Anisotropic proton conduction in double-perovskite GdBaCo2O5.5

The proton-conductive properties of perovskite oxides have attracted attention for applications in sensors and neuromorphic devices. Recently, a unique two-dimensional proton conduction was theoretically predicted in the double-perovskite GdBaCo2O5.5 with a Gd/Ba order along the c-axis. To experimentally confirm the anisotropic proton conductivity, we prepared two types of GdBaCo2O5.5 epitaxial films with different crystal orientations: the films on NdGaO3(110) and SrTiO3(001) substrates had a Gd/Ba order along the in-plane and out-of-plane directions, respectively. Both the GdBaCo2O5.5 films were doped with protons via an electrochemical method under the same conditions at room temperature. From the reaction time dependence of the change in the lattice constants and proton depth profiles of the films, we found that the proton conductivity along the Gd/Ba order was slower than across it, which agreed well with theoretical calculations. The anisotropy of proton conductivity in GdBaCo2O5.5 probably originates from the BaO planes with large Ba2+ ions that hinder proton diffusion.