Thermochemical properties of the gaseous lower valent fluorides of uranium

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Abstract
High temperature gaseous equilibria involving the lower‐valent uranium fluorides UF, UF2, UF3, and UF4 were studied by mass spectrometry, and reaction enthalpies and entropies with estimated uncertainties of ±2 kcal/mol and ± 2 cal/deg mol were derived solely from the temperature coefficients of reaction equilibrium constants. These results yield the following bond dissociation energies at 298 K in kcal/mol: D°(F3U–F) = 148.2; D° (F2U–F) = 149.0; D° (FU–F) = 136.5; and D° (U–F) = 157.5. The sum of these values is compatible with the heat of atomization of UF4(g) evaluated from independent sources. A value of D°(F3U–F) derived from electron impact measurements agrees well with the equilibrium data. The reaction entropy data indicate that the electronic entropies of UF, UF2, and UF3 are comparable to that of atomic uranium in the same temperature range, and provide a basis for estimating the total thermodynamic functions of these species. Experimental entropies obtained for UF4 indicate the likelihood of a distorted tetrahedral structure of effective C2v symmetry. The results provide sufficient information for predicting the equilibrium composition of the gaseous U–F system over a wide temperature range.

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