Spectroscopic and thermal investigations of the fluoroaluminate complex formation in NaF-KF and LiF-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 740degreesC for FLINAK and FNAK mixtures, respectively. Total melting/dissolution of mixtures with 9-11 mol% aluminum fluoro salts added took place near 780degreesC in the FLINAK solvent and at approximately 900degreesC for FNAK solutions. The solidified melts were characterized by Raman bands at 561 (vi), 391 (nu(2)), and 328 cm(-1) (nu(5)) and a Al-27 NMR chemical shift near 0 ppm originating from isolated AIF(6)(3-) octahedral ions. The Raman and NMR signals due to AIF(6)(3-) were also observed at temperatures where the samples were only partly melted. Upon total melting, a pronounced dissociation of AIF(6)(3-) into AIF(5)(2-) and fluoride ions took place. At even higher temperatures, the equilibrium was displaced in favor of AIF(5)(2-) in the FNAK solvent. The AIF(5)(2-) ion was characterized by an intensive Raman band at 558 cm(-1) and an increasingly positive Al-27 chemical shift with raising temperature, e.g., of 16 ppm at 935degreesC.