Structural characterization and oxygen nonstoichiometry of ceria-zirconia (Ce_1-xZr_xO_2-δ) solid solutions

The oxygen nonstoichiometry and crystalline structure of ceria-zirconia Ce_1-xZr_xO_2-δ (CZO) (x = 0.05, 0.1, 0.2, 0.5, and 0.8) solid solutions, commercially used as oxygen storage materials, have been examined using thermogravimetry, X-ray diffraction, and Raman spectroscopy. In this study detailed data describing oxygen vacancy concentrations, obtained at intermediate to high pO₂, are interpreted with the aid of point defect equilibria relationships. For cubic CZO (x ≤ 0.2), the ease of reducibility dramatically increased with increasing Zr content, as reflected by an ultimate >40% decrease in reduction enthalpy, with a corresponding shift in onset of reduction to higher pO₂. The impact of pre-existing oxygen vacancies on the larger reduction enthalpy found for Y doped CZO, as compared with this study, is discussed, as is evidence that Zr increases the electron migration energy in ceria by 50%. The reducibility of tetragonal CZO (x > 0.2) was found to increase following redox cycling. This enhanced reducibility is believed to be related to ordering and is partially negated by a high temperature (1000 C) heat treatment.