Diversity of Iron Oxides: Mechanisms of Formation, Physical Properties and Applications

Iron oxide compounds have naturally formed during the whole of Earth’s history. Synthetic compositions with iron oxides are produced with the use of various techniques and widely used for scientific and applied purposes. This review considers an attempt to classify all the information on different iron oxide compound formation mechanisms and intended applications in biomedicine, catalysis, waste remediation, geochemistry, etc. All the literature references analyzed were divided into several groups by their number of included iron oxide compounds: compositions containing only one compound (e.g., magnetite or wüstite), including various polymorphs of iron(III) oxide (α-, β-, γ-, ε-, ζ-, δ-Fe2O3); compositions with two different distinguishable iron oxide phases (e.g., maghemite and hematite); compositions containing non-crystalline phases (amorphous iron oxide or atomic clusters); and compositions with mixed iron oxide phases (indistinguishable separate iron oxide phases). Diagrams on the distribution of the literature references between various iron oxide compounds and between various applications were built. Finally, the outlook on the perspectives of further iron oxide studies is provided.