High Conductivity and Diffusion Mechanism of Oxide‐Ion in Sillén‐Aurivillius Phase Oxobromide

Oxide‐ion conductors are applicable in fields such as solid oxide fuel cells (SOFC), oxygen sensors and exchange membranes. A pivotal factor in advancing these applications is the discovery of new classes of oxide‐ion conductors. Mixed‐anion compounds, with their diverse coordination environments and orbital interactions, offer opportunities to tailor functional properties. In this work, we report Sillén–Aurivillius phase oxyhalides Bi 4 M O 8 X ( M = Nb 5+ , Ta 5+ ; X = Cl − , Br − , I − ) as a new family of oxide‐ion conductors. Among them, Oxobromide Bi 4 TaO 8 Br exhibits the highest oxide‐ion conductivity, attributed to its expanded lattice and favorable oxygen‐vacancy migration pathways. Furthermore, Sr 2+ substitution at the Bi 3+ sites within the oxide‐ion conducting layers significantly enhances ionic transport, with Bi 3.9 Sr 0.1 TaO 7.95 Br achieving a bulk conductivity of 5.3 × 10 −2 S cm −1 at 700°C, surpassing commercial SOFC electrolytes such as yttria‐stabilized zirconia (YSZ) and La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 2.815 (LSGM). This represents the first example of an oxide‐ion‐conducting oxobromide exhibiting such high oxide‐ion conductivity. These findings not only establish a new class of oxide‐ion conductors, but also highlight a promising design paradigm for enhancing ionic transport in mixed‐anion oxyhalides through halide regulation and targeted doping.