Versatile Cell Design for Molten Fluoride Salt Spectroscopy: Investigating Metal-Ion Speciation in Molten Fluoride Salts

Fluoride-based molten salts are widely used in industrial applications including aluminum production, thermal energy storage, optical crystal growth and advanced nuclear reactor designs. Despite the wide range of uses, fundamental understanding of coordination chemistry and methods for probing molten fluorides are scarce, likely due to the difficulty of probing fluoride melts with spectroscopic probes. Performing spectroscopic measurements of fluoride-based salts is challenging due to the highly corrosive nature of these salts, which can degrade many common optical materials. In this work, we present a versatile optical cell design that enables spectroscopic measurements corrosive melts. This innovative cell design overcomes the challenges posed by the corrosive nature of the salts, allowing for accurate and consistent spectroscopic analysis. This work reports temperature dependent absorption measurements for Co2+, Ni2+, and Cr3+ analytes in LiF-NaF-KF eutectic salt (i.e., FLiNaK), which are common corrosion products originating from structural alloys in molten-fluoride handling. Absorption spectra were used to understand interactions of these analytes with FLiNaK, particularly ligand field coordination. The analysis of absorption spectra was complemented by structural analyses using ab initio molecular dynamics (AIMD) simulations, providing deeper insights into the behavior of the analytes in FLiNaK. Our findings indicate that the analytes studied in this work exist in octahedral or near-octahedral coordination states that remain stable across the temperature range of 500-600°C. This work highlights not only an applied solution to performing optical spectroscopy in corrosive, high-temperature melts, but provides important fundamental insight on coordination behavior of transition metal species in molten fluorides.