The crystal structure and chemical formula of ammonium diuranate (ADU), general formula (xUO₃●yNH₃●zH₂O), is known to vary as a function of the synthetic conditions used to prepare the material. We prepared ADU through an ammonium hydroxide (NH₄OH) precipitation route to form 3UO₃●NH₃●5H₂O, which was verified by powder X-ray diffraction and thermogravimetric analysis. Our interest in doing so, was to determine the exact nature of the uranyl (UO₂²⁺) metal-coordination environment, and the identity of the ligands and non-coordinated species. We have therefore examined the pentahydrate form of ADU using photoluminescent (PL) spectroscopy to study the coordination environment and, moreover, determine if unexpected species like hydroxide or ammonium are present. The PL analysis revealed two distinct emissive components in the blue (440 nm) and green (535 nm) regions. The blue-centered emission is surprising because coordinated water is not sufficiently reducing to cause this behavior. Interestingly, cooling the specimen to 77 K suppressed the blue-centered emission, implying a ligand and/or temperature dependence. This behavior was studied further via the intentional dehydration of 3UO₃●NH₃●5H₂O by heat treatment to form a dehydrated species, presumably 3UO₃●NH₃●1.5H₂O, because the hydration state of ADU is known to causes changes in the UO₂²⁺ coordination environment. Reexamination by PL revealed the loss of the green emissive component at ca. 550 nm, and a notable increase in the intensity in the blue region at ca. 450 nm. This suggests the loss of water caused a redistribution of reducing ligands to the equatorial binding sites on the metal center. Presented here will be our investigation of the uranyl bonding environment and composition of ADU and the associated spectroscopic and structural data.

Parker Okabe

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