Iron(III) oxide—also known as
ferric oxide,
ferric iron,
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.
Different forms
Alpha phase
α-Fe
2O
3 has the rhombohedral,
corundum (α-Al
2O
3) structure and is the most common form. It occurs naturally as the mineral
hematite which is mined as the main
ore of iron. It is
antiferromagnetic below ~260 K (Morin transition temperature), and weak
ferromagnetic between 260 K and 950 K (Neel temperature). 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 and 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.
Uses
Magnetic storage
The magnetic iron(III) oxides are 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.
Magnetic ink character recognition (MICR) 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.
Pigment
Iron(III) oxide is also used as a
pigment, under names
Pigment Brown 6,
Pigment Brown 7, and
Pigment Red 101. Some of them, e.g. 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 some 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.
See also
General references
N. N. Greenwood, A. Earnshaw,
Chemistry of the Elements, Pergamon Press, 1984.
