The Oxygen Partial Pressure Dependence of Space Charges at SrTiO ₃ |Mixed Ionic Electronic Conducting Oxide Heterojunctions

The electrical and electrochemical properties of mixed conducting oxides often depend on the oxygen partial pressure p(O₂) and numerous studies have dealt with the p(O₂) dependencies found in bulk materials. However, measurements regarding the properties of interfaces between two different mixed conducting oxides are much less common. This work investigates the interfacial space charge region in SrTiO₃ (STO) caused by the contact with another mixed ionic and electronic conductor (MIEC), specifically La_0.6Sr_0.4FeO_3-δ (LSF), La_0.65Sr_0.35MnO_3-δ (LSM) and La_0.9Sr_0.1CrO_3-δ (LSCr). The space charge regions were investigated by means of electrochemical impedance spectroscopy at 500 °C in the broad p(O₂) range of 1 bar to 10^-32 bar. All measurable space charge potentials Δϕ show a decrease with decreasing p(O₂). A model correlates the change in Δϕ with the strongly p(O₂) dependent defect concentrations of the MIECs. The measured partial pressure dependencies of Δϕ can be fully attributed to the different p(O₂) dependencies of the respective bulk Fermi levels. The suggested model is broadly applicable to MIEC|MIEC interfaces in general and can also be used to predict transitions of space charges from hole depleted to electron depleted layers.