Iron(III) oxide#Polishing

{{chem2|Fe2O3}} can be obtained in various polymorphs. In the primary polymorph, α, iron adopts octahedral coordination geometry. That is, each Fe center is bound to six oxygen ligands. In the γ polymorph, some of the Fe sit on tetrahedral sites, with four oxygen ligands.

α-{{chem2|Fe2O3}} has the rhombohedral, corundum (α-Al₂O₃) 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 exhibits weak ferromagnetism between 260 K and the Néel temperature, 950 K.{{cite book |first=J. E. |last=Greedan |year=1994 |chapter=Magnetic oxides |title=Encyclopedia of Inorganic chemistry |editor-first=R. Bruce |editor-last=King |publisher=John Wiley & Sons |location=New York |isbn=978-0-471-93620-6 }} 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.

γ-Fe₂O₃ has a cubic structure. It is metastable and converted from the alpha phase at high temperatures. It occurs naturally as the mineral maghemite. It is ferromagnetic and finds application in recording tapes, although ultrafine particles smaller than 10 nanometers are superparamagnetic. It can be prepared by thermal dehydratation of gamma iron(III) oxide-hydroxide. Another method involves the careful oxidation of iron(II,III) oxide (Fe₃O₄). The ultrafine particles can be prepared by thermal decomposition of iron(III) oxalate.

Several other phases have been identified or claimed. The beta phase (β-phase) is cubic body-centered (space group Ia3), metastable, and at temperatures above {{convert|500|°C|°F|-1|abbr=on|lk=off}} converts to alpha phase. It can be prepared by reduction of hematite by carbon,{{Clarify|reason=reduction implies Fe(II) but it should still be Fe(III)|date=October 2020}} pyrolysis of iron(III) chloride solution, or thermal decomposition of iron(III) sulfate.{{Cite web|title=Mechanism of Oxidation & Thermal Decomposition of Iron Sulphides|url=https://core.ac.uk/download/pdf/298011553.pdf}}

The epsilon (ε) phase is rhombic, and shows properties intermediate between alpha and gamma, and may have useful magnetic properties applicable for purposes such as high density recording media for big data storage.{{cite journal |title=Advances in magnetic films of epsilon-iron oxide toward next-generation high-density recording media |url=https://pubs.rsc.org/en/content/articlelanding/2021/dt/d0dt03460f |journal=Dalton Transactions |year=2021 |publisher=Royal Society of Chemistry |doi=10.1039/D0DT03460F |access-date=25 January 2021|last1=Tokoro |first1=Hiroko |last2=Namai |first2=Asuka |last3=Ohkoshi |first3=Shin-Ichi |volume=50 |issue=2 |pages=452–459 |pmid=33393552 |s2cid=230482821 }} Preparation of the pure epsilon phase has proven very challenging. Material with a high proportion of epsilon phase can be prepared by thermal transformation of the gamma phase. The epsilon phase is also metastable, transforming to the alpha phase at between {{convert|500|and|750|°C|°F|-1|abbr=on|lk=off}}. It can also be prepared by oxidation of iron in an electric arc or by sol-gel precipitation from iron(III) nitrate.{{Citation needed|date=July 2011}} Research has revealed epsilon iron(III) oxide in ancient Chinese Jian ceramic glazes, which may provide insight into ways to produce that form in the lab.{{cite journal|doi=10.1038/srep04941 |pmid=24820819 |pmc=4018809 |title=Learning from the past: Rare ε-Fe₂O₃ in the ancient black-glazed Jian (Tenmoku) wares |journal=Scientific Reports |volume=4 |pages=4941 |year=2015 |last1=Dejoie |first1=Catherine |last2=Sciau |first2=Philippe |last3=Li |first3=Weidong |last4=Noé |first4=Laure |last5=Mehta |first5=Apurva |last6=Chen |first6=Kai |last7=Luo |first7=Hongjie |last8=Kunz |first8=Martin |last9=Tamura |first9=Nobumichi |last10=Liu |first10=Zhi }}{{Primary source inline|date=October 2020}}

Additionally, at high pressure an amorphous form is claimed.{{cite web|url = http://atmilab.upol.cz/texty/ultrafine02.pdf|access-date = 2014-07-12|title = Ultrafine Particles of Iron(III) Oxides by View of AFM – Novel Route for Study of Polymorphism in Nano-world|first1 = Milan|last1 = Vujtek|first2 = Radek|last2 = Zboril|first3 = Roman|last3 = Kubinek|first4 = Miroslav|last4 = Mashlan|website = Univerzity Palackého}}{{Primary source inline|date=October 2020}}

Molten {{chem2|Fe2O3}} is expected to have a coordination number of close to 5 oxygen atoms about each iron atom, based on measurements of slightly oxygen deficient supercooled liquid iron oxide droplets, where supercooling circumvents the need for the high oxygen pressures required above the melting point to maintain stoichiometry.{{cite journal |last1=Shi |first1=Caijuan |last2=Alderman |first2=Oliver |last3=Tamalonis |first3=Anthony |last4=Weber |first4=Richard |last5=You |first5=Jinglin |last6=Benmore |first6=Chris |title=Redox-structure dependence of molten iron oxides |journal=Communications Materials |date=2020 |volume=1 |issue=1 |page=80 |doi=10.1038/s43246-020-00080-4 |bibcode=2020CoMat...1...80S |doi-access=free }}

Several hydrates of Iron(III) oxide exist.

When alkali is added to solutions of soluble Fe(III) salts, a red-brown gelatinous precipitate forms. This is not {{chem2|Fe(OH)3}}, but {{chem2|Fe2O3*H2O}} (also written as {{chem2|Fe(O)OH}}).

Several forms of the hydrated oxide of Fe(III) exist as well. The red lepidocrocite (γ-{{chem2|Fe(O)OH}}) occurs on the outside of rusticles, and the orange goethite (α-{{chem2|Fe(O)OH}}) occurs internally in rusticles.

When {{chem2|Fe2O3}}·H₂O is heated, it loses its water of hydration. Further heating at {{val|1670|u=K}} converts {{chem2|Fe2O3}} to black {{chem2|Fe3O4}} ({{chem2|Fe^{II}Fe^{III}2O4}}), which is known as the mineral magnetite.

{{chem2|Fe(O)OH}} is soluble in acids, giving {{chem2|[Fe(H2O)6](3+)}}. In concentrated aqueous alkali, {{chem2|Fe2O3}} gives {{chem2|[Fe(OH)6](3-)}}.{{cite book|title = Inorganic Chemistry|url = https://archive.org/details/inorganicchemist00hous_159|url-access = limited|edition = 3rd|chapter = Chapter 22: d-block metal chemistry: the first row elements|first1 = Catherine E.|last1 = Housecroft|first2 = Alan G.|last2 = Sharpe|publisher = Pearson|year = 2008|isbn = 978-0-13-175553-6|page = [https://archive.org/details/inorganicchemist00hous_159/page/n754 716]}}

The most important reaction is its carbothermal reduction, which gives iron used in steel-making:

:{{chem2|Fe2O3 + 3 CO -> 2 Fe + 3 CO2}}

Another redox reaction is the extremely exothermic thermite reaction with aluminium.{{cite book |last1=Adlam |last2=Price |title=Higher School Certificate Inorganic Chemistry |publisher=Leslie Slater Price |year=1945 }}

:{{chem2|2 Al + Fe2O3 -> 2 Fe + Al2O3}}

This process is used to weld thick metals such as rails of train tracks by using a ceramic container to funnel the molten iron in between two sections of rail. Thermite is also used in weapons and making small-scale cast-iron sculptures and tools.

Partial reduction with hydrogen at about {{val|400|u=degC}} produces magnetite, a black magnetic material that contains both Fe(III) and Fe(II):

:{{chem2|Fe2O3 + H2 -> 2 Fe3O4 + H2O}}

Iron(III) oxide is insoluble in water but dissolves readily in strong acid, e.g., hydrochloric and sulfuric acids. It also dissolves well in solutions of chelating agents such as EDTA and oxalic acid.

Heating iron(III) oxides with other metal oxides or carbonates yields materials known as ferrates (ferrate (III)):

:{{chem2|ZnO + Fe2O3 -> Zn(FeO2)2}}

Iron(III) oxide is a product of the oxidation of iron. It can be prepared in the laboratory by electrolyzing a solution of sodium bicarbonate, an inert electrolyte, with an iron anode:

:{{chem2|4 Fe + 3 O2 + 2 H2O -> 4 FeO(OH)}}

The resulting hydrated iron(III) oxide, written here as {{chem2|FeO(OH)}}, dehydrates around {{val|200|u=degC}}.Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1661.

:{{chem2|2 FeO(OH) -> Fe2O3 + H2O}}

The overwhelming application of iron(III) oxide is as the feedstock of the steel and iron industries, e.g., the production of iron, steel, and many alloys.{{cite book |last1=Greenwood |first1=N. N. |last2=Earnshaw |first2=A. |year=1997 |title=Chemistry of the Element |edition=2nd |location=Oxford |publisher=Butterworth-Heinemann |isbn=978-0-7506-3365-9 }} Iron oxide (Fe2O3) has been used in stained glass since the medieval period, with evidence suggesting its use in stained glass production dating back to the early Middle Ages, where it was primarily used to create yellow, orange, and red colors in the glass, still being used for industrial purposes today.{{Cite web |last=Golchha |first=Vipul |title=About Iron Oxide Pigments |url=https://www.golchhaoxides.com/iron-oxide-pigments.php#:~:text=Iron%20Oxides%20have%20been%20used,drawings%20to%20ensure%20bountiful%20hunting. |access-date=2024-10-29 |website=Golchha Oxides Pvt Ltd |language=en}}{{Cite web |title=Iron(III) Oxide - Structure, Properties, Uses of Fe2O3 |url=https://byjus.com/chemistry/fe2o3/#:~:text=Ferric%20oxide%20(Fe2O,glass,%20diamonds%20and%20precious%20metals. |access-date=2024-10-29 |website=BYJUS |language=en}}

A very fine powder of ferric oxide is known as "jeweler's rouge", "red rouge", or simply rouge. It is used to put the final polish on metallic jewelry 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. Products sold as "stropping compound" are often applied to a leather strop to assist in getting a razor edge on knives, straight razors, or any other edged tool.

{{multiple image|align = left

| footer = Sample of the red α- and yellow β-phases of hydrated of iron(III) oxide; both are useful as pigments.

| width1 = 220|image1 = Iron oxide red y.jpg

| width2 = 178|image2 = Iron oxide yellow.jpg

}}

Iron(III) oxide is also used as a pigment, under names "Pigment Brown 6", "Pigment Brown 7", and "Pigment Red 101".{{cite book |title=Paint and Surface Coatings: Theory and Practice |publisher=William Andrew Inc. |isbn=978-1-884207-73-0 |year= 1999}} Some of them, e.g., Pigment Red 101 and Pigment Brown 6, are approved by the US Food and Drug Administration (FDA) for use in cosmetics. Iron oxides are used as pigments in dental composites alongside titanium oxides.{{cite book|last=Banerjee|first=Avijit|title=Pickard's Manual of Operative Dentistry|year=2011|publisher=Oxford University Press Inc., New York|location=United States|isbn=978-0-19-957915-0|pages=89}}

Hematite is the characteristic component of the Swedish paint color Falu red.

Iron(III) oxide was the most common magnetic particle used in all types of magnetic storage and recording media, including magnetic disks (for data storage) and magnetic tape (used in audio and video recording as well as data storage). Its use in computer disks was superseded by cobalt alloy, enabling thinner magnetic films with higher storage density.{{cite journal|doi=10.1063/1.2750414|title=Perpendicular recording media for hard disk drives|journal=Journal of Applied Physics|volume=102|issue=1|pages=011301–011301–22|year=2007|last1=Piramanayagam|first1=S. N.|bibcode=2007JAP...102a1301P}}

α-{{chem2|Fe2O3}} has been studied as a photoanode for solar water oxidation.{{cite journal|author=Kay, A. |author2=Cesar, I. |author3=Grätzel, M.|title=New Benchmark for Water Photooxidation by Nanostructured α-Fe₂O₃ Films |journal=Journal of the American Chemical Society |volume=128 |issue=49 |pages=15714–15721 |doi=10.1021/ja064380l |pmid=17147381 |year=2006 }} However, its efficacy is limited by a short diffusion length (2–4 nm) of photo-excited charge carriers{{cite journal|author=Kennedy, J.H.|author2=Frese, K.W.|title=Photooxidation of Water at α-Fe₂O₃ Electrodes|journal=Journal of the Electrochemical Society|volume=125|issue=5|pages=709|doi=10.1149/1.2131532|year=1978|bibcode=1978JElS..125..709K }} and subsequent fast recombination, requiring a large overpotential to drive the reaction.{{cite journal|author=Le Formal, F. |title=Back Electron–Hole Recombination in Hematite Photoanodes for Water Splitting |journal=Journal of the American Chemical Society |volume=136 |issue=6 |pages=2564–2574 |doi=10.1021/ja412058x |pmid=24437340 |year=2014 |doi-access=free }} Research has been focused on improving the water oxidation performance of {{chem2|Fe2O3}} using nanostructuring, surface functionalization,{{cite journal|author=Zhong, D.K. |author2=Gamelin, D.R.|title=Photoelectrochemical Water Oxidation by Cobalt Catalyst ("Co−Pi")/α-Fe₂O₃ Composite Photoanodes: Oxygen Evolution and Resolution of a Kinetic Bottleneck |journal=Journal of the American Chemical Society |volume=132 |issue=12 |pages=4202–4207 |doi=10.1021/ja908730h |pmid=20201513 |year=2010 }} or by employing alternate crystal phases such as β-{{chem2|Fe2O3}}.{{cite journal|author=Emery, J.D. |title=Atomic Layer Deposition of Metastable β-Fe₂O₃ via Isomorphic Epitaxy for Photoassisted Water Oxidation|journal=ACS Applied Materials & Interfaces|volume=6|issue=24|pages=21894–21900|doi=10.1021/am507065y|pmid=25490778|year=2014|osti=1355777}}

Calamine lotion, used to treat mild itchiness, is chiefly composed of a combination of zinc oxide, acting as astringent, and about 0.5% iron(III) oxide, the product's active ingredient, acting as antipruritic. The red color of iron(III) oxide is also mainly responsible for the lotion's pink color.

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{{reflist}}

• {{cite book | ref=Haynes | editor= Haynes, William M. | year = 2011 | title = CRC Handbook of Chemistry and Physics | edition = 92nd | publisher = CRC Press | isbn = 978-1439855119| title-link = CRC Handbook of Chemistry and Physics }}

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• [https://www.cdc.gov/niosh/npg/npgd0344.html NIOSH Pocket Guide to Chemical Hazards]

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