Correlation between the electronic structure of uranium and their local environment in the crystal in complex uranium (V) oxides (PhD)

Actinide complex oxides with one or more metal ions in addition to an actinide are found in nuclear fuels as fission products that react with the fuel matrix. A detailed characterization of these oxides is helpful in understanding their behaviours during reactor operation and fuel disposal. This also implies the need for a more fundamental understanding of the actinide properties across its different possible valence states. Besides, preparation and characterization of the actinide complex oxides are academically important for the comprehension of the 5f electron behaviour as some of the uranium complex compounds have been found to exhibit superconductivity, catalytic properties in oxidation of organic compounds, and magnetic ordering. Understanding the relation between the electronic structure and the chemical bonding of actinide elements in compounds is necessary in order to correlate their properties to their chemical and physical origins. Because of the peculiar properties of the 5f electrons, much attention has been devoted to unravelling the interplay between the crystal structure and the actinide valence. Effects such as colossal magnetoresistance, superconductivity, magnetic ordering, intermediate valence behaviour, electron localisation and delocalisation, etc. have all been observed in actinide compounds and are related to the valence states.

Both from a theoretical as well as from an experimental point of view, a systematic study of the coupling between the local environment (and resulting crystal field) of the actinide in its compound and the resulting electronic structure leads to a better understanding of the behaviour of the 5f elements. Of the many uranium complex oxides, the alkali metal uranates MUO₃ (M = Li, Na, K, Rb) have attracted much attention because of the unique magnetic and optical properties of U⁵⁺. The local structure and oxidation state of U in some of these ternary uranium oxides including some ternary U⁵⁺ oxides have been studied by the authors and their colleagues at SCK•CEN [1-5]. In the present proposed research, we intend to extend our research from ternary systems with perovskites structure to quaternary systems with perovskites structure in an effort to systematically derive the relation between crystal chemistry and electronic structure.

The ternary oxide BaU⁴⁺O₃ with ordered perovskite crystal structure is an ideal crystal system to begin such a systematic study, because:

1)    Replacing half of the uranium ions with trivalent rare earth ions forms the ordered perovskites Ba₂M³⁺U⁵⁺O₆ (M = rare earth metal: Sc, Y, Gd, Yb, Mn…), oxidizing the rest of the uranium ions to the pentavalent state.

2)    Replacing one-third of the uranium ions with divalent alkaline earth ions also formed ordered perovskites Ba₃M²⁺(U⁵⁺)₂O₉ (M = alkaline earth metal: Ca, Sr…), while  oxidizing the rest of the uranium ions to the pentavalent state.

Further variations include the extension to other ternary crystal systems, such as fluorite with pentavalent uranium M³⁺U⁵⁺O₄ (M = rare earth metal: Sc, Y, Gd, Yb, Mn…) and M²⁺(U⁵⁺)₂O₆ (M = alkaline earth metal: Ca, Sr…), in which the local uranium environment again differs from the perovskites.

1.         J.-H. Liu, S. Van den Berghe, and M. J. Konstantinović, XPS spectra of the U⁵⁺ compounds KUO₃, NaUO₃ and Ba₂U₂O₇. J. Solid State Chem., 2009. 182: p. 1105-1108.

2.         A.V. Soldatov, D. Lamoen, M.J. Konstantinovic, S. Van den Berghe, A.C. Scheinost, and M. Verwerft, Local structure and oxidation state of uranium in some ternary oxides: X-ray absorption analysis. J. Solid State Chem., 2007. 180: p. 54-61.

3.         S. Van den Berghe, A. Leenaers, and C. Ritter, Antiferromagnetism in MUO₃ (M = Na, K, Rb) studied by neutron diffraction. J. Solid State Chem., 2004. 177: p. 2231-2236.

4.         S. Van den Berghe, J.-P. Laval, B. Gaudreau, H. Terryn, and M. Verwerft, XPS investigations on cesium uranates: mixed valency behaviour of uranium. J. Nucl. Mater., 2000. 277: p. 28-36.

5.         S. Van den Berghe, M. Verwerft, J.-P. Laval, B. Gaudreau, P. G. Allen, and A. Van Wyngarden, The Local Uranium Environment in Cesium Uranates: A Combined XPS, XAS, XRD, and Neutron Diffraction Analysis J. Solid State Chem., 2002. 166: p. 320-329.

Having in mind both industrial and scientific significance of the actinides we propose here to study systematically the correlation between the electronic structure and the local environment of U (and resulting crystal field) in a set of complex U (V) compounds, starting with the  Ba₂M³⁺U⁵⁺O₆ & Ba₃M²⁺(U⁵⁺)₂O₉ systems.

Experimental approach:

·         Compound preparation using the thermochemistry laboratory available at SCK•CEN, including dedicated oxygen potential control of the furnace atmospheres to prepare very pure monophasic samples.  Sufficient experience exists with such preparations and preliminary test on preparations in the desired systems have been successful.

·         XPS (X-ray Photoelectron Spectroscopy) experiments. XPS (UPS) spectra of the actinide 4f core level and valence band to probe the electronic structure. XPS and UPS experiments will be mainly performed at SCK•CEN.

·         Neutron, electron and X-ray diffraction. These techniques will provide structural information on the compounds. Neutron diffraction campaigns can be performed at the Laboratoire Léon Brillouin (LLB) and at the Institut Laue-Langevin (ILL). X-ray and electron diffraction experiments can be done at the laboratories of SCK•CEN.

·         XAS (X-ray absorption spectrometry) experiments. EXAFS and XANES spectra at the actinide L₃ edge will enable to determine the oxidation state and the coordination of the probed actinide atoms in the oxides. These research campaigns are planned at the European Synchrotron Radiation Facility (ESRF) in collaboration with Dr. A.V. Soldatov from the Rostov State University, with whom previous collaborations exist.

·         Optical spectroscopy (Raman, UV) experiments. The sensitivity of optical spectroscopy techniques to the valence state of atoms provides an additional method for investigating the behavior of the f-electrons in actinide compounds. Such experiments can be performed at the laboratories of the Katholieke Universiteit Leuven (KUL) and possibilities for collaboration are under investigation.

Theoretical approach

Because SCK•CEN has no immediate interest to start theoretical work on electronic structure calculations, collaborations have been established in the past with the Universities of Antwerp (Prof. Lamoen) and Groningen (Prof. R. Broer), as well as with the University of Texas (Prof. E. Ilton and P. Bagus) to perform such calculations.  The exchange of information between experimentalists and theoreticians is of course of vital importance to validate theoretical approaches and to pilot experimental efforts, which is why they are included in this project.

·         Calculation of XPS spectra by embedded cluster calculation methods involving fully relativistic Dirac-Fock self-consistent field (DF-SCF) theory and Dirac Configuration Interaction (DCI) wave functions. (University of Groningen)

·         Ab initio calculations of electronic structure and XAS spectra. The calculation of XAS spectra requires a precise treatment of the electronic excited states of the materials. Calculations are performed with the full potential “Linearized Augmented Plane Wave method” (LAPW) within DFT and with the “Multiple Scattering” (MS) approach. Core-hole effects are taken into account. Special attention will be paid to the description of electronic exchange-correlation interactions within the LDA+U approximation for each compound considered. (University of Antwerp)