High temperature stability of oxysulphides containing Ce³⁺ and Y³⁺ ions

Abstract

Standard Gibbs energies of formation of oxysulfides of cerium and yttrium from their respective oxedes were determined using solid oxide galvanic cells incorporating calcia-stabilized zirconia as the electrolyte in the temperature range 870–1120 K. The sulfur potential over the electrode containing the oxide and oxysulfide was fixed by a buffer mixture of Ag + Ag₂S. A small amount of CaH₂ was added to the buffer to generate an equilibrium ratio of H₂S and H₂ species in a closed system containing the buffer and the electrode. The sulfur potential is transmitted to the electrode via the gas phase. The results can be summarized by the equations 2(CeO₂)+1/2(S₂)→(Ce₂O₂S)+(O₂) ΔG°=430600−109·7T(±400)J mol⁻¹ (Y₂O₃)+1/2(S₂)→(Y₂O₂S)+1/2(O₂) ΔG°=114780−1·45T(±200)J mol⁻¹ The values are compared with data reported in the literature. The stability field diagram for the Ce₂O₂S system has been developed using the results of this study for Ce₂O₂S and data for other phases from the literature.