Correlating Composition and Cation Inversion in Ternary Iron Spinel Oxides with Localization of Photoexcited States

Ternary iron spinel oxides are promising materials for photo(electro)catalytic applications. However, like other first-row transition metal oxides, these materials have a propensity to form localized photoexcited states known as polarons that may impact their performance. In this work, we explicitly link computed electronic and vibrational structures to experimental optical dielectric and resonance Raman spectra to establish the optical polaronic properties of three different ternary iron spinel oxides: CoFe₂O₄, NiFe₂O₄ and FeNi₂O₄. Ternary iron spinel oxides are known to crystallize with a range of different distributions of cations among tetrahedral and octahedral sites. Here, we correlate heterogeneous broadening observed in resonance Raman spectra and sub-band gap charge transfer transitions observed in optical dielectric spectra to cation inversion. We use DFT + U + J computations to demonstrate the sensitivity of charge transfer transitions to changes in cation inversion in CoFe₂O₄ and NiFe₂O₄. Experimentally, charge transfer transitions at the band-edge that arise from cation inversion exhibit strong phonon coupling in all three ternary iron spinel oxides, which is indicative of the formation of localized photoexcited states. We establish that, by changing the composition of the ternary iron spinel, the phonon mode that exhibits the strongest coupling to the band-edge absorption also changes, suggesting that the structure of photogenerated polarons in ternary iron spinel oxides can be controlled by tuning their composition. With a fundamental understanding of how composition influences the localization of photogenerated charge carriers, there are opportunities for rational material engineering to harness these carriers in photocatalytic applications.