Dopant Clusterization and Oxygen Coordination in Ta-Doped Bismuth Oxide: A Structural and Computational Insight into the Mechanism of Anion Conduction

Bi2O3 in its fluorite-like form can be obtained either at 730-824 °C, showing the highest oxide-ion conduction known so far, or by doping. We present a comprehensive appraisal of the local atomic structure of Ta-doped Bi2O3 investigating by X-ray absorption spectroscopy the aggregation motifs of Ta5+ and the interaction between dopants and oxygen vacancies. Using periodic density functional theory simulations, we show that the connection of Ta4O18 aggregates is energetically favorable. We find that the local coordination of Bi3+ and its electronic structure, as seen from the calculated density of states (DOS), are invariably determined by the Bi 6s2 lone pair in both doped and undoped Bi2O3. This does not depend on the long-range symmetry that is revealed by X-ray diffraction studies. From the similarity of the DOS of α-Bi2O3 and Ta-doped bismuth oxide, it is inferred that the force governing the local coordination of Bi is essentially the same in all forms of Bi2O3. As the local Bi environment, determined by X-ray absorption spectroscopy, is also found to be very similar in all investigated samples, regardless of the dopant concentration, the local mechanism of oxide ion diffusion is arguably similar in doped and undoped bismuth oxide.