Detection of covalency in Pu(IV) materials: spectroscopic and computational tools

Plutonium especially the tetravalent PuO2 compound plays an important role in many advanced nuclear fuel materials. Other applications are radioisotope batteries for example used as power supply in the mars rover Curiosity. Plutonium has a complex electronic structure, where the role of the 5f orbitals in chemical bonding and the level of covalency has not been understood in detail and is a very active field of research. Resonant inelastic X ray scattering (RIXS) is a valuable tool and can lead to a deeper understanding of the electronic structure of plutonium materials.1 High energy resolution X-ray absorption near edge structure (HR-XANES) has proven powerful in plutonium speciation studies.2
Relativistic quantum chemical computations for the Pu4+ ion and the PuO2 compound were performed. We discuss four computational measures of covalency of the Pu 5f orbitals. The XAS experiments on PuO2 and other Pu4+ compounds under various experimental conditions were conducted at the INE and ACT beamlines at the Karlsruhe research accelerator (KARA) at KIT in Karlsruhe Germany.3,4
We found that for PuO2 the Pu 5f covalent mixing with O valence orbitals overall is relatively small. ... mehrIt is the largest in the 5f(7/2) a2u orbital with the highest orbital energy. The analysis of the Pu M4,5-edge HR-XANES in combination with the calculated results, showed that the spectra can not be described with a simple one electron transition between individual 3d and 5f orbitals applying the dipole selection rule. Instead, because the initial and final states are not represented by single determinants, the dipole selection rule should be applied on the total wavefunctions. There is considerable amount of redistribution of 5f electrons involved in both the Pu M4-edge and the Pu M5-edge absorption processes. We found that 3d -> 5f(7/2) dominate for both M4 and M5 absorption edges. In addition, electrons are excited from the lower energy J = 5f(5/2) into higher orbital energy J = 5f(7/2), i.e. shake-up excitations take place. From the comparison of the computational results with the HR-XANES spectrum, it was found that the second peak in the Pu M4-edge and the shoulder feature in the Pu M5-edge spectrum are probing the 5f(7/2) a2u orbital and are therefore expected to be sensitive to bond variations. It will be discussed how the different interatomic interactions affect the Pu M4,5 edge core-to-core 3d4f RIXS map of PuO2. The here presented spectroscopic and theoretical tools will help to advance the understanding of the electronic structure of Pu materials.
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