Progressive oxidation in the U-O system: Structural relations around the U3O7 phase field

International audience; The physicochemical properties of uranium are being studied in many application fields, far beyond that of its original use as nuclear fuel. Owing to its complex electronic structure, with valence band electronic states associated with 5f and 6d orbital occupancy, uranium-containing compounds can be synthesized with either charge-insulating, semi-conducting, or even superconducting electronic properties [1]. Similarly, uranium shows exciting coordination chemistry based on a wide range of possible oxidation states, leading to the development of novel catalysts [2]. The binary U−O solid-state system has remained of particular interest, but despite almost a decade of ongoing research it continues to challenge us.It is quite well known that uranium(IV)oxide (UO2) is susceptible to oxidation under varying conditions. A wide non-stoichiometric solid solution phase-field (UO2±x) develops at temperatures above 700 K. In the low-temperature region, structural phase transformations occur, leading to the formation of mixed-valence compounds, commonly referred to as U4O9, U3O7, and U3O8. The oxides UO2±x, U4O9, and U3O7 are structurally closely related to a fluorite-type lattice, but the incorporation of excess anions results in local perturbations that eventually develop a long-range periodical order. The transformation to U3O8 induces a more drastic crystallographic reorganization associated with a considerable volume increase.In the progressive oxidation of the U−O system at ambient pressures, U3O7 is the last member with a fluorite-related structure, which makes it a tipping point. An understanding of the U3O8 oxidation reconstruction mechanism can be gained with the accurate characterization of the U3O7 phase. In pursuit of this goal, we recently performed a series of neutron scattering experiments and combined the analysis with electronic structure calculations [3]. The U3O7 phase showed a more pronounced charge-transfer insulating flavor compared to the usual Mott insulator picture of UO2. In accordance with recent X-ray absorption spectroscopy results [4], the structure refined from the present data contains atomic arrangements associated mainly with tetra- and pentavalent uranium states. However, one peculiarly compact uranium environment carries a hexavalent uranium state as indicated by Hirshfeld and bond-valence sum analysis of the charge densities. The localization of these atoms might have significant implications for the physicochemical properties of this uranium oxide compound. In this contribution, we will present new insights on the structural relations between the phases enveloping the U3O7 phase field, which are essential to foster our understanding of the complex oxidation processes occurring in the U−O system.[1] S. Ran, et al., Science, 365 (2019) 684.[2] A.R. Fox, S.C. Bart, K. Meyer, C.C. Cummins, Nature, 455 (2008) 341.[3] G. Leinders, G. Baldinozzi, C. Ritter, R. Saniz, I. Arts, D. Lamoen, M. Verwerft, Manuscript submitted to Inorg. Chem.[4] G. Leinders, R. Bes, K.O. Kvashnina, M. Verwerft, Inorganic Chemistry, 59 (2020) 4576.