Equilibrium Pressures over BeF₂/LiF(Flibe) Molten Mixtures

The pressures of the vapor species in equilibrium with Flibe at ~600°C are determined from work by Buchler and Stauffer and by Baes and coworkers. The former authors show that the principal vapor species are BeF₂(g) and LiBeF₃(g). The measurements and the theoretical model of Baes provide accurate values of the activity coefficient of BeF₂ in Flibe. When combined with the vapor pressure of pure BeF₂, the equilibrium pressure of BeF₂ is determined as a function of melt composition and temperature. The activity coefficient of LiF is not measured, but it is obtained by application of the Gibbs-Duhem equation to the measured activity coefficient of BeF₂. Thus, the partial pressure of LiF(g) is also known. The pressure of the mixed dimer LiBeF₃ is calculated from the gas phase equilibrium for the formation of the dimer from the two monomers, with the equilibrium constant given by Buchler and Stauffer. The vapor pressure at 600°C extrapolated from high-temperature Oak Ridge National Laboratory data is ~60% higher than the predicted values.