Towards a Predictive Thermodynamic Model of Oxidation States of Uranium Incorporated in Fe (hydr) oxides

Technical Report · Tue Jan 01 04:00:00 EST 2013

DOI:https://doi.org/10.2172/1257643· OSTI ID:1257643

Bagus, Paul S. ^[1]

---

1. Univ. of North Texas, Denton, TX (United States); University of North Texas

The theoretical research in this project has been directed toward the interpretation of core-level spectroscopies for systems relevant to the project. For the initial efforts, the focus of our theoretical simulations has been the interpretation of laboratory and synchrotron X-Ray Photoemission Spectra, XPS. In more recent efforts, an increasing emphasis has been placed on developing transparent understandings of X-Ray Adsorption Spectra, XAS . For the XAS, the principal concern is for the near-edge features, either just below or just above, an ionization limit or edge, which are described as Near-Edge X-Ray Adsorption Fine Structure, NEXAFS. In particular, a priority has involved the analysis and interpretation of XPS and NEXAFS spectra, especially of Fe and U systems, as measured by our PNNL collaborators. The overall objective of our theoretical studies is to establish connections between features of the spectra and their origin in the electronic structure of the materials. The efforts for the analysis of XPS have been reviewed in a paper by the PI, C. J. Nelin, and E. S. Ilton from PNNL on “The interpretation of XPS spectra: Insights into materials properties”, Surf. Sci. Reports, 68, 273 (2013). Two materials properties of special interest have been the degree of ionicity and the character of the covalent bonding in a range of oxides formed with transition metal, lanthanide, and actinide cations. Since the systems treated have electrons in open shells, it has been necessary to determine the energetics and the character of the angular momentum coupling of the open shell electrons. In particular, we have established methods for the treatment of the “intermediate coupling” which arises when the system is between the limit of Russell-Saunders multiplets, and the limit of j-j coupling where the spin-orbit splittings of single electrons dominate. A recent paper by the PI, and M. J. Sassi, and K. M. Rosso, (both at PNNL) “Intermediate Coupling For Core-Level Excited States: Consequences For X-Ray Absorption Spectroscopy”, J. Elec. Spectros. and Related Phenom., 200, 174 (2015) describes our first application of these methods. As well as applications to problems and materials of direct interest for our PNNL colleagues, we have pursued applications of fundamental theoretical significance for the analysis and interpretation of XPS and XAS spectra. These studies are important for the development of the fields of core-level spectroscopies as well as to advance our capabilities for applications of interest to our PNNL colleagues. An excellent example is our study of the surface core-level shifts, SCLS, for the surface and bulk atoms of an oxide that provides a new approach to understanding how the surface electronic of oxides differs from that in the bulk of the material. This work has the potential to lead to a new key to understanding the reactivity of oxide surfaces. Our theoretical studies use cluster models with finite numbers of atoms to describe the properties of condensed phases and crystals. This approach has allowed us to focus on the local atomistic, chemical interactions. For these clusters, we obtain orbitals and spinors through the solution of the Hartree-Fock, HF, and the fully relativistic Dirac HF equations. These orbitals are used to form configuration mixing wavefunctions which treat the many-body effects responsible for the open shell angular momentum coupling and for the satellites of the core-level spectra. Our efforts have been in two complementary directions. As well as the applications described above, we have placed major emphasis on the enhancement and extension of our theoretical and computational capabilities so that we can treat complex systems with a greater range of many-body effects. Noteworthy accomplishments in terms of method development and enhancement have included: (1) An improvement in our treatment of the large matrices that must be handled when many-body effects are treated. (2) Improvements and extensions of our capabilities for the calculation of the intensities of XPS and XAS transitions. And (3) ongoing development of flexible programs for the visualization of the theoretical spectra so that they can be compared with experiment. Our efforts on applications and methodology for these and related topics will continue under a sub-contract to PNNL.

• Research Organization: Univ. of North Texas, Denton, TX (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
• DOE Contract Number: FG02-04ER15508
• OSTI ID: 1257643
• Report Number(s): DOE-UNT--15508
• Country of Publication: United States
• Language: English

Related Subjects

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY
ABSORPTION SPECTROSCOPY
BONDING
CLUSTER MODEL
CONFIGURATION MIXING
COVALENCE
CRYSTALS
DIRAC EQUATION
ELECTRONIC STRUCTURE
EXCITED STATES
FINE STRUCTURE
HARTREE-FOCK METHOD
INTERMEDIATE COUPLING
IRON HYDROXIDES
IRON OXIDES
MANY-BODY PROBLEM
MATHEMATICAL SOLUTIONS
MATRICES
MULTIPLETS
ORBITS
OXIDES
PHOTOEMISSION
RARE EARTH COMPOUNDS
REACTIVITY
RELATIVISTIC RANGE
SIMULATION
SPECTRA
SPIN
SPINORS
SURFACES
TRANSITION METAL COMPOUNDS
URANIUM
URANIUM OXIDES
VALENCE
WAVE FUNCTIONS
X RADIATION
X-RAY PHOTOELECTRON SPECTROSCOPY
X-RAY SPECTROSCOPY