Raman spectra of aluminum fluoride containing melts and the ionic equilibrium in molten cryolite type mixtures

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Abstract
The structure of molten aluminum fluoride−alkali fluoride mixtures has been investigated by Raman spectroscopy at 700−900°C using a captive liquid windowless cell. Changes in spectra with temperature and melt composition reflect an equilibrium between the species AlF4− and AlF63−. Complete spectrum of the AlF63− was obtained confirming an octahedral coordination; the frequencies of the ν1, ν2, and ν5 vibrations were found to be 555, 390, and 345 cm−1, respectively. A strong cation effect on the bandwidth of the AlF63− ν1 band was observed when the cation is changed from Li+ to Cs+. This effect is believed to be due to a restriction of the rotational mobility of the anion in the presence of a smaller cation. Quantitative analysis of the intensity ratios of the AlF4− and AlF63− ν1 bands as a function of the composition of the melt permits the calculation of the stoichiometric dissociation constant K = XAlF4−XF−2/XAlF63−. An average value of 3±1×10−2 (concentration scale in mole fractions) was found at 780°C. The spectrum of molten cryolite was reinvestigated and found to be different from the previously published spectrum. By extrapolation of the results obtained at lower temperatures and other compositions, the AlF62− ion in molten cryolite dissociates into AlF4− to the extent of ∼25% at 1015°C. This degree of dissociation and the calculated ΔH of the equilibrium are in good agreement with thermodynamic and other methods of investigation.