Cryoscopic studies in fluoride-oxide-silica systems: Part I. systems containing Li+, Na+ and K+

The depressions of the freezing temperatures of NaF and KF by additions of 3M₂O · 2SiO₂, M₂O -SiO₂, M₂O · 2SiO₂ and SiO₂ (where M = Li, Na and K) have been measured and the variations of the activities of NaF and KF with liquidus composition have been calculated. These activities are shown to be in good agreement with a proposed theoretical model of the anionic constitutions of these melts. In melts of M₂O/SiO₂ > 1 linear chain silicate ions and free F⁻ ions are postulated and in melts of M₂O/SiO₂ < 1 reaction between F⁻ and silicate ions to produce polyfluorosilicate anions is postulated. The effect of alkali cation type on the deviation of the liquidus activities from the proposed ideal behavior in melts of M₂O/SiO₂ > 1 and the degrees of polymerization of the fluorosilicate ions in melts of M₂O/SiO₂ < 1 are explained in terms of coulombic interactions and the effects of cation size on the polarization behavior. In the reciprocal systems MF-M₂ ^′O · SiO₂ and MF-M₂ ^′O · 2SiO₂ deviations from proposed ideality are correlated with the signs and magnitudes of the standard free energy changes for the exchange reactions. The conclusions as to the dependency of solution mechanism of fluorides in their alkali silicates on M₂O/ SiO₂ ratio are in agreement with conclusions drawn from studies of infrared absorption in these systems.