Mg Diffusion-Induced Structural and Property Evolution in Epitaxial FeO Thin Films.

Epitaxial FeO thin films grown on single crystal MgO(001) present well-defined model systems to study fundamental multivalent ion diffusion and associated phase transition processes in transition-metal-oxide-based cathodes. In this work, we show at an atomic scale the Mg diffusion pathways, kinetics, and reaction products at the FeO/MgO heterostructures under different oxygen partial pressures but with the same thermal annealing conditions. Combining microscopic, optical, and spectroscopic techniques, we demonstrate that an oxygen-rich environment promotes facile Mg incorporation into the Fe sites, leading to the formation of MgFeO spinel structures, where the corresponding portion of the Fe ions are oxidized to Fe. Conversely, annealing in vacuum results in the formation of a thin interfacial rocksalt layer (MgFeO), which serves as a blocking layer leading to significantly reduced Mg diffusion to the bulk FeO. The observed changes in transport and optical properties as a result of Mg diffusion are interpreted in light of the electronic structures determined by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Our results reveal the critical role of available anions in governing cation diffusion in the spinel structures and the need to prevent formation of unwanted reaction intermediates for the promotion of facile cation diffusion.