Mixed proton and oxide ion conduction, phase stability, and conducting mechanisms in the Sr₂CeO₄-based materials

Sr₂CeO₄ possesses structural characteristic of one-dimensional (1D) edge-sharing CeO₆ octahedral chains, exhibits superior phase and chemical stability compared to the perovskite SrCeO₃. There have been previous attempts to improve electrical conductivity through proton conduction by acceptor-doping of Yb on the Ce site. However, its electrical conductivity remained low, for instance, at 500 °C, the reported value was 3.16 × 10⁻⁷ S/cm, hence limiting its practical applications. In this paper, we synthesized four new rare earth doped Sr₂Ce_0.95M_0.05O_3.975 (where M = Y, Dy, Er, Tb) by the solid state reaction method. We thoroughly investigated their phases, electrical properties, and stability. Additionally, for the first time, we investigated the proton and oxide ion conduction mechanisms in Sr₂CeO₄-based materials using a bond-valence-based method. This analysis revealed a clear 1D conduction along the c-axis within the CeO₆ tetrahedral chains and 2D conduction within the a-c plane, respectively. The findings revealed that all doped materials showed obvious proton conduction, alongside partial oxide ion conduction. Notably, the Er-, Dy-, and Y-doped samples demonstrated significantly improved conductivity under wet condition (with a water partial pressure of ∼0.03 atm) at 500 °C, reaching approximately ∼ 10⁻⁴ S/cm. This is nearly three orders of magnitude higher than the previously reported Yb-doped Sr₂CeO₄.