Vaporization, thermodynamics, and dissociation energy of gadolinium monosulfide: Systematics of vaporization of the rare earth monosulfides

Abstract

Mass spectrometry studies of the congruent, near-stoichiometric sublimation of gadolinium monosuflide and of the isomolecular gaseous reaction GdS+Y=Gd+YS have been carried out in the range 2005 to 2290 K. Sublimation occurs simultaneously to GdS molecules and to atoms Gd+S. Partial pressures are given by log PGd(atm)=-(29873±182)/T+(7.254±0.084) and log PGdS(atm)=-(30343±221)/T+(7.281±0.102). The indicated uncertainties are standard deviations; the absolute pressures may be low by as much as a factor 2. Enthalpies derived from combined second and third law considerations, with estimated maximum uncertainties due to cross section ratios, ground state degeneracies, and the use of the YS/GdS equilibrium for pressure calibration are (all in kcal/mole): GdS(s)=GdS(g), ΔH° 2150=140±3, ΔH°0=147±4; GdS(s)=Gd(g)+S(g), ΔH°2150=270±4, ΔH°0=272±4; GdS(g)=Gd(g)+s(g), ΔH° 2150=129.8±2.5, D°0(GdS)=125.0±2.5; Gd(s)+S(rh)=GdS(s), ΔH°298=-111±4. Data on the vaporization behavior and thermodynamics of the rare-earth monosulfides from LaS to GdS are assembled and/or estimated and discussed. Heats of atomization of MS(s) and of dissociation of MS(g) follow closely the trend of heats of sublimation of the metals. Heats of formation and sublimation of MS(s) are essentially equal for all the elements. © 1978 American Institute of Physics.