Editing Iron (section)

===Allotropes===
{{Main|Allotropes of iron}}
[[File:Iron-alpha-pV.svg|thumb|left|upright=1.15|Molar volume vs. pressure for α iron at room temperature]]

At least four allotropes of iron (differing atom arrangements in the solid) are known, conventionally denoted [[α]], [[Gamma|γ]], [[Delta (letter)|δ]], and [[Epsilon|ε]].

The first three forms are observed at ordinary pressures. As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a [[body-centered cubic]] (bcc) [[crystal structure]]. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a [[face-centered cubic]] (fcc) crystal structure, or [[austenite]]. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}}

The physical properties of iron at very high pressures and temperatures have also been studied extensively,<ref name="phase-dia-iron-eicore">{{Cite journal|vauthors=Tateno S, Hirose K |title=The Structure of Iron in Earth's Inner Core| journal=Science| volume=330| pages=359–361| publisher=American Association for the Advancement of Science| date=2010| doi=10.1126/science.1194662| issue=6002| pmid=20947762| bibcode=2010Sci...330..359T| s2cid=206528628}}</ref><ref name="fe-innercore-stability">{{Cite journal| first=Gaminchev| last=Chamati| title=Dynamic stability of Fe under high pressure| journal=Journal of Physics| volume=558| pages=012013| publisher=IOP Publishing| date=2014| doi=10.1088/1742-6596/558/1/012013| issue=1| bibcode=2014JPhCS.558a2013G| doi-access=free}}</ref> because of their relevance to theories about the cores of the Earth and other planets. Above approximately 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another [[hexagonal close-packed]] (hcp) structure, which is also known as [[hexaferrum|ε-iron]]. The higher-temperature γ-phase also changes into ε-iron,<ref name="fe-innercore-stability"/> but does so at higher pressure.

Some controversial experimental evidence exists for a stable [[Beta|β]] phase at pressures above 50 GPa and temperatures of at least 1500 K. It is supposed to have an [[orthorhombic]] or a double hcp structure.<ref name="beta-iron">{{Cite journal| first=Reinhard| last=Boehler| title=High-pressure experiments and the phase diagram of lower mantle and core materials| journal =Reviews of Geophysics| volume=38| pages=221–45| publisher=American Geophysical Union| date=2000| doi=10.1029/1998RG000053| issue=2| bibcode=2000RvGeo..38..221B| s2cid=33458168| doi-access=free}}</ref> (Confusingly, the term "β-iron" is sometimes also used to refer to α-iron above its Curie point, when it changes from being ferromagnetic to paramagnetic, even though its crystal structure has not changed.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}})

The [[Earth's inner core]] is generally presumed to consist of an iron-[[nickel]] [[alloy]] with ε (or β) structure.<ref>{{Cite journal |last1=Stixrude |first1=Lars |last2=Wasserman |first2=Evgeny |last3=Cohen |first3=Ronald E. |date=1997-11-10 |title=Composition and temperature of Earth's inner core |journal=Journal of Geophysical Research: Solid Earth |volume=102 |issue=B11 |pages=24729–39 |doi=10.1029/97JB02125 |bibcode=1997JGR...10224729S |doi-access=free}}</ref>