Iron(III) oxide

Iron(III) oxide — also known as ferric oxide, Hematite, red iron oxide, synthetic maghemite, colcothar, or simply rust — is one of the several oxide compounds of iron, and has paramagnetic properties. Its chemical formula is Fe2O3.

Alpha phase
α-Fe2O3 has the rhombohedral, corundum (α-Al2O3) structure and is the most common form. It occurs naturally as the mineral hematite. Is mined as the main ore of iron. It is antiferromagnetic up to its critical temperature of 950 K. It is easy to prepare using both thermal decomposition and precipitation in the liquid phase. Its magnetic properties are dependent on many factors, e.g. pressure, particle size, and magnetic field intensity.

Beta phase
Cubic face centered, metastable, at temperatures above 500 °C converts to alpha phase. It can be prepared by reduction of hematite by carbon, pyrolysis of iron(III) chloride solution, or thermal decomposition of iron(III) sulfate.

Gamma phase
Cubic, metastable, converts to the alpha phase at high temperatures. Occurs naturally as the mineral maghemite. Ferrimagnetic. Ultrafine particles smaller than 10 nanometers are superparamagnetic. Can be prepared by thermal dehydratation of gamma iron(III) oxide-hydroxide, careful oxidation of iron(II,III) oxide. The ultrafine particles can be prepared by thermal decomposition of iron(III) oxalate.

Epsilon phase
Rhombic, shows properties intermediate between alpha and gamma. So far has not been prepared in pure form; it is always mixed with the alpha phase or gamma phases. Material with a high proportion of epsilon phase can be prepared by thermal transformation of the gamma phase. The epsilon phase is metastable, transforming to the alpha phase at between 500–750 °C. Can also be prepared by oxidation of iron in an electric arc or by sol-gel precipitation from iron(III) nitrate.

Other phases
High pressure, amorphous

Magnetic storage
Iron(III) oxide is often used in magnetic storage, for example in the magnetic layer of floppy disks. These consist of a thin sheet of PET film, coated with iron(III) oxide. The particles can be magnetised to represent binary data. MICR (Magnetic Ink Character Recognition) also uses iron(III) oxide compounds, suspended in an ink which can be read by special scanning hardware.

The majority of recorded information on earth (such as text and photographs) is stored in the form of magnetization patterns on a thin layer of iron(III) oxide. This is probably because the cost per bit of iron-based magnetic media is currently far less than the cost per bit of any known alternative, such as optical discs, paper books, or microfilm. More text and photos are stored on magnetic media than all the paper books and paper photographs in the world.

Polishing
A very fine powder of ferric oxide is known as jeweller's rouge, red rouge, or simply rouge. It is used to put the final polish on metallic jewellery and lenses, and historically as a cosmetic.

Rouge cuts more slowly than some modern polishes, such as cerium(IV) oxide, but is still used in optics fabrication and by jewelers for the superior finish it can produce. When polishing gold, the rouge slightly stains the gold, which contributes to the appearance of the finished piece. Rouge is sold as a powder, paste, laced on polishing cloths, or solid bar (with a wax or grease binder). Other polishing compounds are also often called "rouge", even when they do not contain iron oxide. Jewelers remove the residual rouge on jewelry by use of ultrasonic cleaning.

Chemical
Iron(III) oxide is used in the production of pure iron in a blast furnace. Iron(III) oxide is also used in an extremely exothermic reaction called a thermite reaction.

2 Al + Fe2O3 → 2 Fe + Al2O3

Pigment
Iron(III) oxide is also used as a pigment, under names Pigment Brown 6, Pigment Brown 7, and Pigment Red 101Paint and Surface Coatings: Theory and Practice William Andrew Inc. ISBN 1884207731 Some of them, eg. Pigment Red 101 and Pigment Brown 6, are Food and Drug Administration (FDA)-approved for use in cosmetics.

Biomedical
Nanoparticles of iron(III) oxide are biocompatible, non-toxic, are chemically active on their surface, and are magnetic . They find wide use in biomedical applications. Can be used as contrast agents in magnetic resonance imaging, in labeling of cancerous tissues, magnetically controlled transport of pharmaceuticals, localized thermotherapy (where the tissue is labeled by iron oxide nanoparticles, then heated by application of AC field to particles), and preparation of ferrofluids. 

General references
N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, 1984.