Refinement of the uranium dispersion corrections from anomalous diffraction

The evolution of the uranium chemical state in uranium compounds, principally
in the oxides, is of concern in the context of nuclear fuel degradation under
storage and repository conditions, and in accident scenarios. The U–O system
shows complicated phase relations between single-valence uranium dioxide
(UO 2 ) and different mixed-valence compounds (e.g. U 4O 9 , U3 O 7 and U 3 O8 ). To
try resolving the electronic structure associated with unique atomic positions, a
combined application of diffraction and spectroscopic techniques, such as
diffraction anomalous fine structure (DAFS), can be considered. Reported here
is the application of two newly developed routines for assessing a DAFS data
set, with the aim of refining the uranium X-ray dispersion corrections. High-
resolution anomalous diffraction data were acquired from polycrystalline
powder samples of UO 2 (containing tetravalent uranium) and potassium
uranate (KUO 3 , containing pentavalent uranium) using synchrotron radiation in
the vicinity of the U L3 edge (17.17 keV). Both routines are based on an
iterative refinement of the dispersion corrections, but differ in either using the
intensity of a selection of reflections or doing a full-pattern (Rietveld method)
refinement. The uranium dispersion corrections obtained using either method
are in excellent agreement with each other, and they show in great detail the
chemical shifts and differences in fine structure expected for tetravalent and
pentavalent uranium. This approach may open new possibilities for the assess-
ment of other, more complicated, materials such as mixed-valence compounds.
Additionally, the DAFS methodology can offer a significant resource optimi-
zation because each data set contains both structural (diffraction) and chemical
(spectroscopy) information, which can avoid the requirement to use multiple
experimental stations at synchrotron sources