Enhancement of the chemical stability in confined δ-Bi₂O₃

Bismuth-oxide-based materials are the building blocks for modern ferroelectrics¹, multiferroics², gas sensors³, light photocatalysts⁴ and fuel cells^5,6. Although the cubic fluorite δ-phase of bismuth oxide (δ-Bi2O3) exhibits the highest conductivity of known solid-state oxygen ion conductors5, its instability prevents use at low temperature^7–10. Here we demonstrate the possibility of stabilizing δ-Bi2O3 using highly coherent interfaces of alternating layers of Er₂O₃-stabilized δ-Bi2O3 and Gd₂O₃-doped CeO₂. Remarkably, an exceptionally high chemical stability in reducing conditions and redox cycles at high temperature, usually unattainable for Bi₂O₃-based materials, is achieved. Even more interestingly, at low oxygen partial pressure the layered material shows anomalous high conductivity, equal or superior to pure δ-Bi₂O₃ in air. This suggests a strategy to design and stabilize new materials that are comprised of intrinsically unstable but high-performing component materials.