Identification of the Complex Ions in Liquid Cryolite

Abstract

The Raman spectrum of molten cryolite, Na₃AlF₆, at 1030°C has been investigated using laser excitation. A novel “windowless” Raman cell was used in order to overcome the problem of corrosion. One band that was observed at 633 Δ cm⁻¹ (polarized) is assigned to the symmetric stretching frequency ${\mathrm{(\nu }}_1)$ of tetrahedral AlF₄⁻ by comparison with the spectrum of crystalline cryolite and with the spectra of other halide complexes of known structure. A much weaker band at 575 Δ cm⁻¹ (polarized) is assigned to the symmetric stretching frequency ${\mathrm{(\nu }}_1)$ of AlF₆³⁻. The Raman evidence suggests that AlF₆³⁻ dissociates into AlF₄⁻ and F⁻ ions to a considerable extent (70%–75%) in molten cryolite.