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.
Kim et al. (Wed,) studied this question.