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According to defect chemistry, the experimental observations of enhancedcation diffusion in a reducing atmosphere in zirconia, ceria and bariumtitanate are in support of an interstitial mechanism. Yet previouscomputational studies always found a much higher formation energy for cationinterstitials than for cation vacancies, which would rule out the interstitialmechanism. The conundrum has been resolved via first-principles calculationscomparing migration of reduced cations and oxidized ones, in cubic ZrO2, CeO2and BaTiO3. In nearly all cases, reduction alone lowers the migration barrier,and pronounced lowering results if cation's electrostatic energy at the saddlepoint decreases. The latter is most effectively realized when a Ti cation isallowed to migrate via an empty Ba site thus being fully screened all the wayby neighboring anions. Since reduction creates oxygen vacancies as well, whichare highly mobile, we also studied their effect on cation migration, and foundit only marginally lowers the migration barrier. In several cases, however, alarge synergistic effect between cation reduction and oxygen vacancy isrevealed, causing an electron to localize in the saddle-point state at a muchlower energy than normal, signaling that the saddle point is a negative-U statein which the soft environment enables a large electron-phonon interaction thatcan over-compensate the on-site Coulomb repulsion. These general findings areexpected to be applicable to defect-mediated ion migration in most transitionalmetal oxides.

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