Uranium Monosulfide. I. Vaporization, Thermodynamics, and Phase Behavior

Abstract

The rate of evaporation of uranium monosulfide has been measured over the 900-deg temperature range 1840° to 2730°K and a pressure range 10—3 to 10—8 atm, with an estimated accuracy of ±4%, by collection of vapor effusing from tungsten effusion cells containing the solid. The congruently evaporating composition was shown to be S/U=1.00. The effusion rate is expressed in terms of an ``effective'' vapor pressure PE calculated as though the entire vapor consisted of gaseous US molecules. An empirical equation derived by the method of least squares from the data is logPE(atm)=−1.7382+3.127×104/T−1.3181×108/T2+0.093776×1012/T3.Mass spectrometric measurements [E. D. Cater, E. G. Rauh, and R. J. Thorn, J. Chem. Phys. 35, 619 (1961)] show that the vaporization actually occurs both to gaseous US and to gaseous U+S. The present data are treated to yield the heats of sublimation at 2300°K to gaseous molecules, 150.3±2.1, and to gaseous elements, 271.2±4.0 kcal/mole, where the quoted uncertainties are estimated errors. The corresponding entropies of sublimation are: to molecules, 38.4±0.6, and to atoms, 65.5±1.6 cal/degmole. The lattice parameter of uranium monosulfide is 5.4903±0.0002 A. The melting point is 2735+30–5°K. The monosulfide solid phase appears to encompass a small composition range. Values derived from the experimental data and the literature for absolute entropies at 2300°K are 45±2 eu for solid, and 83±3 eu for gaseous US, where estimated errors are given. The heat of formation of solid US at 298°K from the gaseous atoms is estimated to be —273±5 kcal/mole and from the solid elements, —90±5. The free energies of formation of solid and gaseous monosulfide between 2100 and 2400°K are expressed by the equations ΔFf∘(US,s)=64.0T−268000cal/moleΔFf∘(US,g)=38.8T−152000cal/mole A semitheoretical treatment gives nonlinear equations for the temperature dependences of the free energies and entropies of vaporization.