The crystal structure adopted by ceria-zirconia depends on the Zr/Ce ratio and temperature. At very low Zr concentrations, ceria-zirconia exhibits the cubic fluorite structure, which is common to both pure ceria and cubic zirconia (pure zirconia normally only adopts a cubic structure at high temperatures). However, at higher Zr contents, other crystal structures are formed, including two different tetragonal phases at intermediate Zr concentrations, and a monoclinic phase at very high Zr concentrations.[2]
There is both experimental[3][4] and theoretical[5] evidence showing that the decomposition of ceria-zirconia into Ce-rich and Zr-rich oxides is thermodynamically favorable in a wide range of solid solution compositions, indicating that ceria-zirconia is metastable with respect to phase separation.
Technological Importance
Ceria-zirconia is widely used as a component in current three-way catalytic converters.[6] The ceria-based component of the converter has several functions, including promoting the dispersion of the noble metals in the catalyst, but also storing and releasing oxygen.[7] The incorporation of zirconium in modern converters, forming ceria-zirconia, improves the performance of the catalyst by enhancing the resistance of the material to sintering, and simultaneously increasing the ability of the oxide to accommodate oxygen vacancies in its structure.[6]
