A THERMODYNAMIC AND ELECTRICAL-CONDUCTIVITY STUDY OF NONSTOICHIOMETRIC CERIUM DIOXIDE

Thermodynamic and dc electrical conductivity measurements were performed on nonstoichiometric CeO2-x to characterize the electrical behavior and defect structure in the vicinity of the n to p transition. Using the mass action approach, activation energies of 2.57, 1.15 and 0.63 eV were obtained for the electron, hole and ionic partial conductivities respectively (600-1000-degrees-C, 1 < P(O2) < 10(-4) atm). Thermodynamic measurements of 99.99% CeO2-x by electrochemical coulometric titration yielded expressions for point defect concentrations per cm3 and nonstoichiometry in the impurity dominated region at 800-degrees-C as follows: [Ce(Ce)'] = 7.08 x 10(16) P(O2)-1/4 cm-3; [h.] = 9.26 x 10(18) P(O2)+1/4 cm-3; x = 2.03 x 10(-23) (7.08 x 10(16) P(O2)-1/4 -9.26 x 10(18) P(O2)+1/4. Thermodynamic constant composition measurements revealed a decrease in the partial molar enthalpy (from -10 eV) with decreasing stoichiometry for x < 10(-2.8). This decrease is attributed to defect reaction involving both holes and electrons.