High-temperature and solid-state NMR investigation of the structural evolution and special phase transition in LiF–NaF–BeF2 mixed salts

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 using 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 the temperature increases, the rapid dynamics weaken the interactions between Li⁺ ions and Be–F oligomers. This weakening of interactions results in the remarkable phase transformation of Be–F oligomers into polymeric chains and networks.