Oxide-ion conduction via interstitials in scheelite-type LaNbO4: a first-principles study

Abstract

Interstitial oxide-ion conduction in tetragonal scheelite-structured oxides has theoretically been analyzed using first-principles calculations, taking lanthanum niobate (t-LaNbO₄) as a model system. In the crystal, two interstitial sites of oxide ions were identified by exhaustive structural optimizations, which are located between two NbO₄ tetragonal units to form an Nb₂O₉ unit. Among the four elementary paths found by the nudged elastic band method, the three common paths enable interstitial oxide ions to migrate over a long range both in the ab-plane and along the c-axis, leading to the almost isotropic oxide-ion conductivity. However, the oxide-ion conductivities estimated on the basis of the kinetic Monte Carlo method were much higher than the reported experimental values. The apparent activation energies were also different, ∼0.4 eV in the estimated conductivities vs. ∼0.9 eV in the experimental conductivities. As a result of additional evaluation of the trapping effect by dopants, interstitial oxide ions were found to be strongly trapped by W dopants on La sites. The corrected oxide-ion conductivity was drastically reduced by the trapping effect, finally to be in reasonable agreement with the experimental conductivities.