Ionic to Electronic Transport in Ba3Ti3O6(BO3)2 under Reducing Atmosphere

The influence of a reducing atmosphere on transport properties of undoped and doped oxyborate Ba 3 Ti 3 O 6 (BO 3 ) 2 has been studied using various experimental methods and first-principles calculations. We find that the electrical transport mechanism in the material changes from being ionic under oxidizing and soft reducing conditions to mainly electronic under extreme reducing conditions; consequently, the total conductivity is increased by about 200 times. The change is caused by the emergence of electron polarons, associated with the reduction of Ti 4+ to Ti 3+, as the predominant highly mobile current-carrying defects. Hybrid-density-functional electronic structure and defect calculations provide a novel theoretical framework for understanding the observed transport properties, including the ionic to electronic transition and the effects of doping. We also find that Ba 3 Ti 3 O 6 (BO 3 ) 2 is stable in the extreme reducing atmosphere, and the Ti 4+ /Ti 3+ reduction and re-oxidation reactions are reversible. This mixed ionic–electronic conductor can thus have applications in solid–oxide fuel-cell anodes or anode composites.