Oxidation and crystal field effects in uranium

Abstract

An extensive investigation of oxidation in uranium has been pursued. This includes the utilization of soft X-ray Absorption Spectroscopy (XAS), hard X-ray Absorption Near Edge Structure (XANES), Resonant (hard) X-ray Emission Spectroscopy (RXES), Cluster Calculations and a Branching Ratio analysis founded on Atomic Theory. The samples utilized were uranium dioxide (UO 2 ), uranium trioxide (UO 3 ) and uranium tetrafluoride (UF 4 ). A discussion of the role of non-spherical perturbations, i.e. crystal or ligand field effects, will be presented.

Observation of 5f intermediate coupling in uranium x-ray emission spectroscopy

“…U M 4,5 Extended x-ray Absorption Fine Structure (EXAFS) measurements beyond the main (whiteline) absorption features were used as a method to monitor the oxidation condition and sample composition. Although similar, the EXAFS of these two compounds with isoelectronic U 5f 2 sites, UF 4 and UO 2 , are easily differentiated, consistent with earlier studies [11,18]. The x-ray Photoelectron Spectroscopy data used for comparison came from an earlier study of a very pure sample, as described elsewhere [19,20].…”

“…Each of these excitation energies was chosen to be well above threshold for the corresponding transition. The samples used were the same as those used in earlier RXES, XAS and HERFD studies [11,18]. Oxidation and sample corruption can be a problem with uranium samples, but that was not an issue here, in part because both samples were already oxidized by either oxygen or fluorine.…”

“…I d3/2 /I d5/2 , is very large, on the scale of a factor of 5. This ratio is over twice the value of the N 5 :N 4 peak ratio (∼2) that underlies the reported XAS Branching Ratio of UF 4 of 0.68 [11,12]. Below, the analysis process will be discussed, including the comparison to FEFF calculations [13][14][15][16], normalization via the 6p XES features and peak fitting for the extraction of intensities.…”

“…Thus, the impact of Intermediate Coupling has been successfully monitored using XAS as a probe of the U5f Unoccupied Density of States (UDOS) [3,4,11]. These effects manifest themselves as a strong variation of the Branching Ratios (BR) as a function of atomic number and 5f occupation, driven by electric dipole selection rules.…”

Probing5f-state configurations inURu2Si2with ULIII-edge reso

“…EELS experiments suggest that URu 2 Si 2 has 5f states which are more localized than α-U (even at room temperature), but still not completely localized, with a 5f electron count n f = 2.7 ± 0.1 suggesting a mixed valence ground state and/or some 5f -electron itinerancy. 31 However, other interpretations question the usefulness of EELS in this respect in uranium compounds, 32 implying that further support is needed. In addition, the degree of surface sensitivity in this electron spectroscopy technique remains a concern.…”