Structural Characterization of Uranium Tetrafluoride Hydrate (UF₄·2.5H₂O)

Abstract

Uranium tetrafluoride is an important intermediate in the nuclear fuel cycle. Facile synthesis of its hydrate, uranium tetrafluoride hydrate (UF 4 •2.5H 2 O), has recently been reported. The hydrate forms by contacting anhydrous UF 4 with neat H 2 O at room temperature for 24 h or by exposing anhydrous UF 4 to high relative humidity (>90%) conditions for several weeks. These pathways are of clear environmental relevance. Further understanding of the structure and optical spectra of UF 4 •2.5H 2 O, especially of the water molecules, is therefore necessary. Herein, the structure of UF 4 •2.5H 2 O was probed using time-of-flight neutron powder diffraction to improve understanding of the crystalline water environments in the structure. The complete structure was elucidated and compared to a previously reported partial structure for UF 4 •2.5H 2 O and a predicted complete structure from density functional theory. The crystalline structure exhibits three distinct water environments: two of the three water sites are bound to uranium, and the third water is unbound or "free". Furthermore, the completed structure reveals an extensive hydrogen bonding network involving water−fluorine and water−water interactions. One bound water site participates in hydrogen bonding with nearby fluoride ligands (O−H•••F−U), and the second bound water site participates in hydrogen bonding with the unbound water (O−H••• O) and a nearby fluoride ligand (O−H•••F−U); the unbound water participates in hydrogen bonding with bound water (O−H•••O− U). Low-temperature experiments and thermal analysis indicate UF 4 •2.5H 2 O is thermally stable from 10 to 358 K, undergoes dehydration at higher temperatures, and is nearly dehydrated at 473 K. Structural measurements provide foundational understanding and will inform future investigations of the thermal and environmental stability of UF 4 •2.5H 2 O.