NONSTOICHIOMETRY OF CE0.8GD0.2O1.9-X

Oxygen nonstoichiometry of Ce0.8Gd0.2O1.9-chi, was gravimetrically mea sured at temperatures from 700 to 1000 degrees C and oxygen partial pr essures between 10(-24) and 1 atm. Based on the experimental data, the relationship between log chi and log po(2), (where chi is the oxygen deficiency and po(2) the oxygen partial pressure) is discussed and par tial molar enthalpy, Delta H-o, and entropy, Delta (S) over bar o, of oxygen are calculated. In the region of chi less than or equal to 0.04 , chi is proportional to the -1/4 power of po(2) and Delta H-o, and De lta (S) over bar(o), behave as in an ideal solution, showing that oxyg en vacancies are doubly charged and defects in Ce(0.8)Gd(0.2)O(1.9-chi )are randomly distributed. Thus, the mass action law is satisfied in t he area chi less than or equal to 0.04.