Probing the local structure of FLiBe melts and solidified salts by in situ high-temperature NMR

Abstract

The 2LiF-BeF₂ (FLiBe) salt melt is considered the primary choice for a coolant and fuel carrier for the generation IV molten salt reactor (MSR). However, the basics of ionic coordination and short-range ordered structures have been rarely reported due to the toxicity and volatility of beryllium fluorides, as well as the lack of suitable high-temperature in situ probe methods. In this work, the local structure of FLiBe melts was investigated in detail using the newly designed HT-NMR method. It was found that the local structure was comprised of a series of tetrahedral coordinated ionic clusters (e.g., BeF₄²⁻, Be₂F₇³⁻, Be₃F₁₀⁴⁻, and polymeric intermediate-range units). Li⁺ ions were coordinated by BeF₄²⁻ ions and the polymeric Be–F network through the analysis of the NMR chemical shifts. Using solid-state NMR, the structure of solid FLiBe solidified mixed salts was confirmed to form a 3D network structure, significantly similar to those of silicates. The above results provide new insights into the local structure of FLiBe salts, which verifies the strong covalent interactions of Be–F coordination and the specific structural transformation to the polymeric ions above 25% BeF₂ concentration.