Reinvestigation of Molten Fluoroaluminate Raman Spectra: The Question of the Existence of AlF 5 2− Ions

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Abstract
The structure of molten NaF-AlF 3 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-Na 3 AlF 6 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 AlF 6 3- and AlF 4 − ions, the existence of a new species, AlF 5 2- . Consequently, the previous assignment of the main Raman bands has been changed as follows: v 1 AlF 6 3- = 515 cm −1 , v 1 AlF 5 2- = 555 cm −1 , v 1 AlF 4 − = 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-AlF 3 melts molar ratio, has been performed by deconvoluting the Raman-bands envelope. The calculated equilibrium constants (expressed in mole fraction units) for AlF 6 3- ⇌ AlF 5 2- + F − and A1F 5 2- ⇌ A1F 4 − + 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 α F − , the activity in free fluorides [estimated with these constants for molten cryolite ( α F − = 0.41 ± 0.02)], is in good agreement with values found from electrochemical measurements ( α F − = 0.375 ± 0.015) or from vapor pressure measurements ( α F − = 0.38 ± 0.04). Finally, on the basis of this new ionic model for MF-AlF 3 mixtures, an explanation for the variation of alumina solubility in M 3 AlF 6 melts is proposed.

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