Abstract
The structural phase diagram of uranyl fluoride (UO2F2), while incomplete, contains at least one anhydrous crystal structure and a second, zeolite-like structure with the formula [(UO2F2)(H2O)]7 ⋅ (H2O)4 that can be produced by adding water to the anhydrous structure. While traditional diffraction measurements can easily differentiate these crystals, additional aqueous structures (in general of the form UO2F2 + xH2O) have been proposed as well. We present results using a novel sample environment setup to intercalate water during a quasielastic neutron scattering measurement over the course of 86 h. Our sample environment allows low-pressure (<2 atm) humid air flow across the sample coupled with a system to control the relative humidity of this air flow between 10% and 70%. The water dynamics in UO2F2 and [(UO2F2)(H2O)]7 ⋅ (H2O)4 are sufficiently different to distinguish them, with water in the latter executing a restricted diffusion (D = 2.7 × 10−6 cm2/s) within the structure's accessible pores (r = 3.17 Å) such that the dynamics can be used as a fingerprinting tool. We confirm that water vapor pressure is the driving thermodynamic force for the conversion of the anhydrous structure to [(UO2F2)(H2O)]7 ⋅ (H2O)4, and we demonstrate the feasibility of extending this approach to aqueous forms of UO2F2 + xH2O. This method has general applicability to systems in which water content itself is a driving variable for structural or dynamical phase transitions.
Highlights
Uranyl fluoride is a hygroscopic mineral structure with several known crystal hydrates.1–4 The structures of these hydrates were studied as early as 1948: Zachariassen initially solved the high-symmetry anhydrous crystal structure (UO2F2, space group R3m, a 1⁄4 5.755 A, a 1⁄4 42.5) via neutron diffraction.4,5 Anhydrous UO2F2 contains the linear [UO2]2þ ion coordinated equatorially with six fluoride ligands, polymerizing into a hexagonal, three-layer structure
We confirm that water vapor pressure is the driving thermodynamic force for the conversion of the anhydrous structure to [(UO2F2)(H2O)]7 Á (H2O)4, and we demonstrate the feasibility of extending this approach to aqueous forms of UO2F2 þ xH2O
We have previously used quasielastic neutron scattering (QENS), which is sensitive to the dynamics of hydrogen, to identify the relative quantities of bound and free water and their associated diffusion rates in both the anhydrous UO2F2 and its hydrate [UO2F2(H2O)]7 Á (H2O)4.1,2 The water dynamics in each of these crystal forms are of sufficiently different character to use the spectra themselves to identify the underlying crystal structure
Summary
Uranyl fluoride is a hygroscopic mineral structure with several known crystal hydrates. The structures of these hydrates were studied as early as 1948: Zachariassen initially solved the high-symmetry anhydrous crystal structure (UO2F2, space group R3m, a 1⁄4 5.755 A , a 1⁄4 42.5) via neutron diffraction. Anhydrous UO2F2 contains the linear [UO2]2þ ion coordinated equatorially with six fluoride ligands, polymerizing into a hexagonal, three-layer structure. Uranyl fluoride is deliquescent and exists as a saturated salt in the presence of sufficient water.. Uranyl fluoride is deliquescent and exists as a saturated salt in the presence of sufficient water.10,11 The structure of these configurations has been approached primarily from a computational perspective, with only a small number of extended X-ray absorption fine structure measurements composing the experimental structural data.. We have previously used quasielastic neutron scattering (QENS), which is sensitive to the dynamics of hydrogen, to identify the relative quantities of bound and free water and their associated diffusion rates in both the anhydrous UO2F2 and its hydrate [UO2F2(H2O)]7 Á (H2O)4.1,2 The water dynamics in each of these crystal forms are of sufficiently different character to use the spectra themselves to identify the underlying crystal structure. We establish the feasibility of these measurements for future applications to saturated solutions and the aqueous state
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
