IRON OXIDE: STRUCTURE, PROPERTIES, NOMENCLATURE, USES - CHEMISTRY - 2025

An iron oxide is any of the compounds formed between iron and oxygen. They are characterized by being ionic and crystalline, and they lie scattered product of the erosion of their minerals, composing the soils, the vegetal mass and, even, the interior of living organisms.

It is then one of the families of compounds that predominate in the earth's crust. What exactly are they? Sixteen iron oxides are known to date, most of them of natural origin and others synthesized under extreme conditions of pressure or temperature.

Source: five seventh, Flickr.

A portion of powdered ferric oxide is shown in the image above. Its characteristic red color covers the iron of various architectural elements in what is known as rust. Likewise, it is observed in the slopes, mountains or soils, mixed with many other minerals, such as the yellow powder of goethite (α-FeOOH).

The best known iron oxides are hematite (α-Fe ₂ O ₃) and maghemite (ϒ- Fe ₂ O ₃), both polymorphs of ferric oxide; and not least, magnetite (Fe ₃ O ₄). Their polymorphic structures and their large surface area make them interesting materials as sorbents, or for the synthesis of nanoparticles with wide applications.

Structure

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The top image is a representation of the crystal structure of FeO, one of the iron oxides where iron has a valence of +2. The red spheres correspond to the O ^2- anions, while the yellow ones to the Fe ²⁺ cations. Note also that each Fe ²⁺ is surrounded by six O ^2-, forming an octahedral unit of coordination.

Therefore, the structure of FeO can be "broken down" into units of FeO ₆, where the central atom is Fe ²⁺. In the case of oxyhydroxides or hydroxides, the octahedral unit is FeO ₃ (OH) ₃.

In some structures, instead of the octahedron, there are tetrahedral units, FeO ₄. For this reason the structures of iron oxides are usually represented by octahedra or tetrahedra with iron centers.

The structures of iron oxides depend on the conditions of pressure or temperature, on the Fe / O ratio (that is, how many oxygens there are per iron and vice versa), and on the valence of iron (+2, +3 and, very rarely in synthetic oxides, +4).

In general, the bulky O ^2- anions line up to form sheets whose voids house the Fe ²⁺ or Fe ³⁺ cations. Thus, there are oxides (such as magnetite) that have irons with both valences.

Polymorphism

Iron oxides present polymorphism, that is, different structures or crystal arrangements for the same compound. Ferric oxide, Fe ₂ O ₃, has up to four possible polymorphs. Hematite, α-Fe ₂ O ₃, is the most stable of all; followed by maghemite, ϒ- Fe ₂ O ₃, and by synthetic β- Fe ₂ O ₃ and ε- Fe ₂ O ₃.

They all have their own types of crystal structures and systems. However, the 2: 3 ratio remains constant, so there are three O ^2- anions for every two Fe ³⁺ cations. The difference lies in how the FeO ₆ octahedral units are located in space and how they are attached.

Structural links

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The octahedral units FeO ₆ can be visualized with the help of the image above. In the corners of the octahedron are the O ^2-, while in its center the Fe ²⁺ or Fe ³⁺ (in the case of Fe ₂ O ₃). The way these octahedra are arranged in space reveals the structure of the oxide.

However, they also influence how they are linked. For example, two octahedra can be joined by touching two of their vertices, which is represented by an oxygen bridge: Fe-O-Fe. Similarly, octahedra can be joined through their edges (adjacent to each other). It would then be represented with two oxygen bridges: Fe- (O) ₂ -Fe.

And finally, octahedra can interact through their faces. Thus, the representation would now be with three oxygen bridges: Fe- (O) ₃ -Fe. The way in which the octahedra are linked would vary the Fe-Fe internuclear distances and, therefore, the physical properties of the oxide.

Properties

An iron oxide is a compound with magnetic properties. These can be anti, ferro or ferrimagnetic, and depend on the valences of Fe and how the cations interact in the solid.

Because the structures of solids are very varied, so are their physical and chemical properties.

For example, the polymorphs and hydrates of Fe ₂ O ₃ have different values ​​of melting points (which range between 1200 and 1600ºC) and densities. However, they have in common the low solubility due to Fe ³⁺, the same molecular mass, are brown in color and dissolve poorly in acid solutions.

Nomenclature

The IUPAC establishes three ways to name an iron oxide. All three are very useful, although for complex oxides (such as Fe ₇ O ₉) the systematics rules over the others due to their simplicity.

Systematic nomenclature

The numbers of oxygen and iron are taken into account, naming them with the Greek numbering prefixes mono-, di-, tri-, etc. According to this nomenclature, Fe ₂ O ₃ is called: tri oxide of di iron. And for Fe ₇ O ₉ its name would be: hepta-iron nonaoxide.

Stock nomenclature

This considers the valence of iron. If it is Fe ²⁺, it is written iron oxide…, and its valence with Roman numerals enclosed in parentheses. For Fe ₂ O ₃ its name is: iron oxide (III).

Note that Fe ³⁺ can be determined by algebraic sums. If the O ^2- has two negative charges, and there are three of them, they add up to -6. To neutralize this -6, +6 is required, but there are two Fe, so they must be divided by two, + 6/2 = +3:

2X (metal valence) + 3 (-2) = 0

Simply solving for X, the valence of Fe in the oxide is obtained. But if X is not an integer (as is the case with almost all other oxides), then there is a mixture of Fe ²⁺ and Fe ³⁺.

Traditional nomenclature

The suffix –ico is given to the prefix ferr- when Fe has valence +3, and –oso when its valence is 2+. Thus, Fe ₂ O ₃ is called: ferric oxide.

Applications

Nanoparticles

Iron oxides have a high crystallization energy in common, which allows the creation of very small crystals but with a large surface area.

For this reason, they are of great interest in the fields of nanotechnology, where they design and synthesize oxide nanoparticles (NPs) for specific purposes:

-As catalysts.

-As a reservoir of drugs or genes within the body

-In the design of sensory surfaces for different types of biomolecules: proteins, sugars, fats

-To store magnetic data

Pigments

Because some oxides are very stable, they can be used to dye textiles or give bright colors to the surfaces of any material. From the mosaics on the floors; red, yellow and orange (even green) paints; ceramics, plastics, leather, and even architectural works.

References

1. Trustees of Dartmouth College. (March 18, 2004). Stoichiometry of Iron Oxides. Taken from: dartmouth.edu
2. Ryosuke Sinmyo et al. (September 8, 2016). Discovery of Fe ₇ O ₉: a new iron oxide with a complex monoclinic structure. Recovered from: nature.com
3. M. Cornell, U. Schwertmann. The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses.. WILEY-VCH. Taken from: epsc511.wustl.edu
4. Alice Bu. (2018). Iron Oxide Nanoparticles, Characteristics and Applications. Taken from: sigmaaldrich.com
5. Ali, A., Zafar, H., Zia, M., ul Haq, I., Phull, AR, Ali, JS, & Hussain, A. (2016). Synthesis, characterization, applications, and challenges of iron oxide nanoparticles. Nanotechnology, Science and Applications, 9, 49–67.
6. Golchha Pigments. (2009). Iron Oxides: Applications. Taken from: golchhapigments.com
7. Chemical formulation. (2018). Iron (II) oxide. Taken from: formulacionquimica.com
8. Wikipedia. (2018). Iron (III) oxide. Taken from: