High oxygen vacancy concentration and improved electrical conductivity in tetragonal LaNbO4 stabilized by Ga and Mo Co-doping on the Nb site

The LaNbO₄-based materials were well documented to be good ionic conductors with the charge carriers of protons or interstitial oxygen ions. Herein, for the first time, we reported that the high oxide ion conduction, e.g. 3 × 10⁻³ S cm⁻¹ at 900 °C, mediated by oxygen vacancies was achieved in LaNbO₄ via equimolar Ga and Mo co-doping on the Nb site. Such a co-doping effectively stabilize the high temperature tetragonal structure of LaNbO₄ to room temperature, and thus represents the first case of room temperature tetragonal LaNbO₄ with high oxygen vacancy concentration, in contrast with the previously reported tetragonal LaNbO₄ stabilized by isovalent doping free of oxygen vacancies or donor-doping with interstitial oxygen. The local structure and conducting mechanism of oxygen vacancy defects were thoroughly studied by computational simulations. The results revealed that the oxygen vacancy was accommodated by transforming two neighboring isolated NbO₄ tetrahedrons into a corner-sharing Nb₂O₇ unit, and the oxygen ion migrated via a cooperative process involving the breaking and reforming of Nb₂O₇ units, assisted by synergic rotation and deformation of other neighboring NbO₄ tetrahedra. These findings provided us a more comprehensive understanding for the LaNbO₄-based materials and emphasized the possibility of developing LaNbO₄ material as oxide-ion conductors mediated by high concentration of oxygen vacancies.