Metal-to-insulator transitions in 3d-band correlated oxides containing Fe compositions

Metal-to-insulator transitions (MITs), which are achieved in 3d-band correlated transitional metal oxides, trigger abrupt variations in electrical, optical, and/or magnetic properties beyond those of conventional semiconductors. Among such material families, iron (Fe: 3d⁶4s²)-containing oxides pique interest owing to their widely tunable MIT properties, which are associated with the various valence states of Fe. Their potential electronic applications also show promise, given the large abundance of Fe on Earth. Representative MIT properties triggered by critical temperature (T_MIT) were reported for ReFe₂O₄ (Fe^2.5+), ReBaFe₂O₅ (Fe^2.5+), Fe₃O₄ (Fe^2.67+), Re_1/3Sr_2/3FeO₃ (Fe^3.67+), ReCu₃Fe₄O₁₂ (Fe^3.75+), and Ca_1−xSr_xFeO₃ (Fe⁴⁺) (where Re represents rare-earth elements). The common feature of MITs of these Fe-containing oxides is that they are usually accompanied by charge ordering transitions or disproportionation associated with the valence states of Fe. Herein, we review the material family of Fe-containing MIT oxides, their MIT functionalities, and their respective mechanisms. From the perspective of potentially correlated electronic applications, the tunability of the T_MIT and its resultant resistive change in Fe-containing oxides are summarized and further compared with those of other materials exhibiting MIT functionality. In particular, we highlight the abrupt MIT and wide tunability of T_MIT of Fe-containing quadruple perovskites, such as ReCu₃Fe₄O₁₂. However, their effective material synthesis still needs to be further explored to cater to potential applications.