HIGH-TEMPERATURE VAPORIZATION AND THERMODYNAMICS IN THE LUTETIUM-SULFUR SYSTEM

The high temperature vaporizations of Lu3S4 and nonstoichiometric LuSx (0.75<x<1.30) compositions in the lutetium-sulfur system were investigated by mass spectrometric and target collection Knudsen effusion techniques. Lu3S4 vaporizes congruently according to: (1) LuS1.33(s)=Lu(g)+0.33S(g) and (2) LuS1.33(s)=LuS(g)+0.33S(g). The equilibrium partial pressures are: log PLu(atm)=-(29162±239)/T+(7.117±0.112); log Ps(atm)=-(29513±338)/T+(7.104±0.159); and log P LuS(atm)=-(31366±675)/T+(8.052±0.317). The second-law thermodynamic values are: Reaction (1) ΔH°298=317. 6±2.5 kcal mol-1; ΔS°298=79.6±1.5 eu; Reaction (2) ΔH°298=196.9±3.3 kcal mol -1; ΔS°298=55.9≱1.8 eu; and for LuS 1.33(s), ΔH°f,298=-126.5±2.5 kcal mol-1; S°298=18.0±1.5 eu. The dissociation energy of LuS(g) is computed as D°0=120.6±3.2 kcal mol-1 from the mass spectrometric slopes for log ILu +T and log ILuS+T vs 1/T, and with an iterated slope for logIs+T consistent with the congruency of the vaporization reactions. The activity of Lu across the homogeneity range of the monosulfide was measured. Together with the Gibbs-Duhem derived values for the activity of sulfur, the thermodynamic properties of stoichiometric LuS 1.0 were calculated. These are: (in kcal mol-1) ΔH°atom,298 =268.5±3.0; ΔH° subl,298=147.7±3.0; and ΔH°f,298=-99. 6±3.1. © 1979 American Institute of Physics.