Structural Phase Transitions and Water Dynamics in Uranyl Fluoride Hydrates

We report a novel production method for uranium oxyfluoride [(UO2)(7)F-14(H2O)(7)]center dot 4H(2)O, referred to as structure D. Structure D is produced as a product of hydrating anhydrous uranyl fluoride, UO2F2, through the gas phase at ambient temperatures followed by desiccation by equilibration with a dry environment. We follow the structure of [(UO2)(7)F-14(H2O)(7)]center dot 4H(2)O through an intermediate, liquid-like phase, wherein the coordination number of the uranyl ion is reduced to 5 (from 6 in the anhydrous structure), and a water molecule binds as an equatorial ligand to the uranyl ion. Quasielastic neutron scattering results compare well with previous measurements of mineral hydrates. The two groups of structurally distinct water molecules in D perform restricted motion on a length scale commensurate with the O-H bond (r = 0.92 angstrom). The more tightly bound equatorial ligand Waters rotate slower (D-r = 2.2 ps(-1)) than their hydrogen-bonded partners (D-r = 28.7 ps(-1)).