Neptunium (Np) chemistry is underexplored compared to that of uranium, specifically considering the differences in redox properties of the material. Unlike uranium, neptunium is typically stable in as Np(V) in aqueous conditions but can also be stabilized in both the hexavalent and heptavalent states with the addition of oxidants. However, the redox properties of Np are specifically linked to the chemical environment and we are currently lacking in the specific interactions that control this behavior in aqueous systems. We hypothesize that extensive actinyl-cations influence the redox properties of Np in solid-state complexes and solution species. In the current study, we utilize single-crystal X-ray diffraction data in combination with Raman and IR spectroscopy to evaluate the behavior of Np(V), Np(VI), and Np(VII) complexes in the presence of Li+. Single-crystal X-ray diffraction demonstrates strong interactions between the Li+ and Np(VI) and Np(VII) complexes. The influence of Li+ on the actinyl cation symmetric stretch increases in order of U(VI)O22+ &lt; Np(VI)O22+ &lt; Np(V)O2+ , with Np(V)O2+ being the only actinyl cation that engages in disproportionation reactions with increasing LiCl concentration. We also explore the subtle differences in Np-O distances in the Np(VII)O4- units across a range of crystal structures as the result of Li+ interactions, which result in changes between D4h and D2h point groups and affect the number of vibrational bands we observe in vibrational spectra.

Impacts of Li+ interactions on the vibrational features of Np(V), Np(VI), and Np(VII) solutions and solid materials

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