Hidden Symmetry and Charge Disproportionation in U3O8

The room-temperature structure of triuranium octoxide (U3O8) was investigated by combining neutron total scattering with high-resolution neutron powder diffraction to compare its average and local structural organization. Rietveld refinement of the diffraction data confirms that the long-range structure is well-described by the established orthorhombic model (C2mm) without clear structural expression of uranium charge disproportionation. In contrast, analysis of the pair distribution function derived from the total scattering data indicates that the short-range structure is better described by a monoclinic distortion which remains closely related to the primitive cell of the orthorhombic C2mm phase. Within this lower-symmetry framework, the uranium sites separate into distinct coordination environments: two remain similar to one another, whereas the third exhibits a modified geometry characterized by shortened planar U-O bonds. Bond valence sum analysis of the coordination shells yields one U(VI)-like site and two U(V)-like sites, in agreement with prior spectroscopic observations, ab initio calculations, and theoretical treatments of mixed valence in U3O8. These results support the view that the structural articulation of charge disproportionation in U3O8 is expressed more clearly at the local than at the average structural level and illustrates the value of combining diffraction and total scattering approaches for mixed-valence oxides.