ENTHALPIES OF MIXING IN LIQUID BERYLLIUM FLUORIDE-ALKALI FLUORIDE MIXTURES

The integral enthalpies of mixing of the liquid mixtures of beryllium fluoride with lithium fluoride, potassium fluoride, and rubidium fluoride have been determined calorimetrically at 862°. The beryllium fluoride-lithium fluoride system has an S-shaped enthalpy curve with positive values in the high lithium fluoride range. The beryllium fluoride-potassium fluoride system is exothermic at all compositions, but the curvature of the enthalpy of mixing curve is positive at high beryllium fluoride contents. The strong, energetic asymmetry of the considered systems is attributed to the energy associated with the breakdown of the network structure of liquid BeFa. The data also indicate the existence of the complex anionic species BeF42-, particularly in the concentration range 0-50 mol % BeF2 in mixtures with KF and RbF. For the system BeF2-LiF the partial excess enthalpies of BeF2 have been derived from our data for comparison with corresponding excess free energies available in the literature. The comparison shows that the partial excess entropies of BeF2 in this system are positive, rising from zero at NBeF = 1 to values of the order of 3.0 cal/deg mol at about 30% BeF2. The partial excess entropies of BeF2 at high to intermediate contents of BeF2 are to some extent consistent with a model proposed by Fϕrland, based on a random distribution of bridging and nonbridging fluoride ions. © 1969, American Chemical Society. All rights reserved.