Raman Study of the Hexafluoroaluminate Ion in Solid and Molten FLINAK

Abstract

Raman spectra have been obtained for matrix-isolated AlF6(3-) in an LiF/NaF/KF (FLINAK) eutectic mixture. Three Raman bands characteristic of the hexafluoroaluminate ion were identified in the solids formed from FLINAK melts which contained small amounts (5-11 mol%) of either AlF3 or Na3AlF6. The three allowed Raman-active bands of the matrix-isolated octahedral complex ion, nu 1(A1g), nu 2(Eg), and nu 5(F2g), were observed at 560.5, 380, and 325 cm-1, respectively, for the solid sample at 25 degrees C. Wavenumbers and relative intensities were similar to those of Na3AlF6 (cryolite), K3AlF6, and K2NaAlF6 (elpasolite) and other crystals known to contain discrete, octahedral AlF6(3-) ions. Peak positions, half-widths, and relative intensities for the bands were measured for samples at temperatures different from room temperature through the melting transition and into the molten state. The transition from high-temperature solid to molten salt at about 455 degrees C occurred gradually without perceptible change in the peak positions, half-widths, or relative intensities. For a sample in molten FLINAK at 455 degrees C, the nu 1(A1g), nu 2(Eg), and nu 5(F2g) modes of the AlF6(3-) ion were observed at 542, 365, and 324 cm-1, respectively. Raman depolarization experiments were consistent with these assignments, and the low value of the depolarization ratio of the nu 1(A1g) mode at 542 cm-1 indicated that the sample was molten above 455 degrees C. Differential thermal analysis also indicated that the FLINAK samples melted at about 455 degrees C. Raman measurements were performed for samples at temperatures from 25 to 600 degrees C in a silver dish, on a hot stage, in an argon-filled atmosphere, under a microscope. Additional Raman experiments were performed on samples at temperatures from 25 to 750 degrees C in a conventional graphite windowless cell, in an argon-filled quartz tube, in a standard furnace. Over the concentration range 4.8-11 mol% AlF3 (CR 23-8.0) in FLINAK, only bands due to the AlF6(3-) ion were detected. There was no evidence to support the presence of other aluminum complexes in these melts.

Modeling the dependence of alumina solubility on temperature and melt composition in cryolite-based melts

“…[6] through [10] were considered for 1300 K, and similar sets of equations including unknown equilibrium constants were set up for each temperature by using the element balances described by Eqs. [12] through [16]. These equations were solved simultaneously by the least-squares, data-fitting program, and the equilibrium constants were obtained for each temperature, as represented in Table II. From the modeling results, the ⌬G 0 f values for the solutes described by the formation reactions of Eqs.…”

“…For oxygen, [12] where x 3 is the molar fraction of dissolved Al 2 O 3 in a ternary system of NaF-AlF 3 -Al 2 O 3 , as listed in the last column in Table I, and n t the total moles of all the species in the melt. Other equations can be similarly established by element balances.…”

“…From thermodynamic considerations, Raman spectroscopic studies, NMR studies, etc., some authors [3][4][5][6][7][8][9] suggested the existence of Na 2 AlF 5 as a major or minor solute species in the molten NaF-AlF 3 system, but others [10][11][12][13][14] found no evidence to support the presence of this five-fold coordinated aluminum complex in the system. In general, solute species existing in any system should possess the simplest geometric arrangements of large anions surrounding the small highly charged cation, e.g., in this case, AlF 6 3Ϫ (octahedron) and AlF 4 Ϫ (tetrahedron).…”

In situ high temperature Raman and DFT analysis of cerium fluoride and oxyfluoride structures in molten FLiNaK

“…The FLiNaK samples were prepared by mixing LiF (99.99%, Sigma-Aldrich), NaF (99.99%, Sigma-Aldrich), and KF (99.99%, Sigma-Aldrich) in a glassy carbon crucible using the method reported previously. [26,27] The oxygen contents in these eutectics were less than 300 ppm (measured by LECO O836 oxygen analyzer). The molten FLiNaK-CeF 3 and FLiNaK-CeF 3 -Li 2 O samples were prepared by the following procedures: Firstly, the CeF 3 (99.99%, Sigma-Aldrich) and Li 2 O (97%, Sigma-Aldrich), vacuum dried for 8 h at 473 K, were mixed with alkali metal fluorides at a specific molar ratio; then, the mixture was placed in a platinum crucible, which was heated to a certain temperature by a Linkam TS1000 temperature-controlled hotstage (accuracy: ±1 K) under flowing argon condition for spectral acquisition.…”

Existence of Al2F7− in molten MF–AlF3 (M = K, Cs) systems as determined by Raman spectroscopy and structural simulation

“…The ionic structures of the molten systems of alkali metal fluorides and aluminum fluoride have attracted significant interest from the research community. At present, the existence of AlF 6 3 − , AlF 5 2 − , and AlF 4 − complex ions have been proven by Raman spectroscopy [9][10][11][12][13][14] . It is acknowledged that the equilibria among the Al-F complex ions in molten systems exist as AlF 6 3 − = AlF 5 2 − + F − and AlF 5 2 − = AlF 4 − + F − .…”