AVS 58th Annual International Symposium and Exhibition
    Actinides and Rare Earths Focus Topic Thursday Sessions
       Session AC+SS-ThM

Invited Paper AC+SS-ThM1
The XPS of Heavy Metal Oxides: New Insights Into Chemistry

Thursday, November 3, 2011, 8:00 am, Room 207

Session: The Surface Science of Actinides and Rare Earths
Presenter: Paul Bagus, University of North Texas
Authors: P.S. Bagus, University of North Texas
E.S. Ilton, Pacific Northwest National Laboratory
C.J. Nelin, Consultant
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The XPS of Rare Earth and Actinide oxides are commonly used to obtain information about the oxidation state of the metal by taking various features of the spectra as fingerprints of the metal oxidation state. However, it is possible to obtain detailed information about the nature of the chemical interactions from these features by using the predictions of rigorous theoretical analyses. One of our important concerns is to make direct assessments of the covalent character of the metal-ligand interaction; i.e., the mixing of O(2p) with partly occupied, or unoccupied, metal levels to form bonding and anti-bonding orbitals. We relate this covalent character to the XPS features. In particular, we investigate the connection between the covalent character of the interaction and the satellite intensity. We also investigate the characterization of the satellites and discuss their assignment as shake satellites, an assignment that is naturally connected with the extent of the covalent mixing of the metal and oxygen levels. Furthermore, we examine how vibrational excitations can lead to broadening of the XPS features and suggest that the observed broadening of XPS peaks may contain, hitherto not utilized, information about the chemical interactions in an oxide. Our focus will be on the XPS of two Rare Earth oxides, CeO2 and LaAlO3, and two actinide oxides, UO2 and UO3; these systems have different electronic character that permit the mechanisms discussed above to be explored and compared. Our theoretical analyses are based on relativistic molecular orbital wavefunctions, WF’s, for both initial states, before ionization, and final states, after ionization. The WF’s are for materials models that contain explicit cations and anions embedded in a point charge field. With the variationally optimized orbitals for these WF, covalent mixing is naturally taken into account. The cluster WF’s include one-body and many-body effects and do not use parameters that are adjusted to make calculated relative energies and intensities fit to experiment.