Editing Iron(III) oxide (section)

==Structure==
{{chem2|Fe2O3}} can be obtained in various [[polymorphism (materials science)|polymorph]]s. In the primary polymorph, α, iron adopts octahedral coordination geometry. That is, each Fe center is bound to six oxygen [[ligand]]s. In the γ polymorph, some of the Fe sit on tetrahedral sites, with four oxygen ligands.

===Alpha phase===
α-{{chem2|Fe2O3}} has the [[rhombohedral lattice system|rhombohedral]], [[aluminium oxide|corundum]] (α-Al<sub>2</sub>O<sub>3</sub>) 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.<ref>{{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 }}</ref> 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.

===Gamma phase===
[[maghemite|γ-Fe<sub>2</sub>O<sub>3</sub>]] has a [[cubic crystal system|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,<ref name="InorgChem"/> 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]] (Fe<sub>3</sub>O<sub>4</sub>).<ref name="InorgChem"/> The ultrafine particles can be prepared by thermal decomposition of [[iron(III) oxalate]].

===Other solid phases===
Several other phases have been identified or claimed. The beta phase (β-phase) is cubic body-centered (space group Ia3), [[Metastability|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]].<ref>{{Cite web|title=Mechanism of Oxidation & Thermal Decomposition of Iron Sulphides|url=https://core.ac.uk/download/pdf/298011553.pdf}}</ref>

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 [[magnetic storage|recording media]] for [[big data]] storage.<ref>{{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 |url-access=subscription }}</ref> 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 ware|Jian ceramic]] glazes, which may provide insight into ways to produce that form in the lab.<ref>{{cite journal|doi=10.1038/srep04941 |pmid=24820819 |pmc=4018809 |title=Learning from the past: Rare ε-Fe<sub>2</sub>O<sub>3</sub> 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 }}</ref>{{Primary source inline|date=October 2020}}

Additionally, at high pressure an [[amorphous]] form is claimed.<ref name="atmilab">{{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}}</ref>{{Primary source inline|date=October 2020}}

===Liquid phase===
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.<ref name="ShiFeOx2020">{{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 }}</ref>