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

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: v₁ AlF₆^3- = 515 cm⁻¹, v₁ AlF₅^2- = 555 cm⁻¹, v₁ 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 A1F₅^2- ⇌ A1F₄⁻ + 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-AlF₃ mixtures, an explanation for the variation of alumina solubility in M₃AlF₆ melts is proposed.