Probing Oxygen Vacancy Formation in U₁₋ₓLnₓO₂±δ with NIXS and Electronic Structure Calculations

Lanthanides (Ln) are critical resources for renewable energy technologies but their supply is limited, with most reserves located in China. Their recovery from spent nuclear fuel, where they appear as fission products of UO₂, is essential. Within the MaLaR European project, we investigate the selective separation of actinides (An) and Ln by studying their interactions with novel 2D–3D materials. A key step is understanding Ln/An speciation in solid UOₓ and their behavior during dissolution. While incorporation of light Ln in UO₂ is relatively well studied, heavy Ln (beyond Gd) remain poorly explored, particularly regarding incorporation limits, lattice distortions, and defect formation. These features critically influence solid/solution interfacial reactivity. Our work focuses on U₁₋ₓLnₓO₂±δ compounds with the smallest, heaviest Ln, up to Lu, using NIXS at the oxygen K-edge combined with electronic structure calculations to elucidate oxygen vacancy formation mechanisms.