Gamma-Ray-Induced Formation of Uranyl Peroxide Cage Clusters

Aqueous solutions of lithium uranyl triperoxide, Li-4[UO2(O-2)(3)] (LiUT), were irradiated with gamma rays at room temperature and found to form the uranyl peroxide cage cluster, Li-24[(UO2)(O-2)(OH)](24) (Li-U-24). Raman spectroscopy and O-18 labeling were used to identify the Raman-active vibrations of LiUT. With these assignments, the concentration of LiUT was tracked as a function of radiation dose. A discrepancy between monomer removal and cluster formation suggests that the reaction proceeds by the assembly of an intermediate. Non-negative matrix factorization was used to separate Raman spectra into components and resulted in the identification of a unique intermediate species. Much of the conversion appears to be driven by water radiolysis products, particularly the hydroxyl radical. This differs from the O-18-labeled copper-catalyzed formation of U-24, which progresses at a steady rate with no observation of intermediates. Li-U-24 in solution decomposes at high radiation doses resulting in a solid insoluble product similar to Na-compreignacite, Na-2(UO2)(6)O-4(OH)(6)center dot 7H(2)O, which contains uranyl oxyhydroxy sheets.