X-ray photoelectron studies of thorium, uranium, and their dioxides

The valence-band structures of the light actinides thorium, uranium, and their dioxides have been investigated by means of x-ray photoemission spectroscopy. This study shows that the electronic structures of Th${\mathrm{O}}_2$ and U${\mathrm{O}}_2$ are quite similar, except for the presence of $5f$ electrons in U${\mathrm{O}}_2$ which lie close to the Fermi level. It is these low-binding-energy $5f$ electrons that give rise to the most dramatic differences in physical properties, notably the magnetism, electrical conductivity, and color. Conduction in U${\mathrm{O}}_2$ apparently occurs via holes created by excitation of rather localized $5f$ electrons from the uranium to interstitial oxygen-acceptor sites. Sputtering experiments on thin oxide film and bulk samples show Fermi-level shifts in x-ray photoemission spectra associated with doping changes that result from preferential removal of interstitial ("impurity") oxygen atoms. The deep-trap and polaron conduction models have been reexamined. It is our view, at this time, that neither model can be ruled out. For the pure metal thorium, valence-band spectra were compared with the available density-of-states calculations and satisfactory agreement was obtained. For $\alpha -\mathrm{uranium}$, no detailed band calculation exists. However, the results agree with the qualitative density-of-states picture presented by Friedel.