Vaporization reactions in the ytterbium–fluorine system

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 YbF_2.00 is heated in vacuum, it decomposes to give a metal rich vapor and a metal deficient residue until the composition YbF_2.40(4) is reached. The following equilibrium reactions were established: YbF_2.40(s,l) = (0.60)YbF₂(g) + (0.40)YbF₃(g), Reaction (a); and YbF_3.00(s,l) = (0.05)YbF_2.40(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.