On the origin of low-valent uranium oxidation state

Abstract Recently, numerous novel actinide compounds with exotic oxidation states have been reported, highlighting the significant role of 5f electrons in actinide bonding and the formation of new actinide systems. However, efforts to understand 5f electron systems are still hindered by the lack of high-quality experimental data, particularly for low-valent actinide compounds. In this study, we present the first measurements of X-ray absorption near-edge structure (XANES) in the high-energy resolution fluorescence detection (HERFD) mode at the uranium (U) M4 edge for the UIII and UIV halides, namely UX3 and UX4 (X = F, Cl, Br, I). The spectral shapes of these two halide families exhibit clear differences, which can be comprehended within the framework of crystal field multiplet theory. Electronic structure calculations were conducted for the 3d-4f Resonant Inelastic X-ray Scattering (RIXS) process, considering various strengths of electron-electron interactions between 3d, 4f, and 5f states. Our results confirm the capability of the HERFD XANES method at the M4 edges to detect the presence of low-valent compounds. Moreover, contrary to earlier expectations of increased ionicity in low-valent uranium compounds, our study reveals that the electronic structure of low-valent U systems exhibits heightened sensitivity to the influence of ligands surrounding the U atoms.