Vibrational spectroscopy of uranium tetrafluoride hydrates

Abstract

Uranium tetrafluoride (UF4) is an important intermediate in the production of UF6 and nuclear fuel. Historical characterization of UF4 with Raman spectroscopy was plagued with ambiguity until the first accurate Raman spectrum was published by our group in 2016. Although generally considered to be relatively stable, UF4 can hydrolyze to form numerous UF4 hydrates that may play a role in future uranium waste forms. In contrast to anhydrous UF4, the hydrates, with their OH stretch and HOH bending modes, can be spectroscopically characterized by the type and degree of water bonding in the crystal lattice, which can yield additional information about their crystal structure. Herein, vibrational spectroscopy (Raman and infrared) was used to characterize three different UF4 hydrates: UF4(H2O)0.33, U3F12(H2O), and UF4(H2O)2.5. Spectra show the different hydrates vary in the number of observed bands, full-width half-maximum of the bands, and band intensity. These differences are due to varying interactions between the OH stretch and HOH bending modes with UF4 and the polymeric UF4 structure in the crystal lattice. These vibrational data, in combination with spectral fitting and crystallographic structures measured with powder X-ray diffraction and single crystal X-ray diffraction, provide unique details on the location of water molecules in the crystal lattice of hydrated UF4, and provide an interesting contrast to the vibrational spectra of anhydrous UF4.