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.