FREE-ENERGY OF FORMATION OF CE2O2S AND THE NONSTOICHIOMETRY OF CERIUM OXIDES

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 CeO1.72. 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. © 1979 American society for metals and the metallurgical society of aime.