High Oxygen Ion Conductivity in Hexagonal Perovskite Ba₇Nb₄MoO₂₀ via Epitaxy-Assisted Orienting of Two-Dimensional Diffusion Pathways

Oxygen ion conductors are a key component in solid-state ionic devices such as fuel cells, catalysts, sensors, and artificial intelligent devices. The recent discovery of undoped Ba₇Nb₄MoO₂₀ hexagonal perovskites has attracted great attention due to the existence of two-dimensional oxygen diffusion pathways between NbO₄ and MoO₄ tetrahedra. However, there have been rare studies on the control parameters for hexagonal perovskites to further boost oxygen ion transport at lower temperatures. Here, we find significantly higher oxygen ion conductivity (5.6 × 10^-4 S cm^-1 at 340 °C, 3.2 × 10^-1 S cm^-1 at 600 °C) of (001)-oriented Ba₇Nb₄MoO₂₀ epitaxial films by several orders of magnitude than that of sintered pellets. Our report is comparable to the oxygen ion conductivities of conventional doped conductors. X-ray diffraction and atomic-scale characterization with energy-dispersive X-ray spectroscopy reveal that this epitaxy-driven enhancement is attributed to the good alignment of two-dimensional pathways in an ion current direction. Our design principle of hexagonal perovskites will trigger an advanced understanding of the correlation between the crystal structure and ultrahigh oxygen ion conductivity.