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Reinvestigation of Molten Fluoroaluminate Raman Spectra: The Question of the Existence of AlF5 2- Ions

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The structure of molten NaF-AlF3 mixtures has been reinvestigated by Raman spectroscopy, with the use of a captive liquid windowless cell. The previously observed intensity variation of the 555-cm−1 and 622-cm−1 bands with the melt composition has been confirmed. However, by recording spectra of NaF-Na3AlF6 molten mixtures, one can clearly observe a new band located at 515 cm−1. This band increases in intensity with the melt basicity and is not due to an oxide contamination. The most probable explanation is to consider, in addition to the usual AlF6 3− and AlF4 ions, the existence of a new species, AlF5 2−. Consequently, the previous assignment of the main Raman bands has been changed as follows: v 1 AlF6 3− = 515 cm−1, v 1 AlF5 2− = 555 cm−1, v 1 AlF4 = 622 cm−1. This new assignment has been checked by comparison of spectra of similar mixtures with different cations (Li, K). A quantitative analysis of the various components, as a function of the NaF-AlF3 melts molar ratio, has been performed by deconvoluting the Raman-bands envelope. The calculated equilibrium constants (expressed in mole fraction units) for AlF6 3− AlF5 2− + F and AlF5 2− AlF4 + F are K B = 0.29 ± 0.05 and K A = (1.65 ± 0.1).10−2 respectively. Considering the experimental errors, the value of a F, the activity in free fluorides [estimated with these constants for molten cryolite (a F = 0.41 ± 0.02)], is in good agreement with values found from electrochemical measurements (a F = 0.375 ± 0.015) or from vapor pressure measurements (a F = 0.38 ± 0.04). Finally, on the basis of this new ionic model for MF-AlF3 mixtures, an explanation for the variation of alumina solubility in M 3AlF6 melts is proposed.

Keywords: Cryolite; Fluoroaluminates; Molten salts; Raman spectroscopy

Document Type: Research Article

DOI: http://dx.doi.org/10.1366/0003702904085525

Affiliations: Laboratory of Analytical Chemistry and Radiochemistry, University of Liège, B-4000 Sart-Tilman Liège, Belgium

Publication date: February 1, 1990

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