Effusion Studies, Mass Spectra, and Thermodynamics of Beryllium Fluoride Vapor

Torsion—effusion and mass-spectrometric techniques were used to obtain absolute pressures and composition data for vapors over condensed beryllium fluoride and beryllium fluoride-aluminum mixtures. Saturated beryllium fluoride vapor in the effusion pressure range was found to consist predominantly of monomeric BeF2, with less than 1 mole % of Be2F4. When aluminum powder was added to liquid beryllium fluoride, the vapor was found to contain a few mole percent of AlF in addition to the BeF2. The second-law entropy of vaporization of beryllium fluoride is consistent with a low bending frequency (320 cm−1) for BeF2(g). The heat of sublimation of BeF2 at 298°K was derived as 55.3±1 kcal/mole, leading to a value of −187.0±2.2 kcal/mole for the standard heat of formation of BeF2(g). Differential measurements of the AlF pressure over BeF2–Al mixtures yielded a value of −183.8±3 kcal/mole for the heat of formation of BeF2(g) at 298°K, and the corresponding value obtained from mass-spectrometric studies is −187.0±3 kcal/mole. Effusion measurements on Be–BeF2 mixtures indicate the dissociation energy of BeF at 298°K to be less than 6.8 eV, while data on the BeF2–Al system indicate D298<6.4 eV for BeF.