High-temperature NMR Investigation of the Structural Evolution and the Special Phase Transition in LiF-NaF-BeF2 Mixed Salt Melts

Abstract

Molten salt mixtures of LiF, NaF, and BeF2 are widely recognized as potential solvents and coolants in molten salt reactor applications. The structural effects of LiF addition to the ternary salt were investigated using HT-NMR and solid-state NMR techniques. A distinct phase transition was identified via HT-NMR during the melting process of LiF-NaF-BeF2 ternary salts. The results indicated that the addition of LiF facilitates the transition from a crystalline to an amorphous structure. The influence of Li+ and Na+ on the amorphous structure was analyzed, revealing that Li+ ions exhibit relatively strong interactions with Be-F oligomers. Furthermore, as temperature increases, the rapid dynamics weaken the interactions between Li+ ions and Be-F oligomers. This dynamic weakening results in the remarkable phase transformation of Be-F oligomers into polymeric chains and networks.