Oxide Ion Conduction, Defect Chemistry, and the Conducting Mechanism in Ruddlesden–Popper La2–xMxBaIn2O7–0.5x (M = Ca, Sr, Ba)

Abstract

Tetragonal La2BaIn2O7 belongs to the stoichiometric n = 2 members of the Ruddlesden–Popper (RP) structure, with La3+ and Ba2+ cations ordered in the rock salt and octahedral layers, respectively. Herein, a series of alkaline-earth-metal-doped materials La2–xMxBaIn2O7–0.5x (M = Ca, Sr, Ba) were prepared by the traditional solid-state reaction method. The results revealed that the substitutions of Ca/Sr/Ba for La introduced oxygen vacancies and led to good oxide ion conduction, e.g., ∼1.21 × 10–3–1.38 × 10–2 S/cm within 600–900 °C for the specimen La1.7Ba1.3In2O6.85, which was more than 2 orders of magnitude higher than the parent material. However, both the parent and acceptor-doped La2BaIn2O7 showed very limited proton conductions. The energetics of defect formation and oxygen vacancy migrations in La2BaIn2O7-based materials was studied through the static lattice and molecular dynamics (MD) atomistic simulations based on the interatomic potential approach. The defect formation energy calculations for the electrons and holes in La2BaIn2O7 rationalized the experimentally observed pure ionic conduction under low pO2 and mixed ionic and p-type electronic conductions under high pO2, respectively. The MD simulations indicated two-dimensional oxygen vacancy migration within the perovskite slab for the acceptor-doped La2BaIn2O7.