Reinvestigation of Molten Fluoroaluminate Raman Spectra: The Question of the Existence of AlF52− Ions

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: v1 AlF63- = 515 cm−1, v1 AlF52- = 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 AlF63- ⇌ AlF52- + F− and A1F52- ⇌ A1F4− + F− are KB = 0.29 ± 0.05 and KA = (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.