Vaporization reactions in the ytterbium–fluorine system

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Abstract
The vaporization and sublimation reactions for the ytterbium fluoride system have been investigated by mass loss, x‐ray powder diffraction, elemental analysis, and mass spectrometry. When YbF2.00 is heated in vacuum, it decomposes to give a metal rich vapor and a metal deficient residue until the composition YbF2.40(4) is reached. The following equilibrium reactions were established: YbF2.40(s,l) = (0.60)YbF2(g) + (0.40)YbF3(g), Reaction (a); and YbF3.00(s,l) = (0.05)YbF2.40(s,l) + (0.95)YbF3(g) + (0.03) F (g), Reaction (b). % A target collection Knudsen effusion method was used to determine vapor pressures for the above reactions. From the slope–intercept data the following enthalpy and entropy changes with associated standard deviations were derived at the median temperatures: For Reaction (a), ΔH°1597 = (96.9±1.1) kcal mol−1 and ΔS°1597 = (36.83±0.67 %cal mol−1⋅K−1; for Reaction (b), ΔH°1568 = (91.19±0.41) kcal mol−1 and ΔS°1568 = (37.28±0.26) cal mole−1⋅K−1. Estimated and reported thermodynamic functions were employed to reduce the thermodynamic values to 298 K. The reduced values are as follows: For reaction (a), ΔH°298 = (110.7±2.3) kcal mol−1 and ΔS°298 = (50.8±2.2) cal mol−1⋅K−1; for Reaction (b), ΔH°298 = (111.1±1.3) kcal mol−1 and ΔS°298 = (56.2±1.5) cal mol−1⋅K−1. By use of the third‐law procedure, the following ΔH°298 values were derived: for (a), (109.8±2.4) kcal mol−1; for (b), (107.0±3.0) kcal mol−1. The enthalpies and entropies for the hypothetical congruent sublimation reactions of YbF2(s) and YbF3(s) are derived and compared with those of the alkaline earth difluorides and other selected lanthanide fluoride systems.

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