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

Abstract

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.

1. Introduction

The chemistry of iron is of great interest because iron is an abundant element present in various fields [1]. The elemental abundance of oxygen, hydrogen and iron at the surface of and within the Earth’s crust has fostered widespread occurrences of iron oxides and oxyhydroxides in a diverse range of aquatic and terrestrial environments; most of the known iron oxides and oxyhydroxides are known to occur in nature [2]. Iron oxides are formed naturally through the weathering of Fe-containing rocks both on land and in the oceans and play an important role in geochemistry [3]. Iron-rich sedimentary rocks have had important implications in the evolution of Earth’s atmosphere and hydrosphere [4]. Iron oxide copper gold, apatite-magnetite and other ore deposits have very important heavy industrial applications [5,6]. There are iron oxides on the surface of Mars, in the depths of Earth, in old rusting factories, in pigeon brains and magnetotactic bacteria [7]. Despite the precise mechanism of biogenic magnetite mineralization on early Earth still being unknown, the understanding of this mechanism includes the origin of banded iron formations [8]. Iron oxides are also linked to pathological states of the human body, such as iron dysregulation in the brain and neurological disorders [9,10].

Metallurgy has been developed relating to iron and iron oxides and used for various applications including colored pigments, magnetic materials, catalysts, water oxidation, biomedical uses including therapy and diagnostics, etc. [1,11,12,13,14]. The stability of the structural incorporation of uranium into the hematite crystal structure suggests the feasibility of iron oxides for inhibiting the mobility of aqueous uranium (VI) [15]. The use of special stainless steels (i.e., Eurofer steel) for some portions of the main wall of a nuclear fusion experimental reactor may come into consideration in industrial applications of nuclear fusion; therefore, the detailed knowledge and quantification of their interactions between atoms, molecules and plasma, including electron impact ionization cross sections of iron oxide molecules, is of considerable interest [16]. Iron oxides are, furthermore, of great interest with regard to the corrosion and oxidation processes of iron metal and steel, which are mediated by the surface whose structure depends greatly on environmental variables such as temperature, oxygen or water partial pressures [17,18]. Additionally to the various Fe oxidation states, ferric oxide (Fe₂O₃) may be stable or metastable in the known α, β, γ, ε, ζ and δ polymorphs [19,20,21,22]. Polymorphism in this case means a possibility for a compound to exist in two or more solid phases that are isochemical but have distinct crystal structures and thus, different physical properties. Due to their different physical properties, which arise from the differences in their crystal structures, all of the polymorphs have found applications in nanotechnology [20]. Moreover, there are also not simply polymorphs of known iron oxides, but distinct compounds with the formula Fe₄O₅ [23]. Other iron oxides with unconventional stoichiometry, such as Fe₅O₆, Fe₅O₇, Fe₇O₉ and more complicated compounds have been predicted in theory, some of which have been successfully synthesized at pressures of 10 ∼ 80 GPa and annealed from high temperatures [24]. Magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃), including in the form of the oxidized zerovalent iron core-shell structure, are widely used in heterogeneous catalysis processes and have been used as attractive alternatives for the treatment of wastewaters and soils contaminated with organic compounds [25,26,27].

Structures containing the various iron oxide crystalline or amorphous phases can either be macro- (ceramics [18]), micro- (microbial-induced precipitates [28]) or nanometer-sized (nanoparticles and nanoclusters [29]). The nucleation and growth of inorganic crystals, including iron oxides, from solutions occurs throughout geochemical, biological and synthetic systems [30]. Iron oxide biominerals are formed under “green” conditions without a loss of functionality such that they have the potential for numerous scientific as well as industrial applications [31]. Global iron cycling is driven by both abiotic and biotic reactions, and in the presence of oxygen and under circumneutral pH conditions, ferrous iron is quickly oxidized to Fe(III) and precipitates as iron oxides [32,33].

The physical properties of the various iron oxide compounds can be extremely different, e.g., magnetite and maghemite have been commonly used in biomedicine because of their saturation magnetization being the highest [34]. The ε-polymorph of Fe₂O₃ possesses the highest coercive force (up to 2 T at room temperature [20,35,36,37]) among the other ferrimagnetic oxides, while maghemite-magnetite nanoparticles (NPs) can have almost zero coercivity [38] with a very low difference in their average size. In some applications, e.g., in catalysts, amorphous iron oxide NPs can be more active than crystalline polymorphs of the same diameter thanks to their “dangling bonds” and higher surface–bulk ratio in their amorphous phase [39]. Based on the literature data, a generalized scheme illustrating the diversity of the known iron oxide compounds, including stable, metastable, atomic clusters and amorphous, is given in Figure 1.

Figure 1. Diversity of stable (solid line) and metastable (dashed line) iron oxides.

The current review attempts to summarize the main information on the various iron oxide compounds to date. The next section is devoted to a comparison of some of the main physical properties of the various iron oxide compounds.

2. Physical Properties of Various Iron Oxide Compounds

An Fe–O phase diagram is given in Figure 2. According to the ratio of Fe²⁺ and Fe³⁺, the phase diagram of Fe–O can be divided into six phase zones from right to left [40].

Figure 2. Equilibrium phase diagram for Fe–O system. Reprinted from [40], with permission from Elsevier.

The shown phase diagram does not take into account the polymorphisms of ferric oxides. Sakurai et al. [41] discuss the crystal structures and magnetic properties of the four phases are the following: ferrimagnetic γ-Fe₂O₃ with a spinel structure; ferrimagnetic ε-Fe₂O₃ with an orthorhombic structure; antiferromagnetic β-Fe₂O₃ with a bixbyite structure; and weak ferromagnetic α-Fe₂O₃ with a corundum structure. The observed phase transformations for Fe₂O₃ phases inside of the mesoporous silica matrix are due to the surface (or interface) energy G_S contribution to the total free energy G = G_B + (6V_m/d)G_S, where G_B is the free energy in the bulk, V_m is the molar volume and d is the diameter of the NP. The G vs. d curves should appear as shown in Figure 3.

Figure 3. Representation of free energy G vs. particle diameter d curves for the four Fe₂O₃ phases. Gray, blue, green and red lines represent the G values of γ-, ε-, β- and α-Fe₂O₃, respectively. Thick solid lines indicate the most stable Fe₂O₃ phases over the corresponding size ranges. Copyright 2009 by the American Chemical Society. Reprinted with permission from [41].

Such a strong correlation between the most stable iron oxide crystal phase and the matrix makes it possible to create nanomaterials with diametrically different magnetic properties. Our team has provided the studies on the natural and synthetic silica-based systems containing inclusions of iron oxide NPs. Figure 4 illustrates the static magnetic characteristics of the silica-based systems with NPs of ε-Fe₂O₃ with a coercive force H_c = 1.07 T [37], γ-Fe₂O₃–Fe₃O₄ with H_c = 0.5 mT [38] and natural magnetite from the Kovdor deposit with H_c = 4 mT [6]. Thus, despite their almost identical chemical composition, the similar structures with iron oxides can possess different H_c values by more than three orders of magnitude.

Figure 4. Static magnetic properties of various samples of silica–iron oxide systems: (a)—Hysteresis loop and backfield demagnetization curve of a sample containing ε-Fe₂O₃. Reprinted from [37], license CC BY 4.0; (b)—Magnetic hysteresis curve of Fe_mO_n-SiO₂ synthetic colloidal nanoparticles. Reprinted from [38], with permission from Elsevier; (c)—Backfield curve and central part of the hysteresis loop of magnetite ore powder; full loop in the 1.8 T maximum field is shown in the inset. Reprinted from [6], with permission from Elsevier.

Iron oxide compounds can differ not only by their coercive force, as shown above, but also by their other physical characteristics. Some of them are shown in Table 1.

Table 1. Comparison of the physical characteristics of various iron oxide compounds [42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73].

Parameter	Iron Oxide Compound
	FeO	Fe3O4	α-Fe2O3	β-Fe2O3	γ-Fe2O3	ε-Fe2O3
Mineral name	Wüstite	Magnetite	Hematite	–	Maghemite	–
Crystal structure	Cubic [42]	Cubic spinel [43]	Rhombohedral [44]	Cubic [62]	Cubic spinel [45]	Orthorhombic [46]
Static dielectric permittivity	22.6 [47]	20–40 [48,49]	12–26 [48,49,50]	n/a	20 [48]	n/a
High-frequency dielectric permittivity	10.8 [47]	7–16 [51]	7.6–7.9 [50]	n/a	14.2 [52]	4–10 [53,54]
Saturation mass magnetization 1 at 300 K, emu/g	11–18 [55,56]	92–94 [57]	0.3–1.9 [58,59]	0.02–0.05 [63,64]	74–80 [60,61]	15 [46]
Curie/Neel point, K	196 [47,55]	838–856 [66]	948–963 [41,58]	110–119 [62,64]	618–928 [41,67]	480–495 [41,46]
Optical band gap at 300 K, eV	1.0 [47]	0.2 [68,69]	1.9–2.2 [59,70,71,72]	1.7–1.9 [64,65]	2.0 [60]	2.0–2.4 [73]

¹ The values of saturation mass magnetization are given for the bulk materials.

The crystal structure of an iron oxide compound can be attributed to various symmetry groups: cubic for FeO [42], Fe₃O₄ [43] and γ-Fe₂O₃ [45], rhombohedral for α-Fe₂O₃ [44], and orthorhombic for ε-Fe₂O₃ [46]. Crystal structure variation influences the electron zone structure, which can be semi-metallic with an optical band gap (at 300 K) of 0.2 eV for Fe₃O₄ [68,69] and semiconductive for FeO with an optical band gap of 1.0 eV [46], as well as for α-, γ- and ε-Fe₂O₃ with an optical band gap of 1.9–2.4 eV [58,59,70,71,72,73]. There is no clear correlation between the crystal structure of an iron oxide compound and its dielectric permittivity, for which values are in the range of 12–40 for static permittivity [46,47,48,49] and 4–16 for high-frequency permittivity [46,49,50,51,52,53]. Since the various iron oxides can possess antiferromagnetic or ferrimagnetic properties, their value of saturation mass magnetization can differ by a few orders of magnitude: from 0.3 emu/g (at 300 K) for pure hematite [57,58] to 92–94 emu/g for stoichiometric magnetite [56]. Such variation in electrical and magnetic characteristics lies in the versatility of the structures containing iron oxides for their wide spectrum of possible applications. The next section is devoted to the main mechanisms of iron oxide formation disclosed in the scientific literature.

3. Mechanisms of Iron Oxide Formation

The literature data on the iron oxide formation mechanisms were divided into several groups according to the iron oxide compounds in the studied structures: the structures containing pure crystal iron oxide phases, the ones containing iron oxide atomic clusters and amorphous iron oxides, the ones containing two co-existing iron oxide crystal phases, and the ones containing three or more co-existing iron oxide phases. Their formation mechanism, either natural or synthetic, is briefly described based on the data in the cited reference. Furthermore, their existing and potential (declared) applications are also given.

3.1. The Structures Containing Pure Phases of FeO, Fe₄O₅, Fe₃O₄, and α-, β-, γ-, δ-, ε- and ζ-Polymorphs of Fe₂O₃

The information on the structures with a pure iron oxide crystal phase is given separately for each iron oxide compound in Table 2, Table 3, Table 4, Table 5 and Table 6.

Based on the gathered information, it was possible to determine the most frequently obtained form of the structures containing FeO (Table 2). This form is FeO NPs produced for biomedical applications via various chemical or physical routes, but not naturally originated, since this iron oxide compound is metastable under normal conditions [42,74,75,76,77,78,79,80].

As one of the stable iron oxide compounds, magnetite can be obtained from the various structures, including natural abiotic or biogenic and synthetic ones (Table 3). Fe₃O₄ NPs can be considered as one of the most frequently used iron oxide magnetic materials and their production techniques are mostly chemical since this approach is most suitable for biomedical applications [81,82,83,84,85,86,87,88,89]. Additionally, “green” techniques, involving the use of natural plant extracts [90,91], biomimetic formation [92] or microbial mineralization [93,94,95,96,97,98], are widely used. Other commonly described forms of magnetite are its inclusion within ore samples [6,99,100,101,102,103,104] widely used in geosciences and the external magnetite layer of metal surfaces [105,106,107,108,109], which is an important object of iron corrosion studies.

The most stable (under normal conditions) iron oxide compound, hematite, attracts the highest attention of scientists, according to the overall amount of published papers (Table 4). Similarly to magnetite, the most frequently obtained form of α-Fe₂O₃ is a chemically synthesized NP, predominantly used for photocatalysis, biogeochemistry and the bioremediation of toxic compounds [105,106,107,108,109,110,111,112,113,114,115,116,117,118,119]. The second important form of hematite is the natural ore with inclusions of α-Fe₂O₃, which is used in Earth sciences [117,118,119,120,121,122,123,124]. The use of the pure γ-Fe₂O₃ is quite rare. The most frequently produced form of γ-Fe₂O₃ is as NPs (Table 5), including chemically synthesized [125,126,127,128,129] or biogenic [93].

Structures with β-Fe₂O₃, ε-Fe₂O₃, ζ-Fe₂O₃, δ-Fe₂O₃ or Fe₄O₅ metastable phases (Table 6) are much less described compared to the main stable compounds of iron oxides. ε-Fe₂O₃ is the only one seriously applicable in biomedicine [130,131], electronics [132,133] or geosciences [134,135] and can be obtained both synthetically [37,136,137] or by extraction from various archeological objects [138,139,140]. There have also been some attempts to use β-Fe₂O₃ NPs in biomedicine [63], sensors and lithium-ion batteries [20,64].

Table 2. Data on the structures containing an FeO phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
FeO NPs	Thermal decomposition of the iron(II) precursor, mechanochemical reduction of magnetite, flame synthesis, laser target interaction in liquid carrier media	Biomedicine, electronics, spintronics, magnetic force microscopy, metastability studies	XRD 1, UV–Vis 2, MALDI-TOF MS 3, EELS 4, F-AAS 5 HAADF-STEM 6,	[41,74,75,76,77,78,79,80]
Ultra-thin FeO film	Oxidation of iron monocrystal surface	Iron oxidation kinetics study	RMDS 7	[141]
	Electron-beam deposition on Au(111) surface	Iron catalysis, electronics, biomedicine	STM 8	[142]
Millimeter-sized iron oxide particles	Magnetite reduction with iron as reducing agent	Catalysts for ammonia synthesis	TG-DSC 9	[40]
FeO layer on the metal alloy surface	Invar oxidation in a static carbon dioxide atmosphere	Iron oxidation kinetics study	XRD, TG-DSC, TEM 10	[143]
Wüstite inclusions in titanomagnetite particles	Titanomagnetite ironsand-fluidized bed reduction by hydrogen	Commercial iron making	XRD	[144]
FeO powder	Reduction of hematite in a gas-controlled electric furnace	Earth’s mantle sound velocity studies	XRD, IXS 11	[145]
FeO inclusions in the mold flux	Iron oxide formation in molten mold flux	Study of the oxidation mechanism of mold flux-covered molten iron	XRF 12	[146]
FeO inclusions within the dense iron shell	Porous hematite gas reduction under isothermal conditions	Industrial exploitation of low-grade iron ores	TG-DSC, XRD	[147]
FeO clusters within the stable iron oxide matrix	The reduction of magnetite/hematite at temperatures of 400~500 °C	Iron catalysis	Quantitative theoretical analysis	[148]

¹ X-ray powder diffraction. ² UV–visible(-NIR) spectroscopy. ³ Matrix-assisted laser desorption ionization time-of-flight mass spectrometry. ⁴ Electron energy loss spectroscopy. ⁵ Flame atomic absorption spectroscopy. ⁶ High-angle annular dark-field scanning transmission electron microscopy. ⁷ Reactive molecular dynamics simulations. ⁸ Scanning tunneling microscopy. ⁹ Thermogravimetry/differential scanning calorimetry. ¹⁰ Transmission electron microscopy. ¹¹ Inelastic X-ray scattering. ¹² X-ray fluorescence.

Table 2. Data on the structures containing an FeO phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
FeO NPs	Thermal decomposition of the iron(II) precursor, mechanochemical reduction of magnetite, flame synthesis, laser target interaction in liquid carrier media	Biomedicine, electronics, spintronics, magnetic force microscopy, metastability studies	XRD 1, UV–Vis 2, MALDI-TOF MS 3, EELS 4, F-AAS 5 HAADF-STEM 6,	[41,74,75,76,77,78,79,80]
Ultra-thin FeO film	Oxidation of iron monocrystal surface	Iron oxidation kinetics study	RMDS 7	[141]
	Electron-beam deposition on Au(111) surface	Iron catalysis, electronics, biomedicine	STM 8	[142]
Millimeter-sized iron oxide particles	Magnetite reduction with iron as reducing agent	Catalysts for ammonia synthesis	TG-DSC 9	[40]
FeO layer on the metal alloy surface	Invar oxidation in a static carbon dioxide atmosphere	Iron oxidation kinetics study	XRD, TG-DSC, TEM 10	[143]
Wüstite inclusions in titanomagnetite particles	Titanomagnetite ironsand-fluidized bed reduction by hydrogen	Commercial iron making	XRD	[144]
FeO powder	Reduction of hematite in a gas-controlled electric furnace	Earth’s mantle sound velocity studies	XRD, IXS 11	[145]
FeO inclusions in the mold flux	Iron oxide formation in molten mold flux	Study of the oxidation mechanism of mold flux-covered molten iron	XRF 12	[146]
FeO inclusions within the dense iron shell	Porous hematite gas reduction under isothermal conditions	Industrial exploitation of low-grade iron ores	TG-DSC, XRD	[147]
FeO clusters within the stable iron oxide matrix	The reduction of magnetite/hematite at temperatures of 400~500 °C	Iron catalysis	Quantitative theoretical analysis	[148]

¹ X-ray powder diffraction. ² UV–visible(-NIR) spectroscopy. ³ Matrix-assisted laser desorption ionization time-of-flight mass spectrometry. ⁴ Electron energy loss spectroscopy. ⁵ Flame atomic absorption spectroscopy. ⁶ High-angle annular dark-field scanning transmission electron microscopy. ⁷ Reactive molecular dynamics simulations. ⁸ Scanning tunneling microscopy. ⁹ Thermogravimetry/differential scanning calorimetry. ¹⁰ Transmission electron microscopy. ¹¹ Inelastic X-ray scattering. ¹² X-ray fluorescence.

Table 3. Data on the structures containing an Fe₃O₄ phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Fe3O4 NPs	Co-precipitation from iron salt solution, co-precipitation from iron oxyhydroxide solution, solvothermal synthesis, electrochemical formation from a pure iron, thermal decomposition of the iron oleate complex, biomimetic process with use of a leaf extract, nucleation mediated by iron-binding protein Mms6, biogeneration with a use of amyloid peptide Aβ42	Biomedicine, magnetic separation, antimicrobial and antioxidant applications, contaminant removal, black pigment production, ferrofluids	TEM, XRD, SAXS 1, RS 2, FTIR 3, XPS 4, HAADF-STEM, EELS, TG-DSC, UV–Vis, PL 5, MSP 6, SAED 7	[81,82,83,84,85,86,87,88,89,90,91,149,150,151,152,153,154,155,156,157,158,159,160,161,162]
Bacterial magnetosomes	Bacterial biomineralization, transient phosphate-rich ferric hydroxide reduction to magnetite, formation by dissimilatory iron-reducing bacteria	Biomedicine, paleomagnetism, microbial iron cycle studies, bioremediation of toxic compounds	HAADF-STEM, TEM, XAS 8, SAED, XMCD 9	[93,94,95,96,97,98,163]
Inclusions of Fe3O4 within ore samples	Abiotic hydrothermal mineralization, iron oxide formation derived from continental weathering, extrusive magmatic formation from iron oxide-melt liquid	Geochemistry, environmental magnetism studies, early Earth iron cycle studies, origin and evolution of iron oxides studies	XRD, RS, XPS, EDS 10, ICP-AES 11	[99,100,101,102]
External magnetite layer on a metal surface	Oxidation of a steel surface, slow oxidation of green rusts at room temperature, high-temperature corrosion	Corrosion studies	XRD, EDS, XRF, RS, XPS, AES 12	[105,106,107,108,109,110]
Fe3O4 microparticles	Microbial-induced precipitation with the use of Sporosarcina pasteurii	Green synthesis of magnetite	EDS	[28]
	Aging of ferrous hydroxide gels at elevated temperatures	Colloidal crud formation studies	XRD, TEM	[162]
Self-assembled Fe3O4 mesocrystalline films	Heat-up method with the use of iron(III) chloride and sodium oleate	Biomedicine and industrial applications	TEM, SAED, XAS, SAXS	[163]
Fe0/Fe3O4 composite	Controlled reduction of the starting Fe3O4 with H2	Treatment of wastewater	MSP, XRD	[164]
Magnetite nanowires	Supercritical fluid inclusion within a mesoporous silica matrix	Soft magnetic materials	TEM, SAED, XRD, FTIR	[165]
Inclusions of Fe3O4 NPs	Bacterial reduction of amorphous hydrous ferric oxide	Biogeochemistry	TEM, SAED, XRD, EDS	[166]
Fe3O4 layer on the zerovalent iron surface	Surface oxidation of iron by oxygen in an aqueous medium	Organic pollutant removal	EDS, XRD	[167]
Epoxy/magnetite nanocomposites	Reduction of anhydrous ferric chloride by ammonium hydroxide	Marine coatings of steel	FTIR, XRD, TEM	[168]
Iron oxide nanocomposite hydrogel	Co-precipitation process by ammonium hydroxide	Biomedicine	XRD, TEM, TG-DSC, EDS	[169]
Surface film containing Fe3O4 NPs	Bacterial mineralization in the air–water interface in Arctic tundra waters	Anaerobic microbial carbon cycle	TEM, EDS, STEM, EELS, FTIR, RS	[170]
Nanocomposite hydrogel with Fe3O4 NPs	Reduction with ammonia from a remixed solution of FeCl3 and FeCl2	Biomedicine	TEM, XRF, EDS, TG-DSC, FTIR	[171]
Biochar composite with Fe3O4 NPs	One-pot solvothermal method using phoenix tree leaf-derived biochar	Treatment of wastewater	TEM, XRD, FTIR, XPS, ICP-AES	[172]
Fe3O4 NP inclusions in the surface layer	Formation of NPs along with cracks and pores during pre-oxidation	Plasma nitriding of steel	XRD	[173]
Chitosan/graphene oxide composite with Fe3O4	Co-precipitation of Fe3O4 and chitosan/graphene oxide	Organic pollutant removal	XRD, XPS, RS, FTIR	[174]
Fe3O4 layer on carbon fibers of a carbon paper	Deposition on the carbon paper gas diffusion layer at the cathode	Corrosion studies	XRD, EDS	[175]
Mesocrystals assembled from Fe3O4 nanocubes	Heat-up method with the use of iron(III) chloride and sodium oleate	Mesocrystal applications	TEM	[176]
Fe3O4 nanorods	Formation in electron-beam-induced deposition from iron pentacarbonyl	Electronics	TEM, EELS	[177]
Lipase immobilized on coated Fe3O4 NPs	Solvothermal method with the use of FeCl3·6H2O and ethylene glycol	Biodiesel production	TEM, XRD, FTIR	[178]
Spherical mesoporous magnetite aggregates	Precipitation from iron(III) ethoxide with ethanol in the surfactant solution	Catalysis, sustainability	FTIR, XPS, EDS, TEM, MSP	[179]
Perfluorocarbon-loaded hydrogel microcapsules	Coaxial interface shearing double emulsion method	Biomedicine	–	[180]
Mesoporous magnetite	Ball milling of Fe3O4 and SiO2 followed by partial reduction	Recyclable absorbent for toxic Cr(VI) ions	TEM, XRD, XPS, ICP-AES	[181]
Magnetite crystal model	Local spin-density approximation density-functional calculation	Magnetite electron structure studies	Density-functional calculations	[43]
Spherulite nanostructure with inclusions of Fe3O4	Electron-beam irradiation of the precursor solution with iron nitrate	Crystal growth dynamics studies	TEM, STEM, EDS	[182]

¹ Small-angle X-ray scattering. ² Raman spectroscopy. ³ Fourier-transform infrared spectroscopy. ⁴ X-ray photoelectron spectroscopy. ⁵ Photoluminescence spectroscopy. ⁶ Mössbauer spectroscopy. ⁷ Selected area electron diffraction. ⁸ X-ray absorption spectroscopy. ⁹ X-ray magnetic circular dichroism. ¹⁰ Energy-dispersive X-ray spectroscopy. ¹¹ Inductively coupled plasma atomic (optical) emission spectroscopy. ¹² Auger electron spectroscopy.

Table 3. Data on the structures containing an Fe₃O₄ phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Fe3O4 NPs	Co-precipitation from iron salt solution, co-precipitation from iron oxyhydroxide solution, solvothermal synthesis, electrochemical formation from a pure iron, thermal decomposition of the iron oleate complex, biomimetic process with use of a leaf extract, nucleation mediated by iron-binding protein Mms6, biogeneration with a use of amyloid peptide Aβ42	Biomedicine, magnetic separation, antimicrobial and antioxidant applications, contaminant removal, black pigment production, ferrofluids	TEM, XRD, SAXS 1, RS 2, FTIR 3, XPS 4, HAADF-STEM, EELS, TG-DSC, UV–Vis, PL 5, MSP 6, SAED 7	[81,82,83,84,85,86,87,88,89,90,91,149,150,151,152,153,154,155,156,157,158,159,160,161,162]
Bacterial magnetosomes	Bacterial biomineralization, transient phosphate-rich ferric hydroxide reduction to magnetite, formation by dissimilatory iron-reducing bacteria	Biomedicine, paleomagnetism, microbial iron cycle studies, bioremediation of toxic compounds	HAADF-STEM, TEM, XAS 8, SAED, XMCD 9	[93,94,95,96,97,98,163]
Inclusions of Fe3O4 within ore samples	Abiotic hydrothermal mineralization, iron oxide formation derived from continental weathering, extrusive magmatic formation from iron oxide-melt liquid	Geochemistry, environmental magnetism studies, early Earth iron cycle studies, origin and evolution of iron oxides studies	XRD, RS, XPS, EDS 10, ICP-AES 11	[99,100,101,102]
External magnetite layer on a metal surface	Oxidation of a steel surface, slow oxidation of green rusts at room temperature, high-temperature corrosion	Corrosion studies	XRD, EDS, XRF, RS, XPS, AES 12	[105,106,107,108,109,110]
Fe3O4 microparticles	Microbial-induced precipitation with the use of Sporosarcina pasteurii	Green synthesis of magnetite	EDS	[28]
	Aging of ferrous hydroxide gels at elevated temperatures	Colloidal crud formation studies	XRD, TEM	[162]
Self-assembled Fe3O4 mesocrystalline films	Heat-up method with the use of iron(III) chloride and sodium oleate	Biomedicine and industrial applications	TEM, SAED, XAS, SAXS	[163]
Fe0/Fe3O4 composite	Controlled reduction of the starting Fe3O4 with H2	Treatment of wastewater	MSP, XRD	[164]
Magnetite nanowires	Supercritical fluid inclusion within a mesoporous silica matrix	Soft magnetic materials	TEM, SAED, XRD, FTIR	[165]
Inclusions of Fe3O4 NPs	Bacterial reduction of amorphous hydrous ferric oxide	Biogeochemistry	TEM, SAED, XRD, EDS	[166]
Fe3O4 layer on the zerovalent iron surface	Surface oxidation of iron by oxygen in an aqueous medium	Organic pollutant removal	EDS, XRD	[167]
Epoxy/magnetite nanocomposites	Reduction of anhydrous ferric chloride by ammonium hydroxide	Marine coatings of steel	FTIR, XRD, TEM	[168]
Iron oxide nanocomposite hydrogel	Co-precipitation process by ammonium hydroxide	Biomedicine	XRD, TEM, TG-DSC, EDS	[169]
Surface film containing Fe3O4 NPs	Bacterial mineralization in the air–water interface in Arctic tundra waters	Anaerobic microbial carbon cycle	TEM, EDS, STEM, EELS, FTIR, RS	[170]
Nanocomposite hydrogel with Fe3O4 NPs	Reduction with ammonia from a remixed solution of FeCl3 and FeCl2	Biomedicine	TEM, XRF, EDS, TG-DSC, FTIR	[171]
Biochar composite with Fe3O4 NPs	One-pot solvothermal method using phoenix tree leaf-derived biochar	Treatment of wastewater	TEM, XRD, FTIR, XPS, ICP-AES	[172]
Fe3O4 NP inclusions in the surface layer	Formation of NPs along with cracks and pores during pre-oxidation	Plasma nitriding of steel	XRD	[173]
Chitosan/graphene oxide composite with Fe3O4	Co-precipitation of Fe3O4 and chitosan/graphene oxide	Organic pollutant removal	XRD, XPS, RS, FTIR	[174]
Fe3O4 layer on carbon fibers of a carbon paper	Deposition on the carbon paper gas diffusion layer at the cathode	Corrosion studies	XRD, EDS	[175]
Mesocrystals assembled from Fe3O4 nanocubes	Heat-up method with the use of iron(III) chloride and sodium oleate	Mesocrystal applications	TEM	[176]
Fe3O4 nanorods	Formation in electron-beam-induced deposition from iron pentacarbonyl	Electronics	TEM, EELS	[177]
Lipase immobilized on coated Fe3O4 NPs	Solvothermal method with the use of FeCl3·6H2O and ethylene glycol	Biodiesel production	TEM, XRD, FTIR	[178]
Spherical mesoporous magnetite aggregates	Precipitation from iron(III) ethoxide with ethanol in the surfactant solution	Catalysis, sustainability	FTIR, XPS, EDS, TEM, MSP	[179]
Perfluorocarbon-loaded hydrogel microcapsules	Coaxial interface shearing double emulsion method	Biomedicine	–	[180]
Mesoporous magnetite	Ball milling of Fe3O4 and SiO2 followed by partial reduction	Recyclable absorbent for toxic Cr(VI) ions	TEM, XRD, XPS, ICP-AES	[181]
Magnetite crystal model	Local spin-density approximation density-functional calculation	Magnetite electron structure studies	Density-functional calculations	[43]
Spherulite nanostructure with inclusions of Fe3O4	Electron-beam irradiation of the precursor solution with iron nitrate	Crystal growth dynamics studies	TEM, STEM, EDS	[182]

¹ Small-angle X-ray scattering. ² Raman spectroscopy. ³ Fourier-transform infrared spectroscopy. ⁴ X-ray photoelectron spectroscopy. ⁵ Photoluminescence spectroscopy. ⁶ Mössbauer spectroscopy. ⁷ Selected area electron diffraction. ⁸ X-ray absorption spectroscopy. ⁹ X-ray magnetic circular dichroism. ¹⁰ Energy-dispersive X-ray spectroscopy. ¹¹ Inductively coupled plasma atomic (optical) emission spectroscopy. ¹² Auger electron spectroscopy.

Table 4. Data on the structures containing an α-Fe₂O₃ phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
α-Fe2O3 NPs	Hydrothermal synthesis, precipitation from a ferric salt solution using a natural leaf extract, precipitation and aging of ferrihydrite in an oxidized system, direct transformation of α-FeOOH via high-energy ball milling	Biomedicine, bioremediation of toxic compounds, photocatalysis, geochemistry, electronics, antibacterial activity studies, geochemistry	XRD, FTIR, UV–Vis, EDS, TEM, RS, XPS, XAS, ICP-AES, EPR 1, HAADF-STEM, WAXS 2	[105,106,107,108,109,110,111,180,181,182,183,184,185,186,187,188,189,190,191,192]
Inclusions of α-Fe2O3 in ore samples	Precipitation from oxygenated iron-rich water or biomineralization, dissolution of Fe(III) hydroxides by Fe(III)-reducing bacteria, terrestrial subglacial oxidation of glacial iron fluvial deposition	Terrestrial iron oxide concretion studies, Precambrian iron formation studies, Antarctic glacier studies, biogeochemistry	EDS, RS, TEM, HAADF-STEM, XRD, SAED, FTIR, UV–Vis	[112,113,114,115,116,117,118,119]
α-Fe2O3 layer on a metal surface	Anodic potentiostatic oxidation of stainless steel sheet	Anodic passivation of stainless steel	AES	[193]
	Oxidation of steel in an O2-N2 atmosphere at high temperature	Improvement of steel coating quality	TEM, EDS, GD-OES 3	[194]
	Corrosion of chromia-forming alloys in simplified combustion atmosphere	Fireside corrosion studies	EDS, XRD	[195]
Porous α-Fe2O3 nanostructures	Hydrothermal synthesis from FeCl3·6H2O in a microwave reactor	Lithium-ion batteries	XRD, TEM, SAED, XPS, TG-DTG	[196]
	Sol–gel transformations of precursors in self-organized nanocellulose	Energy conversion and storage	XRD, TEM, SAED, XPS, TG-DSC	[197]
Martian hematite deposits	Precipitation from oxygenated iron-rich water or biomineralization	Search for evidence of life on Mars	EDS, TEM	[198]
Hematite layers on sandstone grains	Precipitation from oxidizing iron-saturated fluid	Geochemistry	XRD, ICP-MS 4	[199]
Double-walled hematite nanotubes	Growth of Fe nanowires inside porous templates and oxidation	Photocatalysis, biomedicine	XRD, EELS, HAADF-STEM, RS	[200]
Coral-like and nanowire α-Fe2O3	Thermal oxidation of iron foils in air- and water vapor-assisted conditions	Removal of Cr ions from aqueous systems	XRD, RS, TEM, XPS	[201]
α-Fe2O3 NPs on mineral surfaces	Weathering of Fe-bearing silicate minerals or partial oxidation of Fe3O4	Paleoclimate studies	XRD, TEM, SAED	[202]
α-Fe2O3 nanorods	Controlled aqueous growth from FeCl3·6H2O and NaNO3	Photoelectrochemical water splitting	XRD	[203]
Inclusions of α-Fe2O3 in regolith simulant	Ball milling of commercial α-Fe2O3 samples in isopropyl alcohol	Combustion studies	XRD, TG-DSC	[204]
Inclusions of α-Fe2O3 in stone matrix	Bacterial mineralization	Heritage sciences	XRD, EDS, RS	[205]
Inclusions of α-Fe2O3 in auriferous quartz	Terrigenous abiotic mineralization	Geochemistry	EDS	[206]
Hematite layers on sandstone grains	Terrigenous co-precipitation with sandstone and uranium	Geochemistry of radionuclides	Gamma-ray spectrometry, ICP-MS	[207]
Hematite inclusions encapsulated in chert	Dehydration of the interstitial goethite to hematite microplates	Geochemistry	TEM, XRD, EDS	[208]
Hollow α-Fe2O3 nanofibers	Electrospinning with a use of iron chloride and poly(vinylpyrrolidone)	Photoelectrochemical water splitting	EDS, TEM, SAED, TG-DSC, UV–Vis	[209]
Fossilized bacteria with α-Fe2O3	Biomineralization by anoxygenic photoferrotrophy	Biogeochemistry	RS	[210]
Porous α-Fe2O3 xerogel and aerogel	Sol–gel synthesis from Fe(III) salts with addition of propylene oxide	Catalysis, sensors, biology	TEM	[211]
Iron oxide nanostructures	Microbial Fe(II) oxidation of carbonate green rust by Fe(II)-oxidizing bacteria	Precambrian iron formation studies	MSP	[212]
Iron oxide biogenic precipitates	Bacterial mineralization	Biogenic iron oxide formation studies	XAS	[213]
Steel-wearing ejected debris with α-Fe2O3	Steel fretting wear controlled by oxygen ingress to the contact	Steel fretting wear studies	XRD	[214]
α-Fe2O3 NPs on a steel surface	Oxidation of iron-bonded diamond precision-polishing wheel	Grinding of hard and brittle materials	XRD, XPS, TEM	[215]
Nanostructured α-Fe2O3 films	Electrochemical anodization of steel in an alkaline solution	Photocatalysis, anti-bioadhesion	RS, UV–Vis	[216]
Monodispersed micaceous α-Fe2O3	Hydrothermal synthesis from iron chromium hydroxide precursors	Iron chromium grinding waste recycling	ICP-AES, XRD, XPS	[217]
Nanoporous α-Fe2O3 layer on an iron foil	Anodization of iron is an ethylene glycol and NH4F aqueous solution	Photocatalysis	TEM, RS, XRD, UV–Vis, EDS, EELS	[218]
Natural α-Fe2O3 from the iron deposits	Terrigenous abiotic mineralization	Photocatalytic recycling of toxic wastewater	RS, EDS, UV–Vis	[219]
Nanocomposite containing α-Fe2O3	Wet impregnation of Co3O4 powder with an Fe(NO3)⋅9H2O solution	Catalysis	XPS, XRD, TG-DSC, EDS, TEM	[220]
Stepped α-Fe2O3 (0001) surfaces	First principles spin-polarized density-functional theory simulation	Chloride-induced iron depassivation studies	Density-functional theory calculations	[221]
α-Fe2O3 powder	In situ generation of iron oxide via decomposition of Fe(NO3)3·9H2O	Catalysis	XRD	[222]
α-Fe2O3 nanorods	Hydrothermal precipitation and air calcination of goethite nanorods	Catalysis, lithium-ion batteries, sensors	XRD, MSP, UV–Vis, EDS, TG-DSC	[223]
α-Fe2O3 nano- and microparticles	Chemically synthesized commercial α-Fe2O3 samples	Mechanisms of oxide toxicity toward bacteria	FTIR, XAS	[224]
α-Fe2O3 nanowires	Heating of iron wires suspended between two electric contacts	Vacuum electronic devices	TEM, EDS, XPS, RS	[225]
α-Fe2O3 layer on zerovalent iron NPs	Iron oxide film formation under aerobic conditions	Remediation of water pollutants	TEM, FTIR, XPS, XRD	[226]
Inclusions of α-Fe2O3 in rock varnish	Terrigenous abiotic mineralization or biotic processes	Geomicrobiology	XRD, RS, EDS	[227]
Nanolayers of α-Fe2O3 in polymer composite	Iron pentacarbonyl transformation with diamond anvil cells in Ar gas	High-energy density solid studies	RS, TEM, XRD	[228]
Jian ware blue-colored glaze with α-Fe2O3	Calcination of a milled mix at a high temperature in oxidizing atmosphere	Ancient ceramics studies	XRD, UV–Vis, TEM, XPS	[229]
Inclusions of α-Fe2O3 in sediment samples	Microbial reduction of surface Fe(III) by iron-reducing bacteria	Microbial iron reduction studies	XRD	[230]
Core-shell iron/iron oxide NPs	Zerovalent Fe core-controlled oxidation during deposition	Oxide formation under e-beam radiation studies	TEM, EELS	[231]
α-Fe2O3 film on a dielectric substrate	Liquid-phase atomic layer deposition of crystalline hematite	Catalysis, sensors, lithium-ion batteries	XRD, UV–Vis	[232]
Cube-shaped α-Fe2O3 microstructures	Facile hydrothermal method using hydrated ferric nitrate and NaOH	Ethanol gas sensing	XRD, FTIR, EDS, RS	[233]
Iron oxide/Ti composites	Plasma electrolytic oxidation, impregnation and annealing	Phenol photodegradation	XRD, EDS, FTIR, XPS	[234]
Microporous α-Fe2O3 NPs	Precipitation from iron(II) sulfate using a natural leaf extract	Sustainability	XRD, UV–Vis, XPS, FTIR	[235]
Inclusions of α-Fe2O3 in artificial clay	Fe(OH)3 colloid mixing into chemically pure kaolin	Laterite engineering	XRD	[236]
Iron oxide nanotubes	Potentiostatic anodization of iron foil in electrolytes containing NH4F	Catalysis, sensors, supercapacitors	XRD, TEM, SAED	[237]
α-Fe2O3 thin film	Spray pyrolysis from FeCl3 and methanol solution	Electrochemical supercapacitors	XRD, UV–Vis	[44]
Corroded steel tube samples with α-Fe2O3	Steel corrosion in an aqueous medium with oxygen and chlorine	Pipeline corrosion assessment	XRD, EDS, TEM, SAED	[238]
Inclusions of α-Fe2O3 in stone samples	Formation by washing and leaching of a stone object by rainwater	Limestone artifact studies	RS, FTIR, EDS, XRF	[239]
Iron oxide-loaded slag	Precipitation from FeCl3 solution with NaOH into melted slag	Arsenic removal from water	ICP-AES, XRD	[240]
3D-ordered macroporous α-Fe2O3	Impregnation of polymer matrices and high-temperature calcination	Catalysis	XRD, TG-DSC, FTIR, SAED, UV–Vis, XPS	[241]
α-Fe2O3/mesoporous silica core-shell NPs	Solvothermal synthesis from ferric nitrate with sol–gel silica coating	Catalysis, biomedicine	XRD, TEM, FTIR, UV–Vis	[242]
Spindle-shaped α-Fe2O3 mesocrystal	Interface-driven nucleation by ferrihydrate oxidation and attachment	Thermoelectronics, photonics, catalysis, photovoltaics	TEM, SAED, FTIR, EDS	[243]
Hematite nanopillars	Electron-beam evaporation using anodized aluminum oxide templates with well-defined pore diameters	Photoelectrochemical water splitting	XRD, XPS, UV–Vis	[244]

¹ Electron paramagnetic resonance spectroscopy. ² Wide-angle X-ray scattering. ³ Glow-discharge optical emission spectrometry. ⁴ Inductively coupled plasma mass spectrometry.

Table 4. Data on the structures containing an α-Fe₂O₃ phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
α-Fe2O3 NPs	Hydrothermal synthesis, precipitation from a ferric salt solution using a natural leaf extract, precipitation and aging of ferrihydrite in an oxidized system, direct transformation of α-FeOOH via high-energy ball milling	Biomedicine, bioremediation of toxic compounds, photocatalysis, geochemistry, electronics, antibacterial activity studies, geochemistry	XRD, FTIR, UV–Vis, EDS, TEM, RS, XPS, XAS, ICP-AES, EPR 1, HAADF-STEM, WAXS 2	[105,106,107,108,109,110,111,180,181,182,183,184,185,186,187,188,189,190,191,192]
Inclusions of α-Fe2O3 in ore samples	Precipitation from oxygenated iron-rich water or biomineralization, dissolution of Fe(III) hydroxides by Fe(III)-reducing bacteria, terrestrial subglacial oxidation of glacial iron fluvial deposition	Terrestrial iron oxide concretion studies, Precambrian iron formation studies, Antarctic glacier studies, biogeochemistry	EDS, RS, TEM, HAADF-STEM, XRD, SAED, FTIR, UV–Vis	[112,113,114,115,116,117,118,119]
α-Fe2O3 layer on a metal surface	Anodic potentiostatic oxidation of stainless steel sheet	Anodic passivation of stainless steel	AES	[193]
	Oxidation of steel in an O2-N2 atmosphere at high temperature	Improvement of steel coating quality	TEM, EDS, GD-OES 3	[194]
	Corrosion of chromia-forming alloys in simplified combustion atmosphere	Fireside corrosion studies	EDS, XRD	[195]
Porous α-Fe2O3 nanostructures	Hydrothermal synthesis from FeCl3·6H2O in a microwave reactor	Lithium-ion batteries	XRD, TEM, SAED, XPS, TG-DTG	[196]
	Sol–gel transformations of precursors in self-organized nanocellulose	Energy conversion and storage	XRD, TEM, SAED, XPS, TG-DSC	[197]
Martian hematite deposits	Precipitation from oxygenated iron-rich water or biomineralization	Search for evidence of life on Mars	EDS, TEM	[198]
Hematite layers on sandstone grains	Precipitation from oxidizing iron-saturated fluid	Geochemistry	XRD, ICP-MS 4	[199]
Double-walled hematite nanotubes	Growth of Fe nanowires inside porous templates and oxidation	Photocatalysis, biomedicine	XRD, EELS, HAADF-STEM, RS	[200]
Coral-like and nanowire α-Fe2O3	Thermal oxidation of iron foils in air- and water vapor-assisted conditions	Removal of Cr ions from aqueous systems	XRD, RS, TEM, XPS	[201]
α-Fe2O3 NPs on mineral surfaces	Weathering of Fe-bearing silicate minerals or partial oxidation of Fe3O4	Paleoclimate studies	XRD, TEM, SAED	[202]
α-Fe2O3 nanorods	Controlled aqueous growth from FeCl3·6H2O and NaNO3	Photoelectrochemical water splitting	XRD	[203]
Inclusions of α-Fe2O3 in regolith simulant	Ball milling of commercial α-Fe2O3 samples in isopropyl alcohol	Combustion studies	XRD, TG-DSC	[204]
Inclusions of α-Fe2O3 in stone matrix	Bacterial mineralization	Heritage sciences	XRD, EDS, RS	[205]
Inclusions of α-Fe2O3 in auriferous quartz	Terrigenous abiotic mineralization	Geochemistry	EDS	[206]
Hematite layers on sandstone grains	Terrigenous co-precipitation with sandstone and uranium	Geochemistry of radionuclides	Gamma-ray spectrometry, ICP-MS	[207]
Hematite inclusions encapsulated in chert	Dehydration of the interstitial goethite to hematite microplates	Geochemistry	TEM, XRD, EDS	[208]
Hollow α-Fe2O3 nanofibers	Electrospinning with a use of iron chloride and poly(vinylpyrrolidone)	Photoelectrochemical water splitting	EDS, TEM, SAED, TG-DSC, UV–Vis	[209]
Fossilized bacteria with α-Fe2O3	Biomineralization by anoxygenic photoferrotrophy	Biogeochemistry	RS	[210]
Porous α-Fe2O3 xerogel and aerogel	Sol–gel synthesis from Fe(III) salts with addition of propylene oxide	Catalysis, sensors, biology	TEM	[211]
Iron oxide nanostructures	Microbial Fe(II) oxidation of carbonate green rust by Fe(II)-oxidizing bacteria	Precambrian iron formation studies	MSP	[212]
Iron oxide biogenic precipitates	Bacterial mineralization	Biogenic iron oxide formation studies	XAS	[213]
Steel-wearing ejected debris with α-Fe2O3	Steel fretting wear controlled by oxygen ingress to the contact	Steel fretting wear studies	XRD	[214]
α-Fe2O3 NPs on a steel surface	Oxidation of iron-bonded diamond precision-polishing wheel	Grinding of hard and brittle materials	XRD, XPS, TEM	[215]
Nanostructured α-Fe2O3 films	Electrochemical anodization of steel in an alkaline solution	Photocatalysis, anti-bioadhesion	RS, UV–Vis	[216]
Monodispersed micaceous α-Fe2O3	Hydrothermal synthesis from iron chromium hydroxide precursors	Iron chromium grinding waste recycling	ICP-AES, XRD, XPS	[217]
Nanoporous α-Fe2O3 layer on an iron foil	Anodization of iron is an ethylene glycol and NH4F aqueous solution	Photocatalysis	TEM, RS, XRD, UV–Vis, EDS, EELS	[218]
Natural α-Fe2O3 from the iron deposits	Terrigenous abiotic mineralization	Photocatalytic recycling of toxic wastewater	RS, EDS, UV–Vis	[219]
Nanocomposite containing α-Fe2O3	Wet impregnation of Co3O4 powder with an Fe(NO3)⋅9H2O solution	Catalysis	XPS, XRD, TG-DSC, EDS, TEM	[220]
Stepped α-Fe2O3 (0001) surfaces	First principles spin-polarized density-functional theory simulation	Chloride-induced iron depassivation studies	Density-functional theory calculations	[221]
α-Fe2O3 powder	In situ generation of iron oxide via decomposition of Fe(NO3)3·9H2O	Catalysis	XRD	[222]
α-Fe2O3 nanorods	Hydrothermal precipitation and air calcination of goethite nanorods	Catalysis, lithium-ion batteries, sensors	XRD, MSP, UV–Vis, EDS, TG-DSC	[223]
α-Fe2O3 nano- and microparticles	Chemically synthesized commercial α-Fe2O3 samples	Mechanisms of oxide toxicity toward bacteria	FTIR, XAS	[224]
α-Fe2O3 nanowires	Heating of iron wires suspended between two electric contacts	Vacuum electronic devices	TEM, EDS, XPS, RS	[225]
α-Fe2O3 layer on zerovalent iron NPs	Iron oxide film formation under aerobic conditions	Remediation of water pollutants	TEM, FTIR, XPS, XRD	[226]
Inclusions of α-Fe2O3 in rock varnish	Terrigenous abiotic mineralization or biotic processes	Geomicrobiology	XRD, RS, EDS	[227]
Nanolayers of α-Fe2O3 in polymer composite	Iron pentacarbonyl transformation with diamond anvil cells in Ar gas	High-energy density solid studies	RS, TEM, XRD	[228]
Jian ware blue-colored glaze with α-Fe2O3	Calcination of a milled mix at a high temperature in oxidizing atmosphere	Ancient ceramics studies	XRD, UV–Vis, TEM, XPS	[229]
Inclusions of α-Fe2O3 in sediment samples	Microbial reduction of surface Fe(III) by iron-reducing bacteria	Microbial iron reduction studies	XRD	[230]
Core-shell iron/iron oxide NPs	Zerovalent Fe core-controlled oxidation during deposition	Oxide formation under e-beam radiation studies	TEM, EELS	[231]
α-Fe2O3 film on a dielectric substrate	Liquid-phase atomic layer deposition of crystalline hematite	Catalysis, sensors, lithium-ion batteries	XRD, UV–Vis	[232]
Cube-shaped α-Fe2O3 microstructures	Facile hydrothermal method using hydrated ferric nitrate and NaOH	Ethanol gas sensing	XRD, FTIR, EDS, RS	[233]
Iron oxide/Ti composites	Plasma electrolytic oxidation, impregnation and annealing	Phenol photodegradation	XRD, EDS, FTIR, XPS	[234]
Microporous α-Fe2O3 NPs	Precipitation from iron(II) sulfate using a natural leaf extract	Sustainability	XRD, UV–Vis, XPS, FTIR	[235]
Inclusions of α-Fe2O3 in artificial clay	Fe(OH)3 colloid mixing into chemically pure kaolin	Laterite engineering	XRD	[236]
Iron oxide nanotubes	Potentiostatic anodization of iron foil in electrolytes containing NH4F	Catalysis, sensors, supercapacitors	XRD, TEM, SAED	[237]
α-Fe2O3 thin film	Spray pyrolysis from FeCl3 and methanol solution	Electrochemical supercapacitors	XRD, UV–Vis	[44]
Corroded steel tube samples with α-Fe2O3	Steel corrosion in an aqueous medium with oxygen and chlorine	Pipeline corrosion assessment	XRD, EDS, TEM, SAED	[238]
Inclusions of α-Fe2O3 in stone samples	Formation by washing and leaching of a stone object by rainwater	Limestone artifact studies	RS, FTIR, EDS, XRF	[239]
Iron oxide-loaded slag	Precipitation from FeCl3 solution with NaOH into melted slag	Arsenic removal from water	ICP-AES, XRD	[240]
3D-ordered macroporous α-Fe2O3	Impregnation of polymer matrices and high-temperature calcination	Catalysis	XRD, TG-DSC, FTIR, SAED, UV–Vis, XPS	[241]
α-Fe2O3/mesoporous silica core-shell NPs	Solvothermal synthesis from ferric nitrate with sol–gel silica coating	Catalysis, biomedicine	XRD, TEM, FTIR, UV–Vis	[242]
Spindle-shaped α-Fe2O3 mesocrystal	Interface-driven nucleation by ferrihydrate oxidation and attachment	Thermoelectronics, photonics, catalysis, photovoltaics	TEM, SAED, FTIR, EDS	[243]
Hematite nanopillars	Electron-beam evaporation using anodized aluminum oxide templates with well-defined pore diameters	Photoelectrochemical water splitting	XRD, XPS, UV–Vis	[244]

¹ Electron paramagnetic resonance spectroscopy. ² Wide-angle X-ray scattering. ³ Glow-discharge optical emission spectrometry. ⁴ Inductively coupled plasma mass spectrometry.

Table 5. Data on the structures containing a γ-Fe₂O₃ phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
γ-Fe2O3 NPs	Solvothermal synthesis from iron salts, bacterial mineralization, lepidocrocite calcination in an air atmosphere, hydrothermal and solvothermal synthesis from salt solutions	Catalysis, biomedicine, nucleation and formation of biogenic iron oxide studies, electronics, maghemite to hematite transition studies, sensors	TEM, EDS, SAED, XRD, XPS, EPR, FTIR, UV–Vis, ICP-AES, HAADF-STEM, MSP, in situ total scattering, XAS, SAXS, RS	[93,125,126,127,128,129]
γ-Fe2O3 NPs in silica matrix	Gas-phase synthesis in a furnace aerosol reactor from iron pentacarbonyl	Biomedicine	XRD, TEM, EDS, FTIR, UV–Vis	[245]
	Dehydration of iron(III) hydroxide to magnetite followed by oxidation	Catalysis	XRD, FTIR	[246]
γ-Fe2O3 powder	Chemically synthesized commercial γ-Fe2O3 samples	Catalytic oxidation of S(IV)	ICP-MS, FTIR	[247]
26-faceted maghemite polyhedrons	Direct burning of ferrocene in different solvents in an alcohol lamp	Lithium-ion batteries	XRD, TEM	[248]
Magnetic polymeric NPs with γ-Fe2O3	Co-precipitation of FeCl3/FeCl2·4H2O with NH4OH solution	Biomedicine	TEM, TG-DSC, FTIR	[249]
γ-Fe2O3 NP superlattice thin films	Chemically synthesized commercial γ-Fe2O3 samples	Electronics, optical coatings	Grazing incidence small angle X-ray scattering	[250]
Maghemite-decorated graphene nanoscrolls	Hydrolysis of FeCl3·6H2O and W(CO)6, promoted with hydrazine	Energy storage	TEM, XPS, TG-DSC, RS	[251]
Hollow iron oxide NPs	Gas-phase vaporization synthesis of Fe NPs and oxidation to γ-Fe2O3	Optics, nanoelectronics	TEM, HAADF-STEM, EDS	[252]
Mesoporous iron oxide	Inverse micelle synthesis from Fe(NO3)3·9H2O butanol solution	Arsenic removal from water	XRD, FTIR, RS, XPS	[253]

Table 5. Data on the structures containing a γ-Fe₂O₃ phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
γ-Fe2O3 NPs	Solvothermal synthesis from iron salts, bacterial mineralization, lepidocrocite calcination in an air atmosphere, hydrothermal and solvothermal synthesis from salt solutions	Catalysis, biomedicine, nucleation and formation of biogenic iron oxide studies, electronics, maghemite to hematite transition studies, sensors	TEM, EDS, SAED, XRD, XPS, EPR, FTIR, UV–Vis, ICP-AES, HAADF-STEM, MSP, in situ total scattering, XAS, SAXS, RS	[93,125,126,127,128,129]
γ-Fe2O3 NPs in silica matrix	Gas-phase synthesis in a furnace aerosol reactor from iron pentacarbonyl	Biomedicine	XRD, TEM, EDS, FTIR, UV–Vis	[245]
	Dehydration of iron(III) hydroxide to magnetite followed by oxidation	Catalysis	XRD, FTIR	[246]
γ-Fe2O3 powder	Chemically synthesized commercial γ-Fe2O3 samples	Catalytic oxidation of S(IV)	ICP-MS, FTIR	[247]
26-faceted maghemite polyhedrons	Direct burning of ferrocene in different solvents in an alcohol lamp	Lithium-ion batteries	XRD, TEM	[248]
Magnetic polymeric NPs with γ-Fe2O3	Co-precipitation of FeCl3/FeCl2·4H2O with NH4OH solution	Biomedicine	TEM, TG-DSC, FTIR	[249]
γ-Fe2O3 NP superlattice thin films	Chemically synthesized commercial γ-Fe2O3 samples	Electronics, optical coatings	Grazing incidence small angle X-ray scattering	[250]
Maghemite-decorated graphene nanoscrolls	Hydrolysis of FeCl3·6H2O and W(CO)6, promoted with hydrazine	Energy storage	TEM, XPS, TG-DSC, RS	[251]
Hollow iron oxide NPs	Gas-phase vaporization synthesis of Fe NPs and oxidation to γ-Fe2O3	Optics, nanoelectronics	TEM, HAADF-STEM, EDS	[252]
Mesoporous iron oxide	Inverse micelle synthesis from Fe(NO3)3·9H2O butanol solution	Arsenic removal from water	XRD, FTIR, RS, XPS	[253]

Table 6. Data on the structures containing a β-Fe₂O₃, ε-Fe₂O₃, ζ-Fe₂O₃, δ-Fe₂O₃ or Fe₄O₅ metastable phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Pristine and co-substituted ε-Fe2O3	Simulated crystal structure with a use of density-functional calculations	Magnetoelectric material development	Density-functional theory calculations	[254]
ε-Fe2O3 embedded in biomimetic graphene	Precipitation from ferric and ferrous chloride with a biocompatible polymer	Biomedicine	XRD, TEM, SAED, RS, XPS, TG-DSC, FTIR	[130]
Epitaxially stabilized thin-film ε-Fe2O3	Epitaxy on (100)-oriented yttrium-stabilized zirconia substrates	Electronics, permanent magnets, biomedicine	XRD, HAADF-STEM	[136]
ε-Fe2O3 in ancient black glazed wares	Surface iron enrichment and a firing of wares under reducing conditions	Electronics, spintronics	XRF, XAS, XRD, RS, TEM, EDS	[132]
ε-Fe2O3 NPs	Hydrolysis of tetraethoxysilane in a solution of ferric nitrate and annealing	Wireless technologies, electronics	XRD, TEM, THz-TDS 1	[255]
ε-Fe2O3 inclusions in fired clay samples	Stabilization of ε-Fe2O3 NPs in a matrix of silicates during firing of clays	Paleomagnetism	XRD, EDS	[134]
Y3Fe5O12 matrix including ε-Fe2O3	Formation of ε-Fe2O3 in the Y3Fe5O12 matrix using the sol–gel method	Magnetoelectric material development	XRD, XPS, TG-DSC, FTIR	[256]
δ-Fe2O3 in layered double hydroxyl	Dry impregnation of layered double hydroxyl structure with ferric nitrate	Photocatalysis	XRD, FTIR, XRF, TG-DSC, UV–Vis	[22]
ε-Fe2O3-SiO2	Reverse micelle method with the use of ferric nitrate	Oxidative dehydrogenation of n-butene	XRD	[136]
β-Fe2O3	Milling of Fe2(SO4)3 and NaCl and calcination at 550 °C in air
Ga-substituted ε-Fe2O3 NPs	Calcination of a mesoporous silica impregnated with metal nitrates	Biomedicine	XRD, XRF, TEM, ICP-MS	[131]
ε-Fe2O3 in archeological brick and baked clay	High-temperature firing of bricks and clays in air	Archaeomagnetism, paleomagnetism	RS	[138]
ε-Fe2O3 in archeological samples, ε-Fe2O3 NPs	Sol–gel synthesis from ferric and barium nitrate with tetraethyl orthosilicate		XRD, RS	[135]
ε-Fe2O3 coatings on Si(100) substrates	One-pot sol–gel recipe assisted by glycerol in an acid medium	Paleomagnetism, biomedicine, electronics	RS, XAS, EELS, HAADF-STEM	[257]
ε-Fe2O3/SiO2 composite powder	Sol–gel synthesis from ferric and barium nitrate with tetraethyl orthosilicate	Electronics	XRD, TEM	[133]
ε-Fe2O3 nanorods	Chemical vapor deposition from the Fe organic liquid source	Photocatalysis, electronics	XPS	[258]
ε-Fe2O3/SiO2 composite	Sol–gel synthesis from nitrate with tetraethyl orthosilicate and nitric acid	Electronics, spintronics, magnetizable printing	TG-DSC, XRD, TEM	[259]
ε-Fe2O3 NPs	Immersion of mesoporous silica with an FeSO4 or Fe(C10H9CHO) solution and high-temperature calcination	High-coercivity material development	TEM, XRD, MSP, TEM, SAED	[20,41]
β-Fe2O3 NPs		Sensors, lithium-ion batteries
Epitaxial ε-Fe2O3 films on GaN substrate	Pulsed laser deposition on the Ga-terminated surface of the GaN (0001)	Electronics	XRD, RHEED 2, TEM, XAS, XMCD	[260]
Silica-coated ε-Fe2O3 NPs	Sol–gel treatment of β-FeOOH nanorods with tetraethoxysilane and calcination	Electronics	XRD, TEM, EDS, MSP	[261]
ε-Fe2O3 in a Hare’s Fur Jian ware	High-temperature firing of local iron-rich area on the ceramic glaze	Magnetoresistance materials	XRF, XAS, EDS, XRD, RS	[140]
Metal-substituted ε-Fe2O3	Impregnation of mesoporous silica NPs with rhodium-substituted ε-Fe2O3	Electronics, magnetic force microscopy, biomedicine	XRD	[262]
β-Fe2O3 NPs	Thermally-induced solid-state reaction of NaCl with Fe2(SO4)3 in air	Sensors, lithium-ion batteries	XRD, MSP, TEM, SAED	[20]
ζ-Fe2O3	Pressure treatment of β-Fe2O3 NPs at pressures above 30 GPa	n/a
ε-Fe2O3 in a thin MgO(111) layer	Pulsed laser deposition from MgO and Fe2O3 targets ablated using a KrF laser	Electronics	RHEED, XRD, neutron reflectometry	[263]
Single crystal of Fe4O5	Synthesis in the diamond anvil cell at high pressure after laser heating	Solid Earth studies	Density-functional theory calculations	[23]
Nanometer-scale lamellae of Fe4O5	High-pressure and high-temperature multi-anvil synthesis	Deep Earth studies	XRD, TEM, SAED, EDS, STEM	[264]
Powder of Fe4O5	High-pressure and high-temperature direct synthesis from a mixture of Fe3O4 and Fe	Electronics	XRD, neutron diffraction	[265]
β-Fe2O3 NPs	Thermally-induced solid-state reaction of NaCl with Fe2(SO4)3 in air	Optoelectronics, sensors, lithium-ion batteries	XRD, MSP, TEM	[64]
	Hydrolysis of 2M FeCl3 in boiling water and cooling down slowly at room temperature	Biomedicine	UV–Vis, TEM, XRD, FTIR, EDS, SAED	[63]

¹ Terahertz time-domain spectroscopy. ² Reflection high-energy electron diffraction.

Table 6. Data on the structures containing a β-Fe₂O₃, ε-Fe₂O₃, ζ-Fe₂O₃, δ-Fe₂O₃ or Fe₄O₅ metastable phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Pristine and co-substituted ε-Fe2O3	Simulated crystal structure with a use of density-functional calculations	Magnetoelectric material development	Density-functional theory calculations	[254]
ε-Fe2O3 embedded in biomimetic graphene	Precipitation from ferric and ferrous chloride with a biocompatible polymer	Biomedicine	XRD, TEM, SAED, RS, XPS, TG-DSC, FTIR	[130]
Epitaxially stabilized thin-film ε-Fe2O3	Epitaxy on (100)-oriented yttrium-stabilized zirconia substrates	Electronics, permanent magnets, biomedicine	XRD, HAADF-STEM	[136]
ε-Fe2O3 in ancient black glazed wares	Surface iron enrichment and a firing of wares under reducing conditions	Electronics, spintronics	XRF, XAS, XRD, RS, TEM, EDS	[132]
ε-Fe2O3 NPs	Hydrolysis of tetraethoxysilane in a solution of ferric nitrate and annealing	Wireless technologies, electronics	XRD, TEM, THz-TDS 1	[255]
ε-Fe2O3 inclusions in fired clay samples	Stabilization of ε-Fe2O3 NPs in a matrix of silicates during firing of clays	Paleomagnetism	XRD, EDS	[134]
Y3Fe5O12 matrix including ε-Fe2O3	Formation of ε-Fe2O3 in the Y3Fe5O12 matrix using the sol–gel method	Magnetoelectric material development	XRD, XPS, TG-DSC, FTIR	[256]
δ-Fe2O3 in layered double hydroxyl	Dry impregnation of layered double hydroxyl structure with ferric nitrate	Photocatalysis	XRD, FTIR, XRF, TG-DSC, UV–Vis	[22]
ε-Fe2O3-SiO2	Reverse micelle method with the use of ferric nitrate	Oxidative dehydrogenation of n-butene	XRD	[136]
β-Fe2O3	Milling of Fe2(SO4)3 and NaCl and calcination at 550 °C in air
Ga-substituted ε-Fe2O3 NPs	Calcination of a mesoporous silica impregnated with metal nitrates	Biomedicine	XRD, XRF, TEM, ICP-MS	[131]
ε-Fe2O3 in archeological brick and baked clay	High-temperature firing of bricks and clays in air	Archaeomagnetism, paleomagnetism	RS	[138]
ε-Fe2O3 in archeological samples, ε-Fe2O3 NPs	Sol–gel synthesis from ferric and barium nitrate with tetraethyl orthosilicate		XRD, RS	[135]
ε-Fe2O3 coatings on Si(100) substrates	One-pot sol–gel recipe assisted by glycerol in an acid medium	Paleomagnetism, biomedicine, electronics	RS, XAS, EELS, HAADF-STEM	[257]
ε-Fe2O3/SiO2 composite powder	Sol–gel synthesis from ferric and barium nitrate with tetraethyl orthosilicate	Electronics	XRD, TEM	[133]
ε-Fe2O3 nanorods	Chemical vapor deposition from the Fe organic liquid source	Photocatalysis, electronics	XPS	[258]
ε-Fe2O3/SiO2 composite	Sol–gel synthesis from nitrate with tetraethyl orthosilicate and nitric acid	Electronics, spintronics, magnetizable printing	TG-DSC, XRD, TEM	[259]
ε-Fe2O3 NPs	Immersion of mesoporous silica with an FeSO4 or Fe(C10H9CHO) solution and high-temperature calcination	High-coercivity material development	TEM, XRD, MSP, TEM, SAED	[20,41]
β-Fe2O3 NPs		Sensors, lithium-ion batteries
Epitaxial ε-Fe2O3 films on GaN substrate	Pulsed laser deposition on the Ga-terminated surface of the GaN (0001)	Electronics	XRD, RHEED 2, TEM, XAS, XMCD	[260]
Silica-coated ε-Fe2O3 NPs	Sol–gel treatment of β-FeOOH nanorods with tetraethoxysilane and calcination	Electronics	XRD, TEM, EDS, MSP	[261]
ε-Fe2O3 in a Hare’s Fur Jian ware	High-temperature firing of local iron-rich area on the ceramic glaze	Magnetoresistance materials	XRF, XAS, EDS, XRD, RS	[140]
Metal-substituted ε-Fe2O3	Impregnation of mesoporous silica NPs with rhodium-substituted ε-Fe2O3	Electronics, magnetic force microscopy, biomedicine	XRD	[262]
β-Fe2O3 NPs	Thermally-induced solid-state reaction of NaCl with Fe2(SO4)3 in air	Sensors, lithium-ion batteries	XRD, MSP, TEM, SAED	[20]
ζ-Fe2O3	Pressure treatment of β-Fe2O3 NPs at pressures above 30 GPa	n/a
ε-Fe2O3 in a thin MgO(111) layer	Pulsed laser deposition from MgO and Fe2O3 targets ablated using a KrF laser	Electronics	RHEED, XRD, neutron reflectometry	[263]
Single crystal of Fe4O5	Synthesis in the diamond anvil cell at high pressure after laser heating	Solid Earth studies	Density-functional theory calculations	[23]
Nanometer-scale lamellae of Fe4O5	High-pressure and high-temperature multi-anvil synthesis	Deep Earth studies	XRD, TEM, SAED, EDS, STEM	[264]
Powder of Fe4O5	High-pressure and high-temperature direct synthesis from a mixture of Fe3O4 and Fe	Electronics	XRD, neutron diffraction	[265]
β-Fe2O3 NPs	Thermally-induced solid-state reaction of NaCl with Fe2(SO4)3 in air	Optoelectronics, sensors, lithium-ion batteries	XRD, MSP, TEM	[64]
	Hydrolysis of 2M FeCl3 in boiling water and cooling down slowly at room temperature	Biomedicine	UV–Vis, TEM, XRD, FTIR, EDS, SAED	[63]

¹ Terahertz time-domain spectroscopy. ² Reflection high-energy electron diffraction.

3.2. The Structures Containing Iron Oxide Atomic Clusters and an Amorphous Iron Oxide Phase

In their thermodynamic equilibrium state under normal conditions, iron oxides possess a crystal structure; therefore, there are only a few papers describing compositions with amorphous or poorly crystalline iron oxides, which are obtained via chemical [266,267,268,269] or biomineralization routes [270] and intended for various potential applications (Table 7). Iron oxide atomic clusters are a more frequently studied object and can be obtained either by synthetic chemical [271,272,273,274], physical [275] or biomimetic [276,277] techniques. Potential applications for iron oxide clusters include biomedicine [271,273], electronics [271,276], catalysis [273,274], and natural iron storage process studies [276].

The next type of iron oxide structure is ultra-thin, including two-dimensional films on metal surfaces [142,278,279,280], which are obtained using various chemical and physical techniques and can be applied to the production of molecular hydrogen [278], removal of contaminants [279], catalysis [142] and electronics [280]. Simulated iron oxide atomic clusters [281] and ultra-thin layers on a metal surface [282] are also described and can be useful for the prediction of the magnetic properties of FeO_x NPs [281] and chemical water treatment technique development [282].

3.3. The Structures Containing Two Co-Existing Iron Oxide Crystal Phases

Real structures containing iron oxides in various cases are inhomogeneous, for example, due to the partial oxidation of magnetite to maghemite for synthetic [38,283,284,285,286,287,288,289,290,291,292,293], natural abiotic [6] and biogenic [294] origins. In this section, compositions with iron oxides containing two co-existing crystal phases are described (Table 8, Table 9, Table 10 and Table 11).

Crystal phases of α-Fe₂O₃ and γ-Fe₂O₃ quite rarely co-exist (Table 8), in comparison, for example, with the phases of magnetite and maghemite, according to the analyzed research articles. Nevertheless, such a combination can be found in natural and synthetic objects, including oxidized iron items [295], α/γ-Fe₂O₃ isoelement synthetic heterostructures with different crystal content [296], loess and paleosol [297] and saprolite soils [298] samples, graphene-iron oxide nanotube composites [299] and polyacrylonitrile/iron oxide composites [300]. The main applications of such structures are in the removal of contaminants [299,300,301], pedogenic process studies [297,298] and corrosion studies [302].

A more frequently discovered combination is the co-existence of α-Fe₂O₃ and Fe₃O₄ crystal phases (Table 9). They can be found in natural ore samples and can be explained by abiotic [303,304,305,306,307,308] or biogenic [309,310,311] processes. Besides these natural formations, such combinations of iron oxide phases can be synthetically obtained with the use of chemical [312,313,314], physical [315] or biomimetic [316] techniques. The main applications of the compositions are geosciences [306,307,310,311], biomedicine [312,314,317] and catalysis [313,316,317].

Cubic iron oxides, wüstite and magnetite can be co-existing (Table 10), despite these cases being rare, in comparison to the structures containing co-existing α-Fe₂O₃ and Fe₃O₄ phases [303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328], and can be attributed to applications of iron oxides in Earth’s mantle studies [329], porous iron growth mechanism studies [330], environmental remediation, electronics, catalysis, biomedicine and energy storage [331].

Structures containing iron oxides with a spinel structure, magnetite and maghemite, are widely used in various applications with a predominance of NPs intended for biomedicine [45,332,333,334,335,336,337] (Table 11). It is possible to propose the partial oxidation of magnetite NPs to maghemite in the vast majority of cases (except those with inert atmospheric preservation), but this effect can be distinguished only by the use of some additional instruments, including Mössbauer spectroscopy [6], zero-field and field cooling measurements to reveal a Verwey transition and high-resolution transmission electron microscopy [295] to show the crystal structure, while more widely used X-ray powder diffraction cannot resolve magnetite and maghemite [38]. The most frequently used synthetic routes used to obtain γ-Fe₂O₃–Fe₃O₄ NPs are by thermal decomposition [337,338,339,340] and chemical precipitation [332,336,341,342].

The iron oxide layer on metal surfaces is also a possible structure containing co-existing magnetite and maghemite phases. Such structures can originate from both natural [343,344] and synthetic [345,346,347,348,349] routes, generally implying chemical or electrochemical oxidation in a liquid medium. Overall, such compositions play an important role in iron corrosion studies [344,346,347,350,351]. Finally, compositions with magnetite and maghemite, presumably due to the presence of iron(II) and iron(III) cations, are actively used for the remediation of waste [348,352,353,354,355,356,357].

Table 7. Data on the structures containing iron oxide atomic clusters and an amorphous iron oxide phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Iron oxide atomic clusters	Combustion synthesis from Fe(CO)5 mixed with hydrogen and oxygen, high irradiance laser ionization from pressed Fe2O3 and Fe3O4 tablets, biomineralization inside the ferritin shell, reaction of laser ablated iron foil with 5% O2 seeded in a helium carrier gas	Catalysis, biomedicine, electronics, sensors, prediction of the magnetic properties of FeOx NPs, natural iron storage process studies, photovoltaics	MBMS 1, PMS 2, RMDS 3, TEM, LI-TOFMS 4, density-functional theory calculations, European Synchrotron Radiation Facility	[271,272,273,274,275,276,277,281]
Surface iron oxide layer on metal	Multicycling of an iron foil electrode between the switching potentials, formation of iron oxide species after reaction with Cr(VI) and Cu(II)	Chemical water treatment, production of molecular hydrogen, removal of contaminants	RMDS, XRD, XPS, FTIR, EDS	[278,279,282]
Amorphous ferric oxides	Adding Fe(II) or Fe(III) to seawater	Bioavailable iron studies	XAS, XRD	[266]
	Addition of Fe(III) to synthetic buffered solution or soluble microbial systems	Chemical water treatment	UV–Vis	[267]
Amorphous Fe2O3 in a silica matrix	Impregnation of mesoporous silica with ferric nitrate and calcination	Antibiotic adsorption	TEM, XRD, FTIR, UV–Vis	[268]
Poorly crystalline iron oxides	Iron oxide biomineralization by iron-reducing bacteria	Geochemistry	ICP-MS	[270]
Amorphous iron oxide nanostructures	Photothermal reaction inside a droplet of iron(III) acetylacetonate solution	Electronics, sensors	TEM, SAED, EDS, RS	[269]
Two-dimensional iron oxide on Au(111)	Evaporating iron atoms, annealing and cooling down to 300 K in O2	Catalysis	STM, density-functional theory calculations	[142]
Iron oxide layer on zerovalent iron NPs	Zerovalent iron corrosion in an electrolyte solution	Treatment of contaminated aquifers	UV–Vis, XAS	[283]
Ferric oxide NPs	Protein-promoted conversion of Fe(II) into insoluble ferric iron oxides	Mitochondrial iron mishandling studies	UV–Vis	[284]
Ultra-thin iron oxide nanowhiskers	Iron oleate complex followed by selective decomposition at 150 °C	Biomedicine	TG-DSC, TEM, SAED, RS, XPS, FTIR	[285]
High valent iron oxo complexes	Fluorine-substituted Fe−tetra-amidomacrocyclic ligand oxidation	Photocatalysis	UV–Vis, EPR, high-resolution mass spectrometry	[286]
FeO(111)-like film on Fe(110) surface	Initial oxidation of Fe(110) in oxygen via Frank–Van der Merwe mechanism	Catalysis, pigments, electronics	XPS, XAS, STM, AES, LEED 5, STS 6	[280]
Colloidal Fe-FexOy composite NPs	Oxidation of metal NPs via a nanoscale Kirkendall process	Clean fuels, catalysis, electrochemical energy	TEM, SAXS, WAXS, RMDS	[287]
Biogenic microtubular iron oxides	Biotic formation of organic sheaths and subsequent abiotic deposition of Fe	Catalysis, pigments	EDS, RS, TEM, XRD, STEM	[288]
Iron oxide model thin-film electrodes	Thermal oxidation of pure metal iron substrates at 300 ± 5 °C in air	Lithium-ion batteries	RS, XPS, SIMS 7	[289]
Iron(III) oxide/ hydroxide nanonetworks	Synthesis of iron(III) oxide/hydroxide xerogels from a hydrated ferric nitrate	Electronics, catalysis, sensors	XPS, FTIR, XRD, TEM	[290]
Fe0-iron oxide core-shell NPs	Precipitation from ferrous sulfate with leaf extracts	Removal of nitrate in aqueous solution	EDS, XRD, FTIR	[291]
Soil samples with amorphous iron oxides	Abiotic mineralization in soil pore structures	Soil weathering studies	XRD, ICP-AES	[292]
Reticular pipeline cracks filled with iron oxide	Decarburization and diffusive oxidation of steel matrix	Corrosion resistance studies	EDS	[293]

¹ Molecular beam mass spectrometry. ² Particle mass spectrometry. ³ Reactive molecular dynamics simulations. ⁴ Laser ionization orthogonal time-of-flight mass spectrometry. ⁵ Low-energy electron diffraction. ⁶ Scanning tunneling spectroscopy. ⁷ Secondary ion mass spectrometry.

Table 7. Data on the structures containing iron oxide atomic clusters and an amorphous iron oxide phase.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Iron oxide atomic clusters	Combustion synthesis from Fe(CO)5 mixed with hydrogen and oxygen, high irradiance laser ionization from pressed Fe2O3 and Fe3O4 tablets, biomineralization inside the ferritin shell, reaction of laser ablated iron foil with 5% O2 seeded in a helium carrier gas	Catalysis, biomedicine, electronics, sensors, prediction of the magnetic properties of FeOx NPs, natural iron storage process studies, photovoltaics	MBMS 1, PMS 2, RMDS 3, TEM, LI-TOFMS 4, density-functional theory calculations, European Synchrotron Radiation Facility	[271,272,273,274,275,276,277,281]
Surface iron oxide layer on metal	Multicycling of an iron foil electrode between the switching potentials, formation of iron oxide species after reaction with Cr(VI) and Cu(II)	Chemical water treatment, production of molecular hydrogen, removal of contaminants	RMDS, XRD, XPS, FTIR, EDS	[278,279,282]
Amorphous ferric oxides	Adding Fe(II) or Fe(III) to seawater	Bioavailable iron studies	XAS, XRD	[266]
	Addition of Fe(III) to synthetic buffered solution or soluble microbial systems	Chemical water treatment	UV–Vis	[267]
Amorphous Fe2O3 in a silica matrix	Impregnation of mesoporous silica with ferric nitrate and calcination	Antibiotic adsorption	TEM, XRD, FTIR, UV–Vis	[268]
Poorly crystalline iron oxides	Iron oxide biomineralization by iron-reducing bacteria	Geochemistry	ICP-MS	[270]
Amorphous iron oxide nanostructures	Photothermal reaction inside a droplet of iron(III) acetylacetonate solution	Electronics, sensors	TEM, SAED, EDS, RS	[269]
Two-dimensional iron oxide on Au(111)	Evaporating iron atoms, annealing and cooling down to 300 K in O2	Catalysis	STM, density-functional theory calculations	[142]
Iron oxide layer on zerovalent iron NPs	Zerovalent iron corrosion in an electrolyte solution	Treatment of contaminated aquifers	UV–Vis, XAS	[283]
Ferric oxide NPs	Protein-promoted conversion of Fe(II) into insoluble ferric iron oxides	Mitochondrial iron mishandling studies	UV–Vis	[284]
Ultra-thin iron oxide nanowhiskers	Iron oleate complex followed by selective decomposition at 150 °C	Biomedicine	TG-DSC, TEM, SAED, RS, XPS, FTIR	[285]
High valent iron oxo complexes	Fluorine-substituted Fe−tetra-amidomacrocyclic ligand oxidation	Photocatalysis	UV–Vis, EPR, high-resolution mass spectrometry	[286]
FeO(111)-like film on Fe(110) surface	Initial oxidation of Fe(110) in oxygen via Frank–Van der Merwe mechanism	Catalysis, pigments, electronics	XPS, XAS, STM, AES, LEED 5, STS 6	[280]
Colloidal Fe-FexOy composite NPs	Oxidation of metal NPs via a nanoscale Kirkendall process	Clean fuels, catalysis, electrochemical energy	TEM, SAXS, WAXS, RMDS	[287]
Biogenic microtubular iron oxides	Biotic formation of organic sheaths and subsequent abiotic deposition of Fe	Catalysis, pigments	EDS, RS, TEM, XRD, STEM	[288]
Iron oxide model thin-film electrodes	Thermal oxidation of pure metal iron substrates at 300 ± 5 °C in air	Lithium-ion batteries	RS, XPS, SIMS 7	[289]
Iron(III) oxide/ hydroxide nanonetworks	Synthesis of iron(III) oxide/hydroxide xerogels from a hydrated ferric nitrate	Electronics, catalysis, sensors	XPS, FTIR, XRD, TEM	[290]
Fe0-iron oxide core-shell NPs	Precipitation from ferrous sulfate with leaf extracts	Removal of nitrate in aqueous solution	EDS, XRD, FTIR	[291]
Soil samples with amorphous iron oxides	Abiotic mineralization in soil pore structures	Soil weathering studies	XRD, ICP-AES	[292]
Reticular pipeline cracks filled with iron oxide	Decarburization and diffusive oxidation of steel matrix	Corrosion resistance studies	EDS	[293]

¹ Molecular beam mass spectrometry. ² Particle mass spectrometry. ³ Reactive molecular dynamics simulations. ⁴ Laser ionization orthogonal time-of-flight mass spectrometry. ⁵ Low-energy electron diffraction. ⁶ Scanning tunneling spectroscopy. ⁷ Secondary ion mass spectrometry.

Table 8. Data on the structures containing co-existing α-Fe₂O₃ and γ-Fe₂O₃ phases.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Saprolitic soil samples	Aerobic weathering of Fe-bearing minerals	Pedogenic process studies	XRF, UV–Vis, XRD	[298]
Loess and paleosol samples with iron oxides	Aerobic weathering of Fe-bearing silicate minerals		XRD, UV–Vis	[297]
Oxidized iron items	Soil iron corrosion limited by the diffusion of dissolved oxygen	Heritage science	EDS, XRD, RS	[295]
Surface iron oxide layer on metal	Anodic film formation on steel immersed in sour acid media	Corrosion resistance studies	XRD, EDS	[302]
Graphene-iron oxide nanotube composite	An adept template-free hydrothermal route from ferrous sulfate	Removal of the toxic heavy metal Cr(VI)	EDS, XRD, FTIR, UV–Vis, TEM	[299]
Polyacrylonitrile/iron oxide composite	Hydrothermal method for in situ growth of iron oxide; iron alkoxide hydrolysis	Removal of Congo red dye from water	FTIR, XRD, EDS, ICP-AES	[300]
Carbon/FexOy magnetic composites	Mechanical mixing and thermal treatment under N2 atmosphere	Wastewater treatment	XRD, TG-DSC, EDS, FTIR	[301]
Isoelement synthetic heterostructures	Hydrothermal method combined with controlled partial annealing process	Visible-light photocatalysis	XRD, TEM, XPS, UV–Vis, EPR	[296]

Table 8. Data on the structures containing co-existing α-Fe₂O₃ and γ-Fe₂O₃ phases.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Saprolitic soil samples	Aerobic weathering of Fe-bearing minerals	Pedogenic process studies	XRF, UV–Vis, XRD	[298]
Loess and paleosol samples with iron oxides	Aerobic weathering of Fe-bearing silicate minerals		XRD, UV–Vis	[297]
Oxidized iron items	Soil iron corrosion limited by the diffusion of dissolved oxygen	Heritage science	EDS, XRD, RS	[295]
Surface iron oxide layer on metal	Anodic film formation on steel immersed in sour acid media	Corrosion resistance studies	XRD, EDS	[302]
Graphene-iron oxide nanotube composite	An adept template-free hydrothermal route from ferrous sulfate	Removal of the toxic heavy metal Cr(VI)	EDS, XRD, FTIR, UV–Vis, TEM	[299]
Polyacrylonitrile/iron oxide composite	Hydrothermal method for in situ growth of iron oxide; iron alkoxide hydrolysis	Removal of Congo red dye from water	FTIR, XRD, EDS, ICP-AES	[300]
Carbon/FexOy magnetic composites	Mechanical mixing and thermal treatment under N2 atmosphere	Wastewater treatment	XRD, TG-DSC, EDS, FTIR	[301]
Isoelement synthetic heterostructures	Hydrothermal method combined with controlled partial annealing process	Visible-light photocatalysis	XRD, TEM, XPS, UV–Vis, EPR	[296]

Table 9. Data on the structures containing co-existing α-Fe₂O₃ and Fe₃O₄ phases.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Inclusions of iron oxides in ore samples	Precipitation during protracted hydrothermal fluid/rock interaction, biological oxidation of Fe(II) by photoautotrophs, microbial sedimentary ferric iron flux, infiltration by hypogene and supergene fluids during or after deformation	Banded iron formation studies, geochemistry, late Archean and early Paleoproterozoic studies, iron oxide copper gold system studies	ICP-MS, EDS, XRF, SAED, TEM, XRD, ICP-AES, TG-DSC	[303,304,305,306,307,308,309,310,311]
Surface iron oxide layer on metal	Tribo-oxidation wear of the cast iron disc	Brake system wear studies	EDS, XRD, TEM, SAED	[315]
Iron oxide NPs	Anodization of Fe sheet in ethylene glycol electrolyte and calcination	Biomedicine, catalysis, photovoltaics, electronics	XRD, EDS, XPS, RS, FTIR	[312]
Iron oxide inclusions in concrete samples	Corrosion of a steel-reinforcing bar in air-entrained concrete with chlorides	Corrosion resistance studies	EDS	[318]
Iron oxide nanosheets and nanowires	Thermal oxidation of iron foils in the presence of water vapor	Cr(VI) removal	XRD, TEM, RS, UV–Vis	[319]
Iron oxide hollow spheres	Microwave–hydrothermal ionic liquid method, calcination and autocatalysis	Photocatalysis	XRD, TEM, UV–Vis	[314]
Inclusions of iron oxides in mineralized rocks	Abiotic formation of a mineral deposit	Geochemistry	XRF	[320]
Theoretically calculated iron oxide phases	Radiation-chemical oxidation of Fe depending on pH and oxygen content	Precambrian studies	Kinetics of iron oxidation calculations	[321]
Iron oxide NPs supported on biogenic silica	Iron oxide NP impregnation under hydrothermal conditions and calcination	Rhodamine B photocatalytic degradation	EDS, XRD, UV–Vis, TEM	[316]
Sediment samples with inclusions of iron oxides	Mineralization by variable diagenetic processes	Rock magnetism studies	XRD, EDS	[322]
Iron oxide nanorods	Sols of ferric hydroxide radiolysis in water under gamma irradiation	Electronics, biomedicine	XRD, TEM	[314]
Spinel-bearing peridotite	Oxidation of ferrous iron in olivine and pyroxene into ferric iron	Serpentinization studies	FTIR, EDS	[323]
Iron oxide inclusions in kaolin clay samples	Abiotic chemical precipitation	Clay chemistry and morphology studies	ICP-AES, XRD, XRF, TG-DSC	[324]
Precipitates containing iron oxide inclusions	Biomineralization by photosynthetic Fe(II)-oxidizing bacteria	Banded iron formation studies	XRD, EDS	[325]
Iron-mineralized biofilms	Dissolution and re-precipitation of iron oxide minerals	Bioremediation of iron ore mines	–	[326]
Iron oxide nanotubes	Template-based electrodeposition and calcination under oxidizing atmospheres	Biomedicine, electronics, gas sensors, catalysis	TEM, XRD, SAED	[317]
Iron oxide powder	Hydrothermal process with a use of pyrite cinder lixivium	Pyrite cinder reutilization	FTIR, XRD, TEM, SAED	[327]
Growth model for submarine deposits	Transformation of primary (hydr)oxides via reduction by organic matter	Banded iron formation studies	–	[328]

Table 9. Data on the structures containing co-existing α-Fe₂O₃ and Fe₃O₄ phases.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Inclusions of iron oxides in ore samples	Precipitation during protracted hydrothermal fluid/rock interaction, biological oxidation of Fe(II) by photoautotrophs, microbial sedimentary ferric iron flux, infiltration by hypogene and supergene fluids during or after deformation	Banded iron formation studies, geochemistry, late Archean and early Paleoproterozoic studies, iron oxide copper gold system studies	ICP-MS, EDS, XRF, SAED, TEM, XRD, ICP-AES, TG-DSC	[303,304,305,306,307,308,309,310,311]
Surface iron oxide layer on metal	Tribo-oxidation wear of the cast iron disc	Brake system wear studies	EDS, XRD, TEM, SAED	[315]
Iron oxide NPs	Anodization of Fe sheet in ethylene glycol electrolyte and calcination	Biomedicine, catalysis, photovoltaics, electronics	XRD, EDS, XPS, RS, FTIR	[312]
Iron oxide inclusions in concrete samples	Corrosion of a steel-reinforcing bar in air-entrained concrete with chlorides	Corrosion resistance studies	EDS	[318]
Iron oxide nanosheets and nanowires	Thermal oxidation of iron foils in the presence of water vapor	Cr(VI) removal	XRD, TEM, RS, UV–Vis	[319]
Iron oxide hollow spheres	Microwave–hydrothermal ionic liquid method, calcination and autocatalysis	Photocatalysis	XRD, TEM, UV–Vis	[314]
Inclusions of iron oxides in mineralized rocks	Abiotic formation of a mineral deposit	Geochemistry	XRF	[320]
Theoretically calculated iron oxide phases	Radiation-chemical oxidation of Fe depending on pH and oxygen content	Precambrian studies	Kinetics of iron oxidation calculations	[321]
Iron oxide NPs supported on biogenic silica	Iron oxide NP impregnation under hydrothermal conditions and calcination	Rhodamine B photocatalytic degradation	EDS, XRD, UV–Vis, TEM	[316]
Sediment samples with inclusions of iron oxides	Mineralization by variable diagenetic processes	Rock magnetism studies	XRD, EDS	[322]
Iron oxide nanorods	Sols of ferric hydroxide radiolysis in water under gamma irradiation	Electronics, biomedicine	XRD, TEM	[314]
Spinel-bearing peridotite	Oxidation of ferrous iron in olivine and pyroxene into ferric iron	Serpentinization studies	FTIR, EDS	[323]
Iron oxide inclusions in kaolin clay samples	Abiotic chemical precipitation	Clay chemistry and morphology studies	ICP-AES, XRD, XRF, TG-DSC	[324]
Precipitates containing iron oxide inclusions	Biomineralization by photosynthetic Fe(II)-oxidizing bacteria	Banded iron formation studies	XRD, EDS	[325]
Iron-mineralized biofilms	Dissolution and re-precipitation of iron oxide minerals	Bioremediation of iron ore mines	–	[326]
Iron oxide nanotubes	Template-based electrodeposition and calcination under oxidizing atmospheres	Biomedicine, electronics, gas sensors, catalysis	TEM, XRD, SAED	[317]
Iron oxide powder	Hydrothermal process with a use of pyrite cinder lixivium	Pyrite cinder reutilization	FTIR, XRD, TEM, SAED	[327]
Growth model for submarine deposits	Transformation of primary (hydr)oxides via reduction by organic matter	Banded iron formation studies	–	[328]

Table 10. Data on the structures containing co-existing FeO and Fe₃O₄ phases.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Fe-rich carbonates with inclusions of iron oxides	Laser heating of natural goethite in a diamond anvil cell in CO2	Earth’s mantle studies	XRD, XAS, TEM, EELS, HAADF-STEM, SAED	[329]
Samples with partially reduced FeO and Fe3O4	Porous iron growth from wüstite in CO/CO2 and H2/H2O systems	Porous iron growth mechanism studies	–	[330]
Fe/oxide core-shell NPs	Formation of Fe3O4 during the oxidation of Fe NPs; high-temperature reduction of Fe3O4 to FeO by an electron-beam	Environmental remediation, electronics, catalysis, biomedicine, energy storage	TEM, SAED, EELS, HAADF-STEM, EDS	[331]

Table 10. Data on the structures containing co-existing FeO and Fe₃O₄ phases.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Fe-rich carbonates with inclusions of iron oxides	Laser heating of natural goethite in a diamond anvil cell in CO2	Earth’s mantle studies	XRD, XAS, TEM, EELS, HAADF-STEM, SAED	[329]
Samples with partially reduced FeO and Fe3O4	Porous iron growth from wüstite in CO/CO2 and H2/H2O systems	Porous iron growth mechanism studies	–	[330]
Fe/oxide core-shell NPs	Formation of Fe3O4 during the oxidation of Fe NPs; high-temperature reduction of Fe3O4 to FeO by an electron-beam	Environmental remediation, electronics, catalysis, biomedicine, energy storage	TEM, SAED, EELS, HAADF-STEM, EDS	[331]

Table 11. Data on the structures containing co-existing γ-Fe₂O₃ and Fe₃O₄ phases.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Iron oxide NPs	Thermal decomposition of iron oleate, continuous flow synthesis, co-precipitation of Fe3+/Fe2+ ions, aerosol spray pyrolysis with the use of ferric nitrate and ferric chloride, precipitation from iron salts with natural leaf extract	Biomedicine, soil remediation, metal removal, wastewater treatment, electronics, catalysis, energy storage, groundwater remediation	TEM, XRD, FTIR, SAED, TG-DSC, UV–Vis, SAXS, neutron diffraction, EDS, MSP, EELS, EPR, ICP-MS, XAS, RS	[45,332,333,334,335,336,337,338,339,340,352,353,354,355,358,359,360,361,362]
Surface iron oxide layer on metal	Oxidation of a pure iron surface in oxygen, electrochemical reduction of lepidocrocite and ferrihydrite, in situ formation on an iron surface depending on the applied potential	Iron oxidation studies, atmospheric steel corrosion studies, groundwater remediation, corrosion protection studies	XPS, XRD, XAS, RS, AES, ellipsometry	[345,346,347,348,349]
Oxidation layer on archaeological steel	Combined iron oxidation/iron(III) oxyhydroxide reduction without O2	Corrosion studies on ancient metallic objects	EDS, RS	[343]
Iron oxide-TiO2 nanorod heterostructures	Precipitation by injection of Fe(CO)5 into stirred TiO2 containing mixture	Optoelectronics, biomedicine, catalysis	XRD, XAS, ICP-AES, TEM, UV–Vis	[363]
Iron oxide in nanoscrolls and nanoribbons	Precipitation from ferric and ferrous chloride with ammonia solution	Lithium-ion storage, photocatalysis, biosensors	TEM, FTIR	[364]
Iron oxide hollow core/Shell NPs	Solvothermal synthesis from FeCl3 and urea in ethylene glycol and calcination	Biomedicine	XRD, TEM, TG-DSC, UV–Vis	[365]
Thin-film nanocomposite membrane with iron oxide	In situ synthesis from aqueous solutions containing ferric chloride	Biofouling protection	EDS, TEM, XPS, UV–Vis, XRD, TG-DSC	[366]
Magnetoferritin iron oxide NPs	Controlled mineralization from recombinant human H-chain ferritin	Biomedicine	TEM	[367]
Iron oxide-based hollow magnetic nanoparticles	Synthesis from iron pentacarbonyl in 1-octadecene and oleylamine	Exchange bias studies	XRD, TEM, FTIR, MSP, F-AAS	[368]
Albumin protein-based magnetic NPs	Co-precipitation of FeCl2 and FeCl3 by ammonia in the presence of protein	Biomedicine	TEM, TG-DSC	[369]
Composite of organic matrix and iron oxide NPs	Thermal decomposition of iron(III) oleate complex	Biomedicine	TEM	[370]
Iron oxide powder	Photochemical oxidation of siderite (FeCO3) by ultraviolet radiation	Banded iron formation studies	XRD	[371]
Interfacial iron oxide layer on iron artifacts	Iron corrosion in an anoxic environment after a pH increase at the interface	Anoxic corrosion of archaeological steel studies	HAADF-STEM, RS, EDS, SAED, SIMS	[344]
Iron oxide hydroxyapatite core/shell nanocomposites	Precipitation from ferric and ferrous chloride with ammonia under N2	Biomedicine	TEM, FTIR, XRD, AAS, EDS	[372]
Chitosan-based beads with iron oxide NPs	Co-precipitation from ferric and ferrous chloride with NaOH solution	Remediation of water sources	XRD, FTIR, TG-DSC, EDS	[356]
Silica–iron oxide nanocomposite	Co-precipitation from ferric and ferrous chloride with ammonia solution	Toxic species removal	XRD, TEM, FTIR, UV–Vis, SAED	[357]
Vertical tube-shaped iron-oxide accumulations	Deep water corrosion of carbon steel	Marine corrosion studies	EDS	[351]
Hydrogels with embedded iron oxide NPs	In situ mineralization of iron ions in a hydrogel matrix	Dye removal	XRD, FTIR, TG-DSC, TEM	[373]
Corroded reinforced concrete	Iron corrosion in a laboratory corrosion chamber	Steel rebar corrosion studies	XRD, EDS	[350]
Porous hollow iron oxide NPs on carbon nanotubes	Etching of Fe-FexOy intermediate with nitric acid aqueous solution and drying	Biomedicine, catalysis, separation	TEM, XRD	[374]
Iron oxide embedding of bacterial cells	Biomineralization by thermophilic iron-reducing bacteria	Biogenic iron mineral formation studies	XRD	[375]
Activated carbon aerogel with iron oxide inclusions	Hydrothermal synthesis from ferrous sulfate with ammonia	Catalytic oxidation of pesticides	XRD, FTIR, XPS, TEM	[376]
Polyglycerol-grafted iron oxide NPs	Thermal decomposition of iron(III) acetylacetonate in triethylene glycol	Biomedicine	TEM, TG-DSC, FTIR, ICP-AES	[377]

Table 11. Data on the structures containing co-existing γ-Fe₂O₃ and Fe₃O₄ phases.

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Iron oxide NPs	Thermal decomposition of iron oleate, continuous flow synthesis, co-precipitation of Fe3+/Fe2+ ions, aerosol spray pyrolysis with the use of ferric nitrate and ferric chloride, precipitation from iron salts with natural leaf extract	Biomedicine, soil remediation, metal removal, wastewater treatment, electronics, catalysis, energy storage, groundwater remediation	TEM, XRD, FTIR, SAED, TG-DSC, UV–Vis, SAXS, neutron diffraction, EDS, MSP, EELS, EPR, ICP-MS, XAS, RS	[45,332,333,334,335,336,337,338,339,340,352,353,354,355,358,359,360,361,362]
Surface iron oxide layer on metal	Oxidation of a pure iron surface in oxygen, electrochemical reduction of lepidocrocite and ferrihydrite, in situ formation on an iron surface depending on the applied potential	Iron oxidation studies, atmospheric steel corrosion studies, groundwater remediation, corrosion protection studies	XPS, XRD, XAS, RS, AES, ellipsometry	[345,346,347,348,349]
Oxidation layer on archaeological steel	Combined iron oxidation/iron(III) oxyhydroxide reduction without O2	Corrosion studies on ancient metallic objects	EDS, RS	[343]
Iron oxide-TiO2 nanorod heterostructures	Precipitation by injection of Fe(CO)5 into stirred TiO2 containing mixture	Optoelectronics, biomedicine, catalysis	XRD, XAS, ICP-AES, TEM, UV–Vis	[363]
Iron oxide in nanoscrolls and nanoribbons	Precipitation from ferric and ferrous chloride with ammonia solution	Lithium-ion storage, photocatalysis, biosensors	TEM, FTIR	[364]
Iron oxide hollow core/Shell NPs	Solvothermal synthesis from FeCl3 and urea in ethylene glycol and calcination	Biomedicine	XRD, TEM, TG-DSC, UV–Vis	[365]
Thin-film nanocomposite membrane with iron oxide	In situ synthesis from aqueous solutions containing ferric chloride	Biofouling protection	EDS, TEM, XPS, UV–Vis, XRD, TG-DSC	[366]
Magnetoferritin iron oxide NPs	Controlled mineralization from recombinant human H-chain ferritin	Biomedicine	TEM	[367]
Iron oxide-based hollow magnetic nanoparticles	Synthesis from iron pentacarbonyl in 1-octadecene and oleylamine	Exchange bias studies	XRD, TEM, FTIR, MSP, F-AAS	[368]
Albumin protein-based magnetic NPs	Co-precipitation of FeCl2 and FeCl3 by ammonia in the presence of protein	Biomedicine	TEM, TG-DSC	[369]
Composite of organic matrix and iron oxide NPs	Thermal decomposition of iron(III) oleate complex	Biomedicine	TEM	[370]
Iron oxide powder	Photochemical oxidation of siderite (FeCO3) by ultraviolet radiation	Banded iron formation studies	XRD	[371]
Interfacial iron oxide layer on iron artifacts	Iron corrosion in an anoxic environment after a pH increase at the interface	Anoxic corrosion of archaeological steel studies	HAADF-STEM, RS, EDS, SAED, SIMS	[344]
Iron oxide hydroxyapatite core/shell nanocomposites	Precipitation from ferric and ferrous chloride with ammonia under N2	Biomedicine	TEM, FTIR, XRD, AAS, EDS	[372]
Chitosan-based beads with iron oxide NPs	Co-precipitation from ferric and ferrous chloride with NaOH solution	Remediation of water sources	XRD, FTIR, TG-DSC, EDS	[356]
Silica–iron oxide nanocomposite	Co-precipitation from ferric and ferrous chloride with ammonia solution	Toxic species removal	XRD, TEM, FTIR, UV–Vis, SAED	[357]
Vertical tube-shaped iron-oxide accumulations	Deep water corrosion of carbon steel	Marine corrosion studies	EDS	[351]
Hydrogels with embedded iron oxide NPs	In situ mineralization of iron ions in a hydrogel matrix	Dye removal	XRD, FTIR, TG-DSC, TEM	[373]
Corroded reinforced concrete	Iron corrosion in a laboratory corrosion chamber	Steel rebar corrosion studies	XRD, EDS	[350]
Porous hollow iron oxide NPs on carbon nanotubes	Etching of Fe-FexOy intermediate with nitric acid aqueous solution and drying	Biomedicine, catalysis, separation	TEM, XRD	[374]
Iron oxide embedding of bacterial cells	Biomineralization by thermophilic iron-reducing bacteria	Biogenic iron mineral formation studies	XRD	[375]
Activated carbon aerogel with iron oxide inclusions	Hydrothermal synthesis from ferrous sulfate with ammonia	Catalytic oxidation of pesticides	XRD, FTIR, XPS, TEM	[376]
Polyglycerol-grafted iron oxide NPs	Thermal decomposition of iron(III) acetylacetonate in triethylene glycol	Biomedicine	TEM, TG-DSC, FTIR, ICP-AES	[377]

3.4. The Structures Containing Three or More Co-Existing Iron Oxide Phases

The co-existence of three or more iron oxide compounds in a single heterogeneous composition makes it useless to try to precisely distinguish every standalone phase. Thus, studies devoted to such a case are considered in this section and listed in Table 12. Iron oxide NPs and the surface oxide layer on metal surfaces are the largest groups of papers on the structures containing mixed iron oxide phases. Synthesis techniques for obtaining mixed iron oxide NPs include physical ones (flame synthesis [378], thermal oxidation [379] and laser ignited combustion [376]) and chemical ones (thermal decomposition [82,380] and precipitation [341], including “green” process [381]). The main application of such NPs is in biomedicine [82,378,379,380].

Table 12. Data on the structures containing mixed iron oxide crystal phases [42,79,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,405,406,407].

Composition	Main Mechanisms of Iron Oxide Formation	Declared Applications	Phase Verification Techniques	Refs.
Iron oxide NPs	Thermal decomposition of an iron oleate in the presence of oleic acid, flame synthesis from Fe(CO)5, precipitation from ferric chloride in a natural leaf extract, thermal oxidation of polycrystalline Fe foils	Biomedicine, ferrofluids, electronics, immobilization of Cd in soils, catalysis, metal fuel	PMS, TEM, EDS, TG-DSC, SAED, XRD, XPS, FTIR, UV–Vis, EELS	[79,380,381,382,383,384]
Surface iron oxide layer on metal	Carbon steel corrosion at room temperature, iron carboxylate transformation to iron oxides, electrochemical anodization of metal in simulated acid rain solution	Corrosion resistance studies, railway industry, fireside corrosion studies	EDS, XRD, XRF, RS, XPS, FTIR, TG-DSC	[385,386,387,388,389]
Powder containing iron oxide microparticles	Carbothermal reduction of red mud by heating in a microwave furnace	Alumina production by-product recycling	XRD, EDS, XRF, TG-DSC	[400]
Iron oxide microfibers arranged in a complex hierarchical structure	Thermal decomposition of Fe(CO)5 and silicone oil and microwave vaporization	Environmental safety, biomedicine, sensors	EDS, XRD	[396]
Fe-based nanocomposite catalysts containing agglomerates of the two types	Melting of iron, aluminum and copper salts and reduction	Low-temperature catalytic oxidation of CO	XRD, TEM, XAS, EDS	[393]
Iron oxide powders containing hematite, magnetite and maghemite	Chemical precipitation from ferric nitrate and ferrous sulfate and heating	Inositol phosphate selective retention in soil	XRD	[399]
Silica–iron oxide nanocomposite with hematite, magnetite and wüstite	Silica promotion upon the reduction of amorphous iron oxide in hydrogen	Catalysis	XRD, TEM, EELS, STEM, SAED	[394]
Iron(II) and (III) oxides inclusions in char composites	Microwave pyrolysis of Moso bamboo samples with ferric chloride	Syngas production	XRD	[401]
Ultra-thin magnetic iron oxide films containing Fe3O4, γ-Fe2O3 and FeO	Thermally induced phase transformation of ultra-thin iron oxide films	All oxide heterostructures	XRD, XPS	[402]
Surface iron(II) and (III) oxide layer on iron granules	Atomization of the molten semi-steel with a rotary cup atomizer	Iron powder production	TG-DSC, XRD	[403]
Iron oxide (Fe3O4, γ-Fe2O3 and ɑ-Fe2O3) inclusions in fly ash samples	Coal combustion and flue gas cooling at various temperatures	Selenium adsorption by iron minerals	XRF, MSP, XPS	[404]
High-pressure metastable phases mFeO⋅nFe2O3	Formation of complex iron oxide crystals under high-pressure conditions	Earth and planetary deep interior studies	–	[24]
Iron(III) oxide submicron inclusions in a biofilm on a basalt surface	Microbial direct or non-direct biomineralization	Biovermiculation studies	EDS	[388]
Inclusions of poorly crystalline iron(III) oxides in ore samples	Microbial biomineralization	Mine remediation, waste stabilization	SIMS	[390]
Iron oxide (Fe3O4, γ-Fe2O3 and ɑ-Fe2O3)/iron composite	Reactive spark plasma sintering of mechanically activated Fe powders	Magnetic material development	XRD, TG-DSC	[405]
Core−shell Fe@Fe2O3 nanowires containing Fe(II) and Fe (III) oxides	Ferric chloride reduction with sodium borohydride and surface oxidation	Cr(VI) removal	XRD, UV–Vis, TEM, XPS	[406]
Iron oxide submicron particles accumulated in the cytoplasm of cells	Intracellular or extracellular microbial biomineralization	Hydrothermal vent field studies	XRD, TEM, SAED, EDS	[389]
Deposit samples with inclusions of iron(II) and (III) oxides	Anoxygenic photosynthesis by a photoferrotrophic bacterium	Banded iron formation studies	–	[391]
Bovine serum albumin–iron oxide suspensions	Precipitation from ferric nitrate with NaOH in N2 atmosphere	Boreal forest studies	FTIR	[398]
Iron oxide (Fe3O4, γ-Fe2O3 and ɑ-Fe2O3) magnetic short nanotubes	Anion-assisted hydrothermal route by using phosphate and sulfate ions	Biomedicine, ferrofluids, electronics, spintronics	TEM, SAED, XRD	[407]
Fe-biochar composites containing iron(II) and (III) oxides	Pyrolysis of ferric chloride in a biochar matrix at various temperatures	Arsenic removal, environmental remediation	XRD, XPS, RS, FTIR	[397]
FexOy@C spheres embedded with highly dispersed iron oxide NPs	One-pot hydrothermal cohydrolysis-carbonization using iron	Catalysis	TEM, EDS, XAS, XRD, MSP	[395]
Powder with inclusions of submicron iron oxide particles	Reduction of solid ferric hydroxide by iron-reducing bacteria	Microbial iron reduction studies	EDS, RS	[392]

An iron oxide layer on the metal surface is the second commonly considered structure, which plays an important role in corrosion studies [383,384,385] and can be formed both naturally (e.g., iron carboxylate transformation in a leaf contamination on rails [386] or fireside corrosion of steel in the furnace walls in boilers [384]) and synthetically (e.g., in situ oxidation of the surface of a steel sample in a controlled atmosphere [387] or electrochemical anodization of metal in a simulated acid rain solution [385]).

Compositions with mixed iron oxides can originate via a biogenic route, including microbial direct or non-direct biomineralization [388,389,390], anoxygenic photosynthesis by a photoferrotrophic bacterium [391] and the reduction of solid ferric hydroxide by iron-reducing bacteria [392]. Biogenic iron oxide structures can be used in microbial iron reduction studies [392], banded iron formations [391] or hydrothermal vent field studies [389] and waste remediation [390].

Similarly to the above mentioned iron oxide structures, various compositions containing mixed iron oxide phases can be applied for catalytical purposes. These compositions include iron oxide NPs [379], Fe-based nanocomposites [393], silica–iron oxide nanocomposites [394] and Fe_xO_y@C spheres [395] and can be obtained with the use of chemical [394,395] or physical [379,393] synthesis.

Finally, in some cases, such structures are used for various environmental tasks, including environmental safety [396], environmental remediation [397], boreal forest studies [398], inositol phosphate selective retention in soil [399], biovermiculation studies [388] and the Earth and planetary deep interior studies [24].

3.5. The Main Characterization Techniques Used to Verify Phase Composition

From analyzing Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12, the most frequently used characterization techniques can be revealed. These techniques include X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), UV–visible(-NIR) spectroscopy (UV–Vis), selected area electron diffraction (SAED), Raman spectroscopy (RS), thermogravimetry/differential scanning calorimetry (TG-DSC), X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy (MSP) and X-ray fluorescence (XRF). The diagram showing the partial distribution between these techniques is presented in Figure 5.

Figure 5. Main characterization techniques used to verify phase composition.

In some cases, while the crystal structure of the co-existing iron oxide compounds differs significantly (e.g., for hematite and magnetite), the most common and available techniques are suitable for phase composition verification, including XRD and XRF. For instance, XRF and XRD techniques were used to determine the chemical and crystalline phase composition, accordingly, of clay samples containing iron oxides [324]. Another possible example is ε-Fe₂O₃, a crystal structure that is also quite distant from the other iron(III) oxide polymorphs and can be distinguished with the use of RS and XAS [258], or even with the use of a standalone XRD technique [37]. Contrary to the above, the simultaneous existence of magnetite and maghemite crystal phases cannot be adequately analyzed with XRD, SAED or some other techniques due to the similar crystal structure of these compounds and non-stoichiometry of synthetic [38] or natural [6,295] magnetite. In this case, the MSP technique can give additional information on the crystal structure, including magnetite to maghemite partial transition or the superparamagnetic state of NPs [38]. MSP can be applied to characterize samples composed of homogenously sized iron oxide NPs above the blocking temperature in a superparamagnetic regime [368]. Mössbauer measurements were performed to investigate detailed iron mineralogy compositions in magnetic fractions of fly ashes [405]. Perecin et al. showed that although Mössbauer spectra with two sextets were expected for pure magnetite, an extra sextet suggested the maghemite phase’s presence in the sample, in agreement with the FTIR results [89]. The high oxidation degree of magnetite can be confirmed by low isomer shift values [342]. MSP results can show a shift of the Morin transition in hematite upon increasing Ru³⁺-to-Fe³⁺ substitution, similar to the shift in the Morin transitions occurring in temperature-dependent magnetization measurements [224].

3.6. The Analysis of the Distribution of Iron Oxide Compounds by their Frequency of Mention

Based on the data provided in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12 (in total, more than 300 research articles were analyzed), a histogram was built (Figure 6). Compositions containing magnetite are the most frequently considered in scientific articles. This can be explained by taking into account the highest saturation magnetization of this iron oxide compound among others and also because of the wide biomedical application of magnetite NPs. The second most commonly described compound is hematite, presumably due to its use in photocatalysis and its abundance (the same for magnetite ores) in nature. The third important iron oxide compound is maghemite, since γ-Fe₂O₃ NPs are often used instead of magnetite NPs due to the high oxidation instability of Fe₃O₄ in air atmospheres and, therefore, difficulties in its preservation without an inert atmosphere or a protective shell. Other iron oxide compounds, including FeO, β-Fe₂O₃ and ε-Fe₂O₃, are metastable and/or their synthesis procedure is too complex, therefore scientific studies on them are relatively rare.

Figure 6. Distribution of iron oxide compounds by their frequency of mention (based on the analyzed research articles mentioned in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12).

3.7. The Main Mechanisms of Iron Oxide Formation

Using the information given in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12, the generalized scheme illustrating some widely considered mechanisms of iron oxide formation was drawn (Figure 7). Terrigenous formation implies abiotic mineralization [100,102,206,219,292,324]. Extraterrestrial formation including samples from Mars [113] originated from precipitation from oxygenated iron-rich water [198] and abiotic formation in an aqueous environment of deposition [124].

Figure 7. Mechanisms of iron oxide formation: (a)—Terrigenous formation; (b)—Extraterrestrial formation; (c)—Biomineralization; (d)—Iron and iron alloy corrosion; (e)—Ceramics firing; (f)—Biomimetic synthesis; (g)—Chemical precipitation; (h)—Physical deposition; (j)—Mechanochemical synthesis.

Biomineralization implies bacterial magnetosome formation [94,96,97,98], formation by dissimilatory iron-reducing bacteria [95], bacterial reduction of iron hydroxide [122,166,392], biomineralization by anoxygenic photoferrotrophy [210], biomineralization inside the ferritin shell [276,277], biogeneration of magnetite with a use of the amyloid peptide Aβ₄₂ in the case of brain diseases [160], etc.

Iron and iron alloy corrosion include chemical [106,109,238,350] and electrochemical [86,349,385] processes, either natural or intended. Ceramics firing implies the calcination of a milled mix at a high temperature in an oxidizing atmosphere [229], high-temperature firing of local iron-rich area on a ceramic glaze [140] and surface iron enrichment and firing of wares under reducing conditions [132]. Biomimetic synthesis refers to a process using natural plant extracts [91], iron oxide NP formation on biogenic silica [316], nucleation of Fe₃O₄ NPs mediated by the iron-binding protein Mms6 [92] and protein-promoted conversion of Fe(II) into insoluble ferric iron oxides [284].

Chemical precipitation includes co-precipitation by sodium hydroxide from an iron chloride solution [82], precipitation from Fe³⁺ and Fe²⁺ ions by urea with chitosan [149], precipitation from iron(II) sulfate heptahydrate with NaOH [158] and precipitation from iron(III) ethoxide with ethanol in the surfactant solution [179]. Physical deposition implies electron-beam deposition [142], liquid-phase atomic layer deposition [232], pulsed laser deposition on the Ga-terminated surface of a GaN (0001) [260] and chemical vapor deposition from an Fe organic liquid source [258]. Finally, mechanochemical synthesis includes mechanical mixing and thermal treatment under a N₂ atmosphere [301], reactive spark plasma sintering of mechanically activated Fe powders [405] and milled zerovalent iron corrosion in anaerobic synthetic groundwater [348]. All the listed routes of iron oxide formation can be either intended (controlled) or natural (uncontrolled).

4. The Main Applications of the Structures Containing Iron Oxides

The largest amount of scientific papers being analyzed describes various biomedical applications of iron oxides, mainly magnetite/maghemite NPs. Photocatalytic oxidation and other applications in catalysis are the second major practical use of iron oxides, with a predominance of compositions with hematite. The third important field of use is electronics, including spintronics, data storage development and optoelectronics, mainly for thin ferrimagnetic or antiferromagnetic iron oxide films. Corrosion science is a quite obvious, but still very important area of application, since iron oxide passivation can provide a better reliability of steel pipelines, safety of ancient artifacts for heritage science, etc. Waste remediation is generally based on the possibility of iron oxidation from iron(II) to iron(III) and to absorb or bind inorganic and organic pollutants. Finally, serious attempts in geosciences with the use of various iron oxide compositions are still being made, despite a long history of research. Studies of natural iron ores can give much information about Earth’s evolution, including Precambrian research.

Biomedical applications of iron oxides include T₂ magnetic resonance imaging (MRI). MRI contrast agents are based on superparamagnetic NPs; their nanocluster formation increases the magnetic signal and subsequently enhances imaging sensitivity or cell labeling efficiency [29]. Magnetic hyperthermia with alternating magnetic fields requires magnetic NPs having an effective heating rate to enable therapeutic applications [11]. There are diverse bioinspired approaches for the synthesis of magnetic nanochains with optimal properties for biomedical applications, including magnetically guided drug delivery [13]. The integration of magnetic NPs and organic dyes into single platforms demonstrated their use as bimodal imaging agents for both in vitro and in vivo imaging and in multifunctional platforms that perform several tasks in parallel (e.g., dual-mode imaging and photodynamic therapy or drug delivery) [14].

Iron oxide formation mechanisms are important for understanding formation processes of iron-relevant minerals in Precambrian banded iron formations, granular iron formations and associated iron-poor strata [4,33]. This understanding also includes the origin of probably the first protosensory system evolved on Earth, i.e., magnetotaxis, while the precise mechanism of biogenic magnetite mineralization on early Earth is still unknown [8]. Well-known processes of the biomineralization of iron can help in better understanding human body iron metabolism and in curing diseases linked with iron-damaged regulation [10]. Today, by far the single most important use (by volume) of iron oxides is as a source of Fe, which is subsequently processed to make steel [3]. Another important iron oxide industrial application is photoelectrochemical water splitting. It is a leading strategy for producing a promising renewable store of energy—hydrogen [12]. Iron oxides, including magnetite, maghemite and hematite, are widely used in heterogeneous catalysis processes and have been attractive alternatives for the remediation of polluted soil, groundwater and wastewater based on a heterogeneous Fenton reaction (a combination of a solid Fe-based catalyst and H₂O₂) [25,26]. Partially oxidized zerovalent iron NPs with a core-shell structure can be used to remediate groundwater and wastewater contaminated by chlorinated organic compounds, heavy metals, dyes and phenols [27].

Rare iron(III) oxide polymorphs can also be practically used. Thus, β-Fe₂O₃ has found a few applications in an electro-catalyst for the reduction of hydrogen peroxide, in optoelectronics and in red ferric pigments [19]. A very high room-temperature coercive field makes ε-Fe₂O₃ suitable for use in recording media; its magnetoelectric coupling and millimeter-wave ferromagnetic resonance are useful in electric/magnetic field tunable devices and for millimeter wave absorption on the walls of an interior room or on the body of a car, train or airplane [36]. In some cases, e.g., in catalysts, amorphous Fe₂O₃ NPs can be more active than nanocrystalline polymorphs or particles of metallic iron of the same diameter [39]. The structures of iron oxides are common to many binary systems and complex solid solutions; therefore, a rich set of isostructural compounds and solid solutions with tunable properties may be synthesized [23].

Ceramics, including composites containing inclusions of amorphous iron oxides, are suitable for various industrial applications. The process of transforming iron oxides from a glass network into a crystal nucleus was studied for the novel field of glass ceramics based on waste glass [408]. Prim et al. showed that iron oxide from a metal sheet treatment process may be used as a ceramic pigment by encapsulation in a crystalline and amorphous silica matrix [409]. Intended for hazardous waste incineration, glass ceramics containing hematite exhibited a superior compressive strength, volume density and water absorption [410]. Alumina–zirconia–titania ceramic membranes coated with a nanosized hematite layer can be applied in a combined ozonation–membrane filtration process [411]. The formation of solid solutions between mullite and transition metal cations, including iron, affects the thermal expansion of mullite ceramics through the distortion of the Al–O octahedral [412].

Less than 20 a wt% addition of iron oxide significantly lowered the softening and melting temperatures of CaO–Al₂O₃–MgO–SiO₂-based glass ceramics [413]. The lower melting temperature leads to a significant decrease in the price of the vitrification procedure and to the suppression of heavy metal evaporation during glass melting [414]. Such a class of glass ceramics possessing excellent mechanical characteristics (bending strength of 120 MPa, hardness of 9 GPa and fracture toughness of 1.6 MPa∙m^1/2) was discussed, together with the remarkable effect of their vitrification on heavy metal immobilization [415]. One of the most low-temperature techniques, the sol–gel method, which involves the hydrolysis of the precursors of constituent oxides followed by their gelation, has the potential to yield magnetic ceramics, including bioceramics, with a more flexible composition range, better homogeneity, better bioactivity and controllable porous structure [416]. Nanostructured catalyst-modified composite cathodes can be obtained by infiltrating a metal ion solution into a ceramic scaffold, followed by heating at a high temperature. A 3D heterostructured electrode decorated by amorphous iron oxide that works stably at 650 °C with an oxygen reduction reactivity comparable to that of a Pt-decorated one was obtained [417].

A wide range of metals such as gold, silver, copper, zinc, iron, platinum and palladium are fabricated in the form of NPs using algae and cyanobacteria and can be applied for infection control, diagnosis, drug delivery, biosensing and bioremediation [418]. An approach for the synthesis of highly pure, crystalline and biocompatible hematite NPs through the sole use of Psidium guajava leaf extract was proposed. The antibacterial efficacy of the obtained hematite NPs against Gram-positive as well as Gram-negative bacteria was established [419]. Both plants and microbes offer various ways to synthesize magnetite and maghemite NPs for potential dye degradation from industrial effluents from a variety of routes due to their vast genetic diversity and presence of various enzymes, respectively [420]. Co-substituted magnetite NPs were produced during the enzymatic reduction of a synthetic co-ferrihydrite using Geobacter sulfurreducens as an analogue to bioreduction processes in the natural environment to understand the natural biogeochemical cycling of cobalt in Fe-rich environments undergoing microbially mediated redox transformations [421]. Natural biogenic iron oxide extracted from banded iron formations showed high removal potential with the maximum sorption efficiency of 88.65% at a 30 g/L adsorbent dose [422].

Amorphous iron oxides are the promising material for various energetic and catalytic applications, including biomedicine. Compared to well crystalline Fe₂O₃, amorphous Fe₂O₃/graphene composite nanosheets exhibited superior sodium storage properties such as high electrochemical activity, a high initial Coulombic efficiency of 81.2% and a good rate of performance for sodium-ion batteries [423]. Efficient water oxidation catalysts have nominally amorphous mixed-metal oxide phases on their surface which are responsible for catalytic activity [424]. Amorphous iron oxide-packaged oxaliplatin prodrugs can be effective for cancer treatment, since the Fe²⁺/Fe³⁺ ions released by the amorphous iron oxide NPs produce a large amount of reactive oxygen species through Fenton’s reaction [425]. Nanosized amorphous iron oxide showed higher catalytic activity with lower oxidant consumption in comparison to Fe₃O₄- and Fe₂O₃-based clay composites [426]. Amorphous Fe₂O₃ nanoflakes were biosynthesized by a novel sol–gel method using Aloe vera leaf extract, and their catalytic effect on the thermal decomposition of ammonium perchlorate was investigated [427]. Amorphous Fe₂O₃ NPs can act as efficient and robust photocatalysts for solar H₂ evolution without any cocatalysts [428]. Amorphous Fe₂O₃/reduced graphene oxide/carbon nanofiber films were tested as flexible and freestanding anodes for lithium-ion batteries [429].

Corrosion resistance studies involving iron oxides are also important. Thus, potassium and chlorine may interplay to accelerate the corrosion of Fe-rich oxide scales, and an understanding of this process may open up new ideas for ways to decrease corrosion in highly corrosive environments [430]. Wheat straw fiber-reinforced polyvinyl chloride composites pigmented with iron oxide pigment have better seawater corrosion resistance, including better fiber/matrix interfacial interaction, lower total discoloration and higher surface hydrophobicity, mechanical properties and thermal stability [431]. Hydroxyapatite-bioglass-Fe₃O₄-chitosan coatings showed an effective improvement of the surface properties, hemocompatibility and in vitro corrosion rate of a biodegradable magnesium alloy [432]. The corrosion resistance of the epoxy coating was experimentally improved using micaceous iron oxide and Al pigments [433]. In the case of stainless steel, iron oxide formation corresponds to a low pitting potential and corrosion resistance and leads to the degraded protective property of the oxide film [434].

Based on the data in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12, a diagram illustrating the main declared applications of various compositions with iron oxides was built (Figure 8).

Figure 8. Main applications of iron oxides.

5. Summary and Perspectives

Iron oxide compounds are widely presented in various scientific and industrial areas due to their abundance on Earth. The possibility of changing the iron oxidation state between Fe²⁺ and Fe³⁺ lies in the basement of the biogenic iron cycle, which results in band iron formation accumulations and the deposition of fossilized magnetotactic bacteria, called magnetofossils [435]. The mass production of metallurgy and large iron deposits, e.g., the Kovdor deposit [6], provides a low cost of iron oxides compositions and their applicability for different technical purposes. High biocompatibility and modern synthesis techniques, including continuous-flow, biomimetic and biogenic processes, make it possible to translate academic research to clinical practice [435,436,437].

Integrating magnetic NPs with polymers allows for the fabrication of multifunctional systems for chemotherapy and magnetic hyperthermia therapy, which can also be simultaneously monitored by utilizing the magnetic resonance imaging capabilities of magnetic nanoparticle–polymer conjugates [14]. Such systems, e.g., iron-loaded crosslinked magnetic chitosan/graphene oxide, can also be widely applied for the practical environmental remediation of wastewater effluents containing organic pollutants [174]. The novel iron oxide-based materials can be used to improve solar fuel production [110]. The use of ferritin protein as a carrier of iron oxide NPs renders it more suitable for cancer diagnosis as an effective T₂ contrast agent with an expected reduced toxicity due to the prevention of NP interaction with the environment [367]. Two-phase iron oxide NPs (e.g., magnetite/maghemite core-shell structures) are promising for applications implying an intrinsic exchange bias effect [368].

As paleoenvironmental proxies, the iron abundance, speciation and isotopic composition recorded for an Archean ocean analogue in the future can assist in understanding the iron biogeochemistry in the water column and explain the information recorded in sedimentary rocks of the Precambrian ocean [104]. More empirical and experimental research is needed to quantify controlling factors of fractionation that occur with iron oxide crystallization in hydrothermal mineral systems [303]. Future work that reconstructs Archean seawater iron and Si concentrations will be crucial in evaluating the extent to which ferrous hydroxide auto-oxidation controlled the Archean iron cycle and the oxidation of the young Earth [103]. The crystal structure of iron oxides synthesized under high pressures, their bonding nature and build-up structural motifs may guide us in discovering novel iron oxide phases and will be useful in revealing the chemistry and physics of Earth and planetary deep interiors [24].

Despite the long history of iron oxide research, they continue to attract the high attention of scientists all over the world; therefore we can suppose the future fundamental and applied discoveries in this field. The main possible tendencies, which can be predicted from the current state of the research, include the further integration of various scientific analytical approaches, e.g., well-developed in geosciences and nanotechnologies [37,38,294,438], a wider implementation of “green” and biomimetic technologies [439] and a combined use of natural iron oxides and synthetic components in a single structure [6].