HT-NMR Studies of the Be-F Coordination Structure in FNaBe and FLiBe Mixed Salts

Molten NaF-BeF₂ salt is widely considered a promising candidate to replace FLiBe in molten salt reactor applications, which is crucial to reducing the operating costs of the molten salt reactor. Studies on beryllium compounds are rarely conducted due to their volatility and high toxicity. Herein, the Be-F coordination structure of NaF/BeF₂ mixed salts was investigated in-depth through various HT-NMR and solid-state NMR methods, which are optimized to be appropriate for the detection of beryllium compounds. It was found that Na₂BeF₄ and NaBeF₃ crystals were transformed into amorphous tetrahedral coordinated networks when there was an increase in the BeF₂ concentration in the mixed salts. The main coordinate structure comparisons between FNaBe and FLiBe were analyzed, which exhibit high similarity due to the covalent effect of Be-F bonding, demonstrating the theoretical feasibility of applying FNaBe salts as a substitute for FLiBe in MSR systems. In addition, the transition from the crystal phase to the amorphous phase occurred at a lower BeF₂ concentration for FNaBe than that for FLiBe. This was further verified by the results of ab initio molecular dynamics (AIMD) simulation that FNaBe melts had more disordered structures, thus causing slight changes in their physical properties.