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

Abstract

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 ThO, and UO, are quite similar, except for the presence of Sf electrons in UO, 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 UO2 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 e-uranium, no detailed band calculation exists. However, the results agree with the qualitative density-of-states picture presented by Friedel.

Bonding of Metal Oxides in Sodium Silicate Glass

“…The peak near the valence band edge [see curves (n)-vacuum meet sample] is a measure of the occupancy of localized 5f electrons (uranium ions in the +4 ionizntion state). No 5f occupancy appears in the air melt sample [curves (b)] nor in a glass sample prepared with UO3, indicating that for these glasses, the uranium ions are in the +6 oxidation state (16). Similar conclusions for the valence state of uranium in the glasses can be drawn by examining the 4f core levels.…”

Electronic band structure and properties of α-U

“…which has relevance to specific heat and susceptibility determinations in alloys of uranium and to optical absorption measurements. The 1 = 3 DOS correlates well with the XPS data of Veal and Lam [6] whereas the total DOS does not. This result is consistent with the fact that the higher angular momentum (more localized) f orbitals have much larger transition matrix elements.…”

“…This scheme has been successfully applied to the study of some actinide compounds, notably UGe 3 [4], UIr 3 [2], and URh 3 [5]. We have determined the energy band structure, density of states (DOS), orbital angular momentum projected DOS (for use in analysing the XPS experiments of Veal and Lam [6], detailed Fermi surface cross-sections in close collaboration with the work of Arko and Schirber [7] reported elsewhere at this conference), wavefunctions and magnetic field induced spin densities, neutron magnetic form factors (in close collaboration with and to understand the measurements of Maglic et al [8]), and generalized susceptibilities, ^(q) (for investigating possible electronically driven phonon anomalies and charge density waves). Because of space limitations we are able only to give a brief indication of this extensive work here.…”

Photoelectron Spectroscopy of U Oxide at LLNL

“…Under conditions such as these, thin layers composed of materials that would normally be insulating in the bulk can continue to exhibit a Fermi edge, owing to the thinness of the film and the underlying conductor. [11,12] III Discussion To begin, the valence band spectra will be considered. A comparison of the spectra for the LLNL sample versus the earlier work of Veal and Lam is shown in Figure 2.…”