High proton conduction in Ba 2 LuAlO 5 with highly oxygen-deficient layers

Proton conductors have found diverse applications, such as electrolytes in proton ceramic fuel cells, which require high ionic conductivity at low temperatures and high chemical stability. Here, we report the oxide, Ba 2 LuAlO 5 , which exhibits proton conductivities of 10 − 2 S cm − 1 at 487 °C and 1.5 × 10 − 3 S cm − 1 at 232 °C, high diffusivity and high chemical stability without chemical doping. Ba 2 LuAlO 5 is a hexagonal perovskite-related oxide with highly oxygen-deficient hexagonal close-packed h′ layers, which enables a large amount of water uptake x = 0.50 in Ba 2 LuAlO 5 · x H 2 O. Ab initio molecular dynamics simulations and neutron diffraction show the hydration in the h′ layer and proton migration mainly around cubic close-packed c layers existing at the interface of octahedral LuO 6 layers. These results demonstrate that the high proton conduction allowed by the highly oxygen-deficient and cubic close-packed layers is a promising strategy for the development of high-performance proton conductors.