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

Abstract

Uranium trichloride (UCl 3 ) 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 UCl 3 , at both ambient condition and in situ high temperatures up to 871 K. The frequencies of five of the Raman-active vibrational modes (v i ) of UCl 3 exhibit a negative temperature derivative ((∂ν i / ∂T) P ) with increasing temperature. This red-shift behavior is likely due to the elongation of U−Cl bonds. The average isobaric mode Gruneisen parameter (γ iP = 0.91 ± 0.02) of UCl 3 was determined through use of the coefficient of thermal expansion published in Vogel et al. (2021) and the (∂ν i /∂T) P 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 UCl 3 to those of isostructural lanthanide (La−Eu) and actinide chlorides (Am−Cf) has been made.

Transient Covalency in Molten Uranium(III) Chloride

“…The important feature of covalent interactions is their directionality, usually giving rise to absorption bands in the infrared or Raman spectra . Our attempts to obtain experimental Raman results for the molten UCl 3 were unsuccessful, due to the strong self-absorption by the sample . Nevertheless, to investigate whether the dynamic, metastable coordination environment of molten UCl 3 , especially the transient inner subshell, is spectroscopically resolvable, we computed Raman spectra from the AIMD trajectory using the Berry phase formalism .…”

“…9 Our attempts to obtain experimental Raman results for the molten UCl 3 were unsuccessful, due to the strong self-absorption by the sample. 32 Nevertheless, to investigate whether the dynamic, metastable coordination environment of molten UCl 3 , especially the transient inner subshell, is spectroscopically resolvable, we computed Raman spectra from the AIMD trajectory using the Berry phase formalism. 33 This method has previously shown very good reproducibility of the experimental data for various molten salt systems.…”