Structural Phase Transitions and Water Dynamics in Uranyl Fluoride Hydrates

Abstract

We report a novel production method for uranium oxyfluoride [(UO2)7F14(H2O)7]·4H2O, 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)7F14(H2O)7]·4H2O 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 Å). The more tightly bound equatorial ligand waters rotate slower (Dr = 2.2 ps(-1)) than their hydrogen-bonded partners (Dr = 28.7 ps(-1)).

Analysis of Water Coupling in Inelastic Neutron Spectra of Uranyl Fluoride

“…The existence of higher energy molecular vibrations and a hydrogen recoil peak confirm the presence of water in the experimental sample. As surface-adsorbed water was removed prior to experiment, the remaining water is further evidence that the layered UO 2 F 2 crystal supports the localization of water from the environment, either during formation or immediately afterwards 1,3 . In Fig.…”

“…When exposed to atmospheric water vapor UF 6 converts into uranyl fluoride (UO 2 F 2 ), releasing hydrogen fluoride (HF). Recently, we showed that the hygroscopic UO 2 F 2 adsorbs environmental water molecules in a crystal hydrate form 1 . These waters have a significant effect on spectroscopic signatures, in particular the Raman spectra, but accessing this information quantitatively proves difficult for a number of experimental techniques due to various intrinsic limitations based on cross sections or selection rules for scattering.…”

“…At ambient conditions anhydrous uranyl fluoride is a hygroscopic powder that can undergo a hydration reaction resulting in uranyl fluoride hydrate 2 . The transition to the hydrated state can be reversed through annealing under vacuum conditions at 115 °C 1 . The exact locations of retained water molecules in the anhydrous structure were heretofore unknown, but the appearance of stacking disorder in x-ray and neutron diffraction patterns offered tantalizing hints of interlayer waters 2,3 .…”

A NanoSIMS 50 L Investigation into Improving the Precision and Accuracy of the 235U/238U Ratio Determination by Using the Molecular 235U16O and 238U16O Secondary Ions

“…Materials analyzed in this study: A variety of uranium compounds, all produced at Oak Ridge National Laboratory [17][18][19][20], (UO 2 , UO 2 F 2 , UO 3 , UO 2 (NO 3 ) 2 •6(H 2 O), and UF 4 ) as well as two silicate reference materials of known uranium isotope composition (the NIST-610 glass as well as the Plesovice zircon [17]). The NIST-610 glass and Plesovice zircons are widely used reference materials in the geosciences.…”

“…UF 6 was released into a chamber at approximately 23% relative humidity at 24 • C. Particulate UO 2 F 2 was collected on vinyl collection surfaces. The matter was calcined overnight under a stream of 10 mL/s N 2 gas at 150 • C, which has previously been shown to produce anhydrous uranyl fluoride [18].…”