Radiation-Induced Solid-State Transformations of Uranyl Peroxides

Single-crystal X-ray diffraction studies of pristine and gamma-irradiated Ca-2[UO2(O-2)(3)]center dot 9H(2)O reveal site-specific atomic-scale changes during the solid-state progression from a crystalline to X-ray amorphous state with increasing dose. Following gamma-irradiation to 1, 1.5, and 2 MGy, the peroxide group not bonded to Ca2+ is progressively replaced by two hydroxyl groups separated by 2.7 angstrom (with minor changes in the unit cell), whereas the peroxide groups bonded to Ca2+ 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 H2O molecules and ionization of the peroxide O-O bond, which produces two hydroxyls, and allows isolation of the important monomer building block, UO2(O-2)(2)(OH)(2)(4-), 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, gamma-irradiation of solid Li-4[(UO2)(O-2)(3)]center dot 10H(2)O 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 H2O 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 Ca2+ countercations. Highly hydratable and mobile Li+ yields to uranyl hydrolysis reactions, while Ca2+ provides lattice rigidity, allowing observation of the first steps of radiation-promoted transformation of uranyl triperoxide.

Automated summary

Single-crystal X-ray diffraction studies of untreated and gamma-irradiated Ca-2[UO2(O-2)(3)]·9H2O show atomic-level changes as the material transitions from crystalline to amorphous with increasing irradiation dose. Irradiation leads to conversion of peroxide groups to hydroxyl groups through interactions with neighboring lattice water molecules. Comparisons of irradiation products of two different uranyl triperoxide monomers provide insights into the influence of Li+ and Ca2+ countercations on the transformation process.