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

Mark D Scafetta; Tiffany C Kaspar; Mark E Bowden; Steven R Spurgeon; Bethany Matthews; Scott A Chambers

doi:10.1021/acsomega.9b03299

Reversible Oxidation Quantified by Optical Properties in Epitaxial Fe₂CrO_4+δ Films on (001) MgAl₂O₄

ACS Omega. 2020 Feb 14;5(7):3240-3249. doi: 10.1021/acsomega.9b03299. eCollection 2020 Feb 25.

Authors

Mark D Scafetta¹, Tiffany C Kaspar¹, Mark E Bowden², Steven R Spurgeon³, Bethany Matthews³, Scott A Chambers¹

Affiliations

¹ Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland 99532, Washington, United States.
² Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland 99532, Washington, United States.
³ Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland 99532, Washington, United States.

Abstract

We report on the structural and optical properties of Fe₂CrO_4+δ epitaxial films grown by molecular beam epitaxy on MgAl₂O₄ (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 Fe₂CrO₄ 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 Fe₂CrO_4+δ. This research provides new insights into the atomic structure, optical processes, oxidation states, electronic structure, and application potential of Fe₂CrO_4+δ.