Effect of Electronic Interactions on Oxide-Ion Mobility in Solid Electrolytes with a Fluorite Structure

The high oxide-ion transport in three fluorite-structured oxide electrolytes is examined by using density functional theory. Our study elucidates the oxide-ion diffusion mechanism in yttrium (Y)-doped ZrO2, Y-doped CeO2, δ-Bi2O3, and α-Bi2O3, including the major change in oxygen mobility that occurs in Bi2O3 at the δ–α phase transition. This research focuses on the partial covalent interactions, often neglected in atomistic simulations, and their effect on the migrating oxygen ion. We found a direct relationship between the degree of hybridization (i.e., partial covalent interactions) in the resting state, transition state, and the activation barrier for oxygen migration. From this, we can understand the oxygen-ion migration energetics by analyzing the resulting electron density. The oxide ions migrate nonlinearly between two adjacent lattice sites to maximize partial covalent interactions with the neighboring cations. We also provide an explanation for the enhanced oxide-ion conductivity of δ-Bi2O3 compared to α-Bi2O3, related to the electron density due to different 6s lone-pair orientations. The disordered δ-phase has a complex lone-pair orientation, which also changes as the oxygen ion is migrating, resulting in a very low barrier to oxygen-ion migration. In contrast, α-Bi2O3, a material with exactly the same stoichiometry, has a well-ordered lattice, and the lone-pair orientation is always arranged to hinder the oxygen diffusion pathway.