Application of 19F NMR for the Structure Investigation of Fluoride Molten Salts

Molten fluorides (e.g., FLiBe) are widely employed in molten salt reactors as coolants and fuel solvents due to their excellent heat transfer properties and favorable neutronic characteristics. [Purpose]: This study aims to utilize the unique chemical characteristics of fluoride salts and apply high-resolution 19F nuclear magnetic resonance (NMR) techniques to investigate their microscopic structures and temperature-driven transformation mechanisms. [Methods]: High-resolution 19F NMR was combined with magic-angle spinning (MAS), high-temperature, and variable-temperature experiments to systematically characterize molten fluorides. Furthermore, the solid–liquid phase transition was investigated, and a quantitative 19F NMR method based on cooled solid samples was established to speculate the distribution and concentration of ionic structures in the molten state. [Results]: The results demonstrate that variations in chemical shifts observed in high-temperature 19F NMR spectra provide key structural information and reveal temperature-dependent transformation mechanisms. In addition, the proposed quantitative solid-state NMR approach successfully enabled the analysis of ionic structures of molten salt in high temperature, showing the promising applicability for evaluating the Lewis acidity and basicity of fluoride ions. [Conclusions]: By combining multiple 19F NMR analytical strategies, this study provides comprehensive insights into the composition, structural evolution mechanisms of molten fluorides, thereby offering a solid foundation for establishing component-structure–property relationships in molten salt systems.