Abstract

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 AlF₆³⁻ and AlF₄⁻ ions, the existence of a new species, AlF₅²⁻. Consequently, the previous assignment of the main Raman bands has been changed as follows: <i>v</i>₁ AlF₆³⁻ = 515 cm⁻¹, <i>v</i>₁ AlF₅²⁻ = 555 cm⁻¹, <i>v</i>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 AlF₆³⁻ ⇌ AlF₅²⁻ + F⁻ and AlF₅²⁻ ⇌ AlF₄⁻ + F⁻ are <i>K</i>_<i>B</i> = 0.29 ± 0.05 and <i>K</i>_<i>A</i> = (1.65 ± 0.1).10⁻² respectively. Considering the experimental errors, the value of <i>a</i> F⁻, the activity in free fluorides [estimated with these constants for molten cryolite (<i>a</i> F⁻ = 0.41 ± 0.02)], is in good agreement with values found from electrochemical measurements (<i>a</i> F⁻ = 0.375 ± 0.015) or from vapor pressure measurements (<i>a</i> 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 <i>M</i>₃AlF₆ melts is proposed.

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