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 x 10(-3) S cm(-1) at 900 degrees 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.