Spectroscopic and Thermal Investigations of the Fluoroaluminate Complex Formation in NaF-KF and UF-NaF-KF Eutectics

The dissolution and complex formation of fluoroaluminates in two eutectic alkalifluoride mixtures, NaF-KF (FNAK) and LiF-NaF-KF (FLINAK), have been investigated by Raman, NMR, and thermal analysis. Melting and dissolution took place stepwise. The eutectic alkalifluoride mixtures with minor amounts of dissolved fluoroaluminate salts started melting at around 460 and 740 °C for FLINAK and FNAK mixtures, respectively. Total melting/dissolution of mixtures with 9-11 mol % aluminum fluoro salts added took place near 780 °C in the FLINAK solvent and at approximately 900 °C for FNAK solutions. The solidified melts were characterized by Raman bands at 561 (v1), 391 (v2), and 328 cm-1 (v5) and a 27Al NMR chemical shift near ppm 0 originating from isolated AIF6^3- octahedral ions. The Raman and NMR signals due to AIF6^3- were also observed at temperatures where the samples were only partly melted. Upon total melting, a pronounced dissociation of AIF6^3- into AIF5^2 and fluoride ions took place. At even higher temperatures, the equilibrium was displaced in favor of AlF5^2- in the FNAK solvent. The AlF5^2- ion was characterized by an intensive Raman band at 558 cm~" and an increasingly positive 27Al chemical shift with raising temperature, e.g., of 16 ppm at 935 °C.