Cryoscopic studies with molten fluorides. I. Depression of the freezing point of calcium fluoride by alkaline-earth fluorides and oxides and calcium silicates

Abstract

The depression of the freezing point of CaF2 by BaF2 and MgF2 is ideal in the limit of low concentrations, and excess partial free energies of CaF2 at finite concentrations are in accord with data for other systems. Limiting depressions due to SrF2, CaO, and SrO are each approximately 1/4, and due to BaO and MgO, 1/2, of the ideal values. Calcium orthosilicate behaves ideally on the basis of 1 SiO44− ion per molecule. For calcium metasilicate, the results are interpreted in terms of a distribution of fluorosilicate ions of general formula (SiO3)nF(2n+1)− in the melts. For Ca3SiO5, the depressions are complicated by the nonideal behavior of oxygen ions. From the phase diagrams at low concentrations, CaO behaves ideally in CaCl2, and MgO is ideal in MgF2. The results are discussed in terms of possible formation of solid solutions in some of these systems.