Recently, we published an article on the efficacy of a model for the unoccupied 5f electronic structure of the early actinides1 based upon the bremsstrahlung isochromat spectroscopy (BIS) of Th by Baer and Lang.2 The comparisons were for systems with n = 2, 3, and 5, where n is the 5f occupation number. The question has arisen: Will the Th model work for n < 2? The short answer is yes, as can be seen in Fig. 1, with the caveat that the system must contain neither U multiple bonds nor U multiple oxidation states.
As can be seen in Fig. 1, there is substantial agreement between the Th model and the spectra for UO2 (Ref. 3) and U3O8 (Ref. 4). The U3O8 spectrum of Le Pape et al.4 is almost identical to that published earlier by Kvashnina and co-workers.5 However, there is no agreement between the U(VI) uranyl cases4–6 and the Th model. This is not surprising. For the uranyl U(VI) oxidation state, n = 0 in the completely ionized limit and arguably this should correspond to the Th case with n5/2 = 0. However, the uranyl linear moiety, O=U=O, is a unique commodity with multiple bonds (possible bond order of 2–3) and additional chelation in the U plane perpendicular to the uranium–oxygen bonds.7 A sketch of a representative uranyl M4 HERFD spectrum is shown as an inset in Fig. 1. This sketch is very strongly similar to each of the uranyl spectra in Refs. 4–6, although each of these are from a different chemical sample. The multiple peak structure rules out any agreement with the Th model.
UO2 and U3O8 are what might be considered simple cases. Both are thermodynamically favored.8–11 UO2 has a particularly simple structure with only one U site in the fluorite lattice.12 It has been suggested that U3O8 has two sites but with the same oxidation state.11 Thus, there may be broadening in the U3O8 HERFD from the two sites. They may have the same oxidation state but slightly different energies.
The importance of one oxidation state is driven home by the case of U4O9. The HERFD of U4O9 of Kvashnina et al.5 also has more than one peak, thus rendering agreement with the Th model impossible without additional information. β-U4O9 is a modified form of the UO2 fluorite structure, with the additional oxygens driving the generation of a cuboctahedron substructure within the extended lattice.13–15 α-U4O9 has a trigonal distortion of β-U4O9.14 For β-U4O9, there are multiple inequivalent sites for U.15
Stanford Synchrotron Radiation Light-source is a national user facility operated by Stanford University on behalf of the DOE and OBES. LLNL is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration, under Contract No. DE-AC52-07NA27344.
The data that support the findings of this study are available from the corresponding author upon reasonable request.