Bulk Analysis of Oxidation State Changes of doped Ba2In2O5-based Oxygen Transport Membrane Materials

Ba2In2O5 is an interesting ceramic material as it features different conductivities. Below 600 K it is proton conducting, above water is released and it becomes n-type mixed ionic-electronic conducting, enabling usage as oxygen transport membrane. However, there is great need to understand and predict doping effects in future, as e.g., both Cr-/Mn-doped Ba2In2O5 exhibit a p-to-n transition. The reason remains yet unclear. We have developed a concept based on equilibrium thermodynamics tracing back all defect formation energies to a common energy scale, the Fermi level, potentially enabling predictive doping strategies. The Fermi level and oxidation states can be directly measured by XPS; however, the analyzed volume is restricted to the sample surface region. XAS can perfectly complemented to monitor the dopants’ oxidation state changes in the bulk. Using the EXAFS range probes the direct dopant environment; hence, tracking ionic dopant compensation by oxygen interstitials in Ba2In2O5.