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

Journal Article · Mon Mar 07 00:00:00 EST 2016 · Journal of Applied Physics

DOI:https://doi.org/10.1063/1.4943164· OSTI ID:22597013

Trowbridge, L.; Anderson, B. ^[1]; Kirkegaard, M. C. ^[1]; Herwig, K. W.; Mamontov, E. ^[2]

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1. Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (United States)
2. Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (United States)

The structural phase diagram of uranyl fluoride (UO{sub 2}F{sub 2}), while incomplete, contains at least one anhydrous crystal structure and a second, zeolite-like structure with the formula [(UO{sub 2}F{sub 2})(H{sub 2}O)]{sub 7} ⋅ (H{sub 2}O){sub 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 UO{sub 2}F{sub 2} + xH{sub 2}O) 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 UO{sub 2}F{sub 2} and [(UO{sub 2}F{sub 2})(H{sub 2}O)]{sub 7} ⋅ (H{sub 2}O){sub 4} are sufficiently different to distinguish them, with water in the latter executing a restricted diffusion (D = 2.7 × 10{sup −6} cm{sup 2}/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 [(UO{sub 2}F{sub 2})(H{sub 2}O)]{sub 7} ⋅ (H{sub 2}O){sub 4}, and we demonstrate the feasibility of extending this approach to aqueous forms of UO{sub 2}F{sub 2} + xH{sub 2}O. This method has general applicability to systems in which water content itself is a driving variable for structural or dynamical phase transitions.

• OSTI ID: 22597013
• Journal Information: Journal of Applied Physics, Vol. 119, Issue 9; Other Information: (c) 2016 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-8979
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
AIR FLOW
CLATHRATES
CRYSTAL STRUCTURE
CRYSTALS
DIFFRACTION
HUMIDITY
HUMIDITY CONTROL
NEUTRON REACTIONS
NEUTRONS
PHASE DIAGRAMS
PHASE TRANSFORMATIONS
QUASI-ELASTIC SCATTERING
URANYL FLUORIDES
VAPOR PRESSURE
WATER
WATER VAPOR
ZEOLITES