Oxygen Potentials for the Oxidation of Rare Earth Oxysulfides to Oxysulfates

The equilibrium oxygen potentials corresponding to the oxidation of rare earth oxysulfides to their respective oxysulfates are measured in the temperature range 900 to 1480 K using the solid-state cell Pt, Ni + $NiO/(CaO)Zr0_2/R_20_2S + R_20_2S0_4$, Pt, where R = La, Pr, Nd, Sm, Eu, Gd, Tb, or Dy. The reference electrode consisting of a mixture of Ni + NiO is selected as its oxygen potential lies close to those of the rare earth systems studied here. At a constant temperature, the oxygen potentials for the oxidation of the oxysulfides to oxysulfates are found to increase monotonically with the atomic number of the rare earth element. The “second law” enthalpy change for the oxidation reaction also shows a similar trend, albeit a larger uncertainty. Based on the empirical trend, the changes in the standard Gibbs’ energy accompanying the conversion of oxysulfides of Ho, Er, and Tm to their oxysulfates are estimated. The higher polarizing powers of the heavier trivalent rare earth ions appear to destabilize the relatively open structure oxysulfates more than the close-packed oxysulfides