Radiation-Induced Solid-State Transformations of Uranyl Peroxides.

Single-crystal X-ray diffraction studies of pristine and γ-irradiated Ca[UO(O)]·9HO reveal site-specific atomic-scale changes during the solid-state progression from a crystalline to X-ray amorphous state with increasing dose. Following γ-irradiation to 1, 1.5, and 2 MGy, the peroxide group not bonded to Ca is progressively replaced by two hydroxyl groups separated by 2.7 Å (with minor changes in the unit cell), whereas the peroxide groups bonded to Ca cations are largely unaffected by irradiation prior to amorphization, which occurs by a dose of 3 MGy. The conversion of peroxide to hydroxyl occurs through interaction of neighboring lattice HO molecules and ionization of the peroxide O-O bond, which produces two hydroxyls, and allows isolation of the important monomer building block, UO(O)(OH), that is ubiquitous in uranyl capsule polyoxometalates. Steric crowding in the equatorial plane of the uranyl ion develops and promotes transformation to an amorphous phase. In contrast, γ-irradiation of solid Li[(UO)(O)]·10HO results in a solid-state transformation to a well-crystallized peroxide-free uranyl oxyhydrate containing sheets of equatorial edge and vertex-sharing uranyl pentagonal bipyramids with likely Li and HO in interlayer positions. The irradiation products of these two uranyl triperoxide monomers are compared via X-ray diffraction (single-crystal and powder) and Raman spectroscopy, with a focus on the influence of the Li and Ca countercations. Highly hydratable and mobile Li yields to uranyl hydrolysis reactions, while Ca provides lattice rigidity, allowing observation of the first steps of radiation-promoted transformation of uranyl triperoxide.