Mass-Spectrometric Determination of the Dissociation Energy of Beryllium Monofluoride

A mass spectrometer was used to study high-temperature equilibria among vapor species produced by the fluorination of elemental beryllium with CaF2 and BF3 in a Knudsen effusion cell. The beryllium species Be, BeF, and BeF2 were identified in the effusing vapor from appearance potential measurements; Ca and CaF were also found in the Be–CaF2 system. Ion intensities were measured at a number of temperatures in the range 1320° to 1600°K with low ionizing voltages, and the intensities were used to calculate the equilibrium constants of several isomolecular reactions involving BeF. Derived thermodynamic data are for Be(g)+BeF2(g)=2 BeF(g), ΔH298=28.7±2 kcal, and for Be(g)+CaF(g)=BeF(g)+Ca(g), ΔH298=−8.0±2 kcal. These data yield a heat of formation at 298°K of −40.0±1.5 kcal/mole and a dissociation energy D0° of 5.85±0.10 eV for BeF(g). Several check measurements of equilibria in the Be–Al–F system confirmed these results. The derived dissociation energy is discussed in terms of corresponding values obtained from electronic spectra.