Experimental and theoretical study on the complete phase separation of ceria-zirconia solid solution into two end members, ceria and zirconia

Solid solution CeO₂-ZrO₂ has long been used as a non-noble metal oxide promoter for three-way catalysts owing to its high oxygen storage capacity. However, the stability issue of the CeO₂-ZrO₂ has been controversial for a long time. In particular, the phenomena observed by phase instability are so diverse and inconsistent that the related causal analysis is still a matter of debate. In this study, for the first time, it was demonstrated theoretically and experimentally that a Ce_0.75Zr_0.25O₂ (CZO) solid solution must be completely separated into CeO₂ and ZrO₂ phases owing to its inherent thermodynamic instability. According to an extensive evaluation via defect chemical calculations and well-controlled model experiments with grain-boundary-free epitaxial thin film samples, CZO materials undergo phase separation until they are completely separated, and the separation rate is particularly high in a reducing atmosphere. The underlying inherent stability problem and enhanced phase separation kinetics of the CZO material are attributed to the enhanced cation diffusion in a reducing atmosphere, where more mobile cationic defects (interstitial cations) are generated and an easier pathway with a lower migration energy is available.