High Oxide‐Ion Conductivity through the Interstitial Oxygen Site in Sillén Oxychlorides

Abstract Oxide‐ion conductors are gaining attention as future materials in energy applications, such as solid oxide fuel cells. Many Bi‐containing compounds exhibit high oxide‐ion conductivity via conventional vacancy mechanism. However, interstitial oxide‐ion conduction is rare in Bi‐containing materials. Herein, high oxide‐ion conductivity is reported through interstitial oxygen sites in Sillén oxychlorides, LaBi_2−xTe_xO_4+x/2Cl (Bi₂LaO₄Cl‐based oxychlorides). Oxide‐ion conductivity of LaBi_1.9Te_0.1O_4.05Cl is 20 mS cm⁻¹ at 702 °C, and higher than best oxide‐ion conductors as Bi₂V_0.9Cu_0.1O_5.35 below 201 °C. Despite of the presence of Bi and Te species, LaBi_1.9Te_0.1O_4.05Cl shows extremely high chemical and electrical stability at 400 °C from oxygen partial pressure 10⁻²⁵ to 0.2 atm and high chemical stability under CO₂ flow, wet 5% H₂ in N₂ flow, and air with natural humidity. Neutron scattering length density analysis, DFT calculations, and ab initio molecular dynamics simulations indicate that the extremely high oxide‐ion conduction is attributed to cooperative diffusion through interstitial oxygen sites (interstitialcy diffusion mechanism) in triple fluorite‐like layers. The present findings demonstrate the ability of LaBi_2−xTe_xO_4+x/2Cl as superior oxide‐ion conductors, which can open new horizons for oxide‐ion conductors.