Anisotropic Proton and Oxygen Ion Conductivity in Epitaxial Ba2In2O5 Thin Films

TL;DR

This study demonstrates that both proton and oxygen ion conductivities in epitaxial Ba2In2O5 thin films are anisotropic, with conduction properties depending on crystallographic orientation, revealing the influence of layered structures and defect ordering.

Contribution

It provides experimental evidence of anisotropic proton and oxygen ion conduction in BIO thin films, highlighting the role of layered structures and defect ordering in charge transport.

Findings

Oxygen ion conduction is anisotropic due to layered oxygen sublattice.

Proton conduction anisotropy supports proton defect ordering.

Charge transport varies with crystallographic orientation in epitaxial films.

Abstract

Solid oxide oxygen ion and proton conductors are a highly important class of materials for renewable energy conversion devices like solid oxide fuel cells. Ba2In2O5 (BIO) exhibits both oxygen ion and proton conduction, in dry and humid environment, respectively. In dry environment, the brownmillerite crystal structure of BIO exhibits an ordered oxygen ion sublattice, which has been speculated to result in anisotropic oxygen ion conduction. The hydrated structure of BIO, however, resembles a perovskite and the protons in it were predicted to be ordered in layers. To complement the significant theoretical and experimental efforts recently reported on the potentially anisotropic conductive properties in BIO, we measure here the proton and oxygen ion conductivity along different crystallographic directions. Using epitaxial thin films with different crystallographic orientations the charge transport for both charge carriers is shown to be anisotropic. The anisotropy of the oxygen ion conduction can indeed be explained through the layered structure of the oxygen sublattice in brownmillerite BIO. The anisotropic proton conduction however, further supports the suggested ordering of the protonic defects in the material. The differences in proton conduction along different crystallographic directions attributed to proton ordering in BIO are of a similar extent as those observed along different crystallographic directions in materials where proton ordering is not present but where protons find preferential conduction pathways through chain-like or layered structures.