Investigating Oxides by Electrochemical Projection of the Oxygen Off-Stoichiometry Diagram Onto a Single Sample

The oxygen stoichiometry is key to tune functional properties of advanced oxides and has motivated numerous studies of the oxygen off-stoichiometry diagram, aiming to determine and control structural and functional (electronic/ionic, electrochemical, optical) properties, as well as the oxygen storage capacity. Here, a novel approach is developed, allowing to project selected oxygen chemical potential regions onto a single thin film sample with unprecedented control. Therefore, a specifically designed electrochemical cell geometry is deployed, resulting in a well-defined, in-plane oxygen concentration gradient, whose endpoints can be flexibly controlled via the pO₂ and applied overpotentials, and which is independent of variations in the materials electrical resistivity. This allows for an unparalleled study of materials properties as continuous function of the oxygen content using spatially resolved tools (spectroscopic, diffraction, microscopy, etc.) and thereby greatly reduces experimental efforts while avoiding sample-to-sample variability, multi-step treatments, degradation effects, etc. This work presents the proof-of-concept of in-plane oxygen gradients, based on spatially resolved ex/in situ and novel fixed-energy X-ray absorption near edge spectroscopy (XANES), X-ray diffraction, ellipsometry, and electrical measurements in hyper-stoichiometric La₂NiO_4+δ and sub-stoichiometric (La, Sr)FeO_3-δ thin films. It demonstrates the readiness and wide applicability of this innovative approach, highly relevant for fundamental as well as applied research.