Abstract
The influence of a reducing atmosphere on transport properties of undoped and doped oxyborate Ba3Ti3O6(BO3)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 Ti4+ to Ti3+, 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 Ba3Ti3O6(BO3)2 is stable in the extreme reducing atmosphere, and the Ti4+/Ti3+ 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.
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