Standard free energies of formation of rare earth sesquisulphides

Abstract

An e.m.f. technique is used for the measurement of the Gibbs' energy changes involved in the conversion of the rare earth oxysulphides to the corresponding sesquisulphides in the temperature range 870 to 1120 K. The cell arrangement is Au, Ag + Ag₂S|R₂O₂S + R₂S₃|| (Y₂O₃)ThO₂|| Fe + Fe_0.95O, Au where R = La, Pr, Nd, Sm, Eu and Gd. Yttria-doped thoria is used as the solid electrolyte because it has predominant ionic conduction at the low oxygen potentials developed at the R₂O₂S + R₂S₃ electrode, especially for systems containing heavier rare earth elements. The sulphur potential at the left-hand electrode is established by the dissociation of Ag₂S to silver in a closed system. The changes in the standard free energy, enthalpy and entropy associated with the conversion of the rare earth oxysulphide to the sesquisulphide all show a smooth variation with rare earth atomic number. Based on these empirical trends, the corresponding thermodynamic functions for terbium, dysprosium and holmium systems are estimated. The measured free energy values are combined with recent data on the standard free energies of formation of the rare earth oxysulphides to obtain the Gibbs' energies of formation of the corresponding sesquisulphides.