The dissociation of fluoroaluminates in FLiNaK and CsF-KF molten mixtures: A Raman spectroscopic and solubility study

By using a new furnace design, M3AlF6 (M = Na, K, Cs) and mixtures of small amounts of AlF3 in FLiNaK (46.5 mol % LiF, 11.5 mol % NaF, 42 mol % KF) and CsF-KF eutectic have been investigated over a wide temperature range (25-1050 degreesC) by Raman spectroscopy. The peak positions and their relative intensities have been measured as a function of temperature. In FLiNaK, up to 750 degreesC, the bands shift gradually to lower wavenumbers, and their halfwidths increase in agreement with published data. However, it is shown from solubility measurements and Raman data that, in these conditions, the mixture is not totally molten and the spectra correspond mainly to AlF63- in the solid state. When the mixture is completely molten, a new band appears clearly on the high-frequency side of the main band of the spectrum, and its intensity grows up when the temperature is increased. The present results are a clear confirmation of the dissociation of AlF63- into AlF52- and AlF4- that our study of the Raman bands of the fully melted systems MF-AlF3 (M = Na, K, Li) previously suggested. On these systems, it is then important to know if the spectra belong mainly to solid or liquid fluoroaluminates before drawing any conclusion concerning the liquid phase structure.