Reversible Oxidation Quantified by Optical Properties in Epitaxial Fe₂CrO₄₊ Films on (001) MgAl₂O₄

We report on the structural and optical properties of Fe2CrO4+delta epitaxial films grown by molecular beam epitaxy on MgAl2O4 (001) as a function of delta (average cation valence). The average Fe valence is linked to the out-of-plane lattice parameter and the extent of light absorption in the infrared spectral region. Over-oxidized films (0 < delta < 0.5) exhibit smaller lattice parameters and suppressed infrared absorption. The lattice parameter is found to differ for films of equivalent oxidation state but different thermal histories. We discuss the behavior of a novel infrared transition present at similar to 0.6 eV in Fe2CrO4 films deposited at or above 400 degrees C. An optical transition found in all films at 0.9 eV independent of the synthesis temperature can be used to quantify the oxidation state of Fe2CrO4+delta. This research provides new insights into the atomic structure, optical processes, oxidation states, electronic structure, and application potential of Fe2CrO4+delta.