Segregation and Migration of the Oxygen Vacancies in the Σ3 (111) Tilt Grain Boundaries of Ceria

In nanocrystalline materials, defect grain boundary (GB) interactions play a key role in determining structural stability, as well as size-dependent ionic, electronic, magnetic, and chemical properties. In this study, using density functional theory, we systematically investigated the segregation and migration of oxygen vacancies at the Sigma 3 [1 (1) over bar0]/(111) grain boundary of ceria. Three oxygen layers near the GB are predicted to be segregation sites for oxygen vacancies. Moreover, the presence of oxygen vacancies stabilizes this tilt GB at a low Fermi level and/or under oxygen-poor conditions. An atomic strain model is proposed to rationalize the layer dependency of the relaxation energy for a 2+-charged oxygen vacancy. The structural origin of the large relaxation energies at layers 1 and 2 was determined to be free-volume space, which induces ion relaxation toward the GB. Our results not only pave the way for improving oxygen transport near GBs in ceria, but also provide important insights into the engineering of the GB structure of nanocrystalline ceria for better ionic, magnetic, and chemical properties.