Raman study of the hexafluoroaluminate ion in solid and molten FLINAK

Raman spectra have been obtained for matrix-isolated AlF63- in an LiF/NaF/K F (FLINAK) eutectic mixture. Three Raman bands characteristic of the hexafl uoroaluminate ion were identified in the solids formed from FLINAK melts wh ich contained small amounts (5-11 mol %) of either AlF3 or Na3AlF6. The thr ee allowed Raman-active bands of the matrix-isolated octahedral complex ion , nu(1)(A(1g)), nu(2)(E-g), and nu(5)(F-2g), were observed at 560.5, 380, a nd 325 cm(-1), respectively, for the solid sample at 25 degrees C. Wavenumb ers and relative intensities were similar to those of Na3AlF6 (cryolite), K 3AlF6, and K2NaAlF6 (elpasolite) and other crystals known to contain discre te, octahedral AlF63- ions. Peak positions, half-widths, and relative inten sities for the bands were measured for samples at temperatures different fr om room temperature through the melting transition and into the molten stat e. The transition from high-temperature solid to molten salt at about 455 d egrees C occurred gradually without perceptible change in the peak position s, half-widths, or relative intensities. For a sample in molten FLINAK at 4 55 degrees C, the nu(1)(A(1g)), nu(2)(E-g), and nu(5)(F-2g) modes of the Al F63- ion were observed at 542, 365, and 324 cm(-1), respectively. Raman dep olarization experiments were consistent with these assignments, and the low value of the depolarization ratio of the nu(1)(A(1g)) mode at 542 cm(-1) i ndicated that the sample was molten above 455 degrees C. Differential therm al analysis also indicated that the FLINAK samples melted at about 455 degr ees C. Raman measurements were performed for samples at temperatures from 2 5 to 600 degrees C in a silver dish, on a hot stage, in an argon-filled atm osphere, under a microscope. Additional Raman experiments were performed on samples at temperatures from 25 to 750 degrees C in a conventional graphit e windowless cell, in an argon-filled quartz tube, in a standard furnace. O ver the concentration range 4.8-11 mol % AlF3 (CR 23-8.0) in FLINAK, only b ands due to the AlF63- ion were detected, There was no evidence to support the presence of other aluminum complexes in these melts.