Quasielastic neutron scattering with in situ humidity control: Water dynamics in uranyl fluoride

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.