Uncovering the Electronic Effects of Cr Doping in Ba₂In₂O₅

Up to now, the material’s response of ceramic oxygen transport membranes has been mainly described by crystal structure changes and defect chemistry models, which largely neglect the changes in the electronic structure implied by the executed doping routines. To improve this situation, we examine the influence of partially replacing indium with chromium in brownmillerite-type Ba2In2O5 on the electronic structure and Fermi level position by X-ray photoelectron spectroscopy (XPS). The XPS data clearly revealed the presence of Cr6+ as the main chromium species, pointing to an n-type doped Ba2In2O5 with an enhanced electronic conductivity. However, in contrast to In2O3, the Fermi level shift upon heavy donor doping is insignificant. This strongly suggests that the necessary compensation mechanism in chromium-doped Ba2In2O5 is mainly achieved by ionic charge compensation due to a filling of initially empty oxygen interstitial sites in the structure. The latter was already previously obtained by neutron diffraction. The results of this study suggest that an intensified analysis of the Fermi level changes in functional oxides upon doping provides direct access to the underlying charge compensation mechanisms that mainly control the resulting material properties.