The free energy of formation of Ce₂O₂S and the nonstoichiometry of cerium oxides

Abstract

The free energy of formation of Ce₂O₂S was determined by equilibrating CeO₂with COCO₂-SO₂ gas mixtures in the temperature range 900 to 1400°C. There was no significant sulfur solubility in cerium oxide and the free energy of formation of Ce₂O₂S was considerably less than previously estimated; it is 12 kcal (50 kJ) lower at 1300°C The oxygen dissociation pressures for the nonstoichiometric cerium oxide was also measured by equilibrating CeO₂ with CO-CO₂ gas mixtures in the temperature range 1000 to 1450°C and oxygen pressures from 10^-6 to 10^-18 atm. The results indicate a very large range of nonstoichiometry from CeO₂ to at least CeO_1.72. The new thermodynamic information was used to estimate the phase equilibria for the Fe-Ce-S-O system. The calculations indicate that for a steel with normal sulfur levels treated with aluminum and rare earth deoxidizers very little Ce₂O₃will form but rather Ce₂O₂S or cerium Sulfides. Also the stability range of Ce₂O₂S is smaller than previously believed.