Reversible Oxidation Quantified by Optical Properties in Epitaxial Fe(2)CrO(4+δ) Films on (001) MgAl(2)O(4)

[Image: see text] We report on the structural and optical properties of Fe(2)CrO(4+δ) epitaxial films grown by molecular beam epitaxy on MgAl(2)O(4) (001) as a function of δ (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 < δ < 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 ∼0.6 eV in Fe(2)CrO(4) films deposited at or above 400 °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 Fe(2)CrO(4+δ). This research provides new insights into the atomic structure, optical processes, oxidation states, electronic structure, and application potential of Fe(2)CrO(4+δ).