Electron Localization Enhances Cation Diffusion in Reduced ZrO2, CeO2 and BaTiO3

TL;DR

This study uses first-principles calculations to show that electron localization and reduction significantly lower cation migration barriers in ZrO2, CeO2, and BaTiO3, explaining enhanced diffusion in reducing atmospheres.

Contribution

It reveals the role of electron localization and reduction in facilitating cation diffusion, resolving previous computational discrepancies and highlighting the importance of saddle-point energetics.

Findings

Reduction lowers migration barriers for cations.

Electron localization at saddle points creates negative-U states.

Synergistic effects of oxygen vacancies and reduction further enhance diffusion.

Abstract

According to defect chemistry, the experimental observations of enhanced cation diffusion in a reducing atmosphere in zirconia, ceria and barium titanate are in support of an interstitial mechanism. Yet previous computational studies always found a much higher formation energy for cation interstitials than for cation vacancies, which would rule out the interstitial mechanism. The conundrum has been resolved via first-principles calculations comparing migration of reduced cations and oxidized ones, in cubic ZrO2, CeO2 and BaTiO3. In nearly all cases, reduction alone lowers the migration barrier, and pronounced lowering results if cation's electrostatic energy at the saddle point decreases. The latter is most effectively realized when a Ti cation is allowed to migrate via an empty Ba site thus being fully screened all the way by neighboring anions. Since reduction creates oxygen vacancies as well, which are highly mobile, we also studied their effect on cation migration, and found it only marginally lowers the migration barrier. In several cases, however, a large synergistic effect between cation reduction and oxygen vacancy is revealed, causing an electron to localize in the saddle-point state at a much lower energy than normal, signaling that the saddle point is a negative-U state in which the soft environment enables a large electron-phonon interaction that can over-compensate the on-site Coulomb repulsion. These general findings are expected to be applicable to defect-mediated ion migration in most transitional metal oxides.