence state, Pr is stable mostly in the +4 valence state up to ~50% dopant concentration. For larger Pr amounts, doping promotes a fluorite to monoclinic phase transformation. A straightforward method for extracting the dopant oxidation state by the cell parameter dependence on the ionic radius is also proposed.