In Situ High-Temperature Raman Spectroscopy of UCl₃: A Combined Experimental and Theoretical Study

Uranium trichloride (UCl₃) has received growing interest for its use in uranium-fueled molten salt reactors and in the pyrochemical processing of used fuel. In this paper, we report for the first time the experimentally determined Raman spectra of UCl₃, at both ambient condition and in situ high temperatures up to 871 K. The frequencies of five of the Raman-active vibrational modes (v) of UCl₃ exhibit a negative temperature derivative ((∂ν/∂T)) with increasing temperature. This red-shift behavior is likely due to the elongation of U-Cl bonds. The average isobaric mode Grüneisen parameter (γ = 0.91 ± 0.02) of UCl₃ was determined through use of the coefficient of thermal expansion published in Vogel et al. (2021) and the (∂ν/∂T) values determined in this study. These results are in general agreement with those calculated here by density functional theory (DFT+U). Finally, a comparison of the ambient band positions of UCl₃ to those of isostructural lanthanide (La-Eu) and actinide chlorides (Am-Cf) has been made.