High temperature 35Cl nuclear magnetic resonance study of the LiCl–KCl system and the effect of CeCl3 dissolution

Abstract

This paper examines the dynamics of the LiCl–KCl system over a range of temperatures in order to understand the local structure surrounding chlorine, which is the common ion in these systems, during molten salt pyro-processing. Chlorine-35 nuclear magnetic resonance (NMR) is sensitive to the local environments of the resonant nuclei and their motion on a diffusive timescale. Thus, it is a good probe of the atomic scale processes controlling the viscosities, diffusivities and conductivities of these molten salts. The average isotropic chemical shifts (^35Clδ) and spin-lattice relaxation times (T₁) of ³⁵Cl in (Li,K)Cl salt mixtures have been obtained over a compositional range of 0–100 mol% KCl with an interval of 10 mol% using high temperature nuclear magnetic resonance (NMR) spectroscopy from room temperature up to 890 °C. The ^35Clδ in the two end member salts are consistent with the cation–anion radius ratio as previously measured on the solid halides and the average radius ratio of cation to anion, can be used to explain the variation of ^35Clδ with composition. The quadrupolar interaction is found to be responsible for the spin-lattice relaxation of the ³⁵Cl, and the activation energies for T₁ relaxation have been obtained for all compositions. The measured T₁ (³⁵Cl) activation energies do not vary linearly with composition and peak at 50% KCl, which also coincides with the Chemla point for this system. They also are in good agreement with the values from equivalent conductivity measurements. To investigate the response of the system to solutes, 8 wt% of CeCl₃ was added to the pure LiCl as a surrogate actinide. The shift induced was 120 ppm and the activation energy for the T₁ (³⁵Cl) increased by a factor of four. This is a promising preliminary result for probing the effect of actinide dissolution on the dynamics of these pyro-processing salts.