Unraveling the Negative Role of Oxygen-Vacancy Cluster in Ionic Conductivity in CeO₂: Hybrid Functional Study

Oxygen-vacancy formation and migration play a critical role in the high performance of CeO₂ as a highly promising material for solving the environmental and energy-related issues. However, most of the associated works were directed toward the single or isolated oxygen vacancies. In this contribution, the formation and migration of an oxygen-vacancy cluster in CeO₂ have been presented in detail using the Heyd-Scuseria-Ernzerhof hybrid functional method. The results demonstrate that oxygen vacancies exhibit a strong tendency to cluster in the 111 direction of CeO₂. The detailed analyses of formation energy reveal the favorability for forming such a vacancy cluster under O-poor conditions. By means of the climbing-image nudged elastic band method and molecular dynamics simulations, the vacancy cluster is found to have a high kinetic stability and low mobility, thus becoming a challenge in achieving high ionic conductivity in CeO₂. Attempts have been made to unravel the negative effect of vacancy clustering on ionic conductivity at an atomistic level, and possible means for avoiding or eliminating the vacancy clustering are proposed. The current work has implications for tailoring or optimizing the ionic conductivity in CeO₂-based materials or devices for environmentally friendly applications.