Editing Iron (section)

==Characteristics==
===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>

===Melting and boiling points===
[[File:Pure iron phase diagram (EN).svg|thumb|left|upright=1.15|Low-pressure [[phase diagram]] of pure iron]]
The melting and boiling points of iron, along with its [[enthalpy of atomization]], are lower than those of the earlier 3d elements from [[scandium]] to [[chromium]], showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus;{{sfn|Greenwood|Earnshaw|1997|p=1116}} however, they are higher than the values for the previous element [[manganese]] because that element has a half-filled 3d sub-shell and consequently its d-electrons are not easily delocalized. This same trend appears for [[ruthenium]] but not [[osmium]].{{sfn|Greenwood|Earnshaw|1997|pp=1074–75}}

The melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data (as of 2007) still varies by tens of gigapascals and over a thousand kelvin.<ref name="melting">{{Cite book| pages=527–41 |doi=10.1016/B978-044452748-6.00047-X|title =Mineral Physics|first1 = Reinhard|last1 = Boehler|first2= M.|last2 = Ross|chapter = Properties of Rocks and Minerals_High-Pressure Melting|publisher = Elsevier| date = 2007| series = Treatise on Geophysics| volume = 2|isbn=9780444527486}}</ref>

===Magnetic properties===
[[File:Magnetization curves.svg|thumb|upright=1.15|left|Magnetization curves of 9 ferromagnetic materials, showing saturation. 1.{{nbsp}}Sheet steel, 2.{{nbsp}}Silicon steel, 3.{{nbsp}}Cast steel, 4.{{nbsp}}Tungsten steel, 5.{{nbsp}}Magnet steel, 6.{{nbsp}}Cast iron, 7.{{nbsp}}Nickel, 8.{{nbsp}}Cobalt, 9.{{nbsp}}Magnetite<ref>{{cite book|first=Charles |last=Steinmetz |year=1917 |title=Theory and Calculation of Electric Circuits |url=https://archive.org/details/in.ernet.dli.2015.162619 |publisher=McGraw-Hill|section=fig. 42}}</ref>]]
Below its [[Curie point]] of {{Convert|770|C|F K|abbr=on}}, α-iron changes from [[paramagnetic]] to [[ferromagnetic]]: the [[Spin (physics)|spins]] of the two unpaired electrons in each atom generally align with the spins of its neighbors, creating an overall [[magnetic field]].<ref name="cullity">{{cite book |last=Cullity |author2=C. D. Graham |title=Introduction to Magnetic Materials, 2nd|publisher=Wiley–IEEE|year=2008 |location=New York |page=116 |url=https://books.google.com/books?id=ixAe4qIGEmwC&pg=PA116 |isbn=978-0-471-47741-9}}</ref> This happens because the orbitals of those two electrons (d<sub>''z''<sup>2</sup></sub> and d<sub>''x''<sup>2</sup> − ''y''<sup>2</sup></sub>) do not point toward neighboring atoms in the lattice, and therefore are not involved in metallic bonding.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}}

In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into [[magnetic domain]]s, about 10 micrometers across,<ref name="Metallo">{{Cite book| chapter-url={{Google books|hoM8VJHTt24C|page=PA24|keywords=|text=|plainurl=yes}}|pages=24–28|title =Metallographer's guide: practice and procedures for irons and steels|first1 = B.L.|last1 = Bramfitt|first2= Arlan O.|last2 = Benscoter|chapter = The Iron Carbon Phase Diagram|publisher = ASM International| date = 2002| isbn = 978-0-87170-748-2}}</ref><!--https://books.google.com/books?id=brpx-LtdCLYC--> such that the atoms in each domain have parallel spins, but some domains have other orientations. Thus a macroscopic piece of iron will have a nearly zero overall magnetic field.

Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field. This effect is exploited in devices that need to channel magnetic fields to fulfill design function, such as [[electrical transformer]]s, [[magnetic recording]] heads, and [[electric motor]]s. Impurities, [[lattice defect]]s, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists even after the external field is removed – thus turning the iron object into a (permanent) [[magnet]].<ref name="cullity" />

Similar behavior is exhibited by some iron compounds, such as the [[Ferrite (magnet)|ferrites]] including the mineral [[magnetite]], a crystalline form of the mixed iron(II,III) oxide {{chem2|Fe3O4}} (although the atomic-scale mechanism, [[ferrimagnetism]], is somewhat different). Pieces of magnetite with natural permanent magnetization ([[lodestone]]s) provided the earliest [[compass]]es for navigation. Particles of magnetite were extensively used in magnetic recording media such as [[computer memory|core memories]], [[magnetic tape]]s, [[floppy disk|floppies]], and [[hard disk drive|disk]]s, until they were replaced by [[cobalt]]-based materials.

===Isotopes===
{{Main|Isotopes of iron}}
Iron has four stable [[isotope]]s: <sup>54</sup>Fe (5.845% of natural iron), [[iron-56|<sup>56</sup>Fe]] (91.754%), <sup>57</sup>Fe (2.119%) and <sup>58</sup>Fe (0.282%). Twenty-four artificial isotopes have also been created. Of these stable isotopes, only <sup>57</sup>Fe has a [[nuclear spin]] (−{{frac|1|2}}). The [[nuclide]] <sup>54</sup>Fe theoretically can undergo [[double beta decay|double electron capture]] to <sup>54</sup>Cr, but the process has never been observed and only a lower limit on the half-life of 4.4×10<sup>20</sup> years has been established.<ref>{{cite journal | last1=Bikit | first1=I. | last2=Krmar | first2=M. | last3=Slivka | first3=J. | last4=Vesković | first4=M. | last5=Čonkić | first5=Lj. | last6=Aničin | first6=I. | title=New results on the double β decay of iron | journal=Physical Review C | volume=58 | issue=4 | date=1998-10-01 | issn=0556-2813 | doi=10.1103/PhysRevC.58.2566 | pages=2566–2567| bibcode=1998PhRvC..58.2566B }}</ref>

<sup>60</sup>Fe is an [[extinct radionuclide]] of long [[half-life]] (2.6 million years).<ref name="RugelFaestermann2009">{{cite journal |last1=Rugel |first1=G. |last2=Faestermann |first2=T. |last3=Knie |first3=K. |last4=Korschinek |first4=G. |last5=Poutivtsev |first5=M. |last6=Schumann |first6=D. |last7=Kivel |first7=N. |last8=Günther-Leopold |first8=I. |last9=Weinreich |first9=R.|last10=Wohlmuther |first10=M. |title=New Measurement of the <sup>60</sup>Fe Half-Life |journal=Physical Review Letters |volume=103 |issue=7 |page=072502 |date=2009 |doi=10.1103/PhysRevLett.103.072502 |pmid=19792637 |bibcode=2009PhRvL.103g2502R |url=https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A17743/datastream/PDF/view}}</ref> It is not found on Earth, but its ultimate decay product is its granddaughter, the stable nuclide [[nickel-60|<sup>60</sup>Ni]].{{NUBASE2020|ref}} Much of the past work on isotopic composition of iron has focused on the [[nucleosynthesis]] of <sup>60</sup>Fe through studies of [[meteorite]]s and ore formation. In the last decade, advances in [[mass spectrometry]] have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the [[stable isotope]]s of iron. Much of this work is driven by the [[Earth science|Earth]] and [[planetary science]] communities, although applications to biological and industrial systems are emerging.<ref>{{Cite journal|last1=Dauphas|first1=N.|last2=Rouxel|first2=O.|date=2006|title=Mass spectrometry and natural variations of iron isotopes|journal=Mass Spectrometry Reviews |volume=25 |issue=4 |pages=515–50 |doi=10.1002/mas.20078 |url=https://geosci.uchicago.edu/~dauphas/OLwebsite/PDFfiles/Dauphas_Rouxel_MSR06.pdf |pmid=16463281 |bibcode=2006MSRv...25..515D |url-status=dead|archive-url=https://web.archive.org/web/20100610095913/https://geosci.uchicago.edu/~dauphas/OLwebsite/PDFfiles/Dauphas_Rouxel_MSR06.pdf |archive-date=10 June 2010}}</ref>

In phases of the meteorites ''Semarkona'' and ''Chervony Kut,'' a correlation between the concentration of <sup>60</sup>Ni, the [[daughter product|granddaughter]] of <sup>60</sup>Fe, and the abundance of the stable iron isotopes provided evidence for the existence of <sup>60</sup>Fe at the time of [[Formation and evolution of the Solar System|formation of the Solar System]]. Possibly the energy released by the decay of <sup>60</sup>Fe, along with that released by [[aluminium-26|<sup>26</sup>Al]], contributed to the remelting and [[planetary differentiation|differentiation]] of [[asteroid]]s after their formation 4.6 billion years ago. The abundance of <sup>60</sup>Ni present in [[wikt:extraterrestrial|extraterrestrial]] material may bring further insight into the origin and early history of the [[Solar System]].<ref>{{cite journal |title=Evidence for live 60Fe in meteorites |date=2004 |last1=Mostefaoui |first1=S. |last2=Lugmair |first2=G.W. |last3=Hoppe |first3=P. |last4=El Goresy |first4=A. |journal=New Astronomy Reviews |volume=48 |issue=1–4 |pages=155–59 |doi=10.1016/j.newar.2003.11.022 |bibcode=2004NewAR..48..155M}}</ref>

The most abundant iron isotope <sup>56</sup>Fe is of particular interest to nuclear scientists because it represents the most common endpoint of [[nucleosynthesis]].<ref>{{cite journal|last1=Fewell|first1=M. P.|title=The atomic nuclide with the highest mean binding energy |journal=American Journal of Physics|volume=63|issue=7 |page=653|date=1995|doi=10.1119/1.17828 |bibcode=1995AmJPh..63..653F}}</ref> Since <sup>56</sup>Ni (14 [[alpha particle]]s) is easily produced from lighter nuclei in the [[alpha process]] in [[nuclear reaction]]s in supernovae (see [[silicon burning process]]), it is the endpoint of fusion chains inside [[Population III stars|extremely massive stars]]. Although adding more alpha particles is possible, but nonetheless the sequence does effectively end at <sup>56</sup>Ni because conditions in stellar interiors cause the competition between [[photodisintegration]] and the alpha process to favor photodisintegration around <sup>56</sup>Ni.<ref>{{cite journal |last=Fewell |first=M.P. |date=1995-07-01 |title=The atomic nuclide with the highest mean binding energy |journal=American Journal of Physics |volume=63 |issue=7 |pages=653–658 |doi=10.1119/1.17828 |bibcode=1995AmJPh..63..653F |issn=0002-9505}}</ref><ref>{{cite journal |last1=Burbidge |first1=E. Margaret |author-link1=Margaret Burbidge |last2=Burbidge |first2=G.R. |author-link2=Geoffrey Burbidge |last3=Fowler |first3=William A. |author-link3=William Alfred Fowler |last4=Hoyle |first4=F. |author-link4=Fred Hoyle |date=1957-10-01 |title=Synthesis of the elements in stars |journal=Reviews of Modern Physics |volume=29 |issue=4 |pages=547–650 |bibcode=1957RvMP...29..547B |doi=10.1103/RevModPhys.29.547 |doi-access=free}}</ref> This <sup>56</sup>Ni, which has a half-life of about 6 days, is created in quantity in these stars, but soon decays by two successive positron emissions within supernova decay products in the [[supernova remnant]] gas cloud, first to radioactive <sup>56</sup>Co, and then to stable <sup>56</sup>Fe. As such, iron is the most abundant element in the core of [[red giant]]s, and is the most abundant metal in [[iron meteorite]]s and in the dense metal [[Planetary core|cores of planets]] such as [[Earth]].{{sfn|Greenwood|Earnshaw|1997|p=12}} It is also very common in the universe, relative to other stable [[Metallicity|metals]] of approximately the same [[Atomic mass|atomic weight]].{{sfn|Greenwood|Earnshaw|1997|p=12}}<ref>{{cite journal |last1=Woosley |first1=S. |last2=Janka |first2=T. |title=The physics of core collapse supernovae |year=2006 |arxiv=astro-ph/0601261 |doi=10.1038/nphys172 |volume=1 |issue=3 |journal=Nature Physics |pages=147–54| bibcode=2005NatPh...1..147W |s2cid=118974639}}</ref> Iron is the sixth most [[Abundance of the chemical elements|abundant element]] in the [[universe]], and the most common [[refractory]] element.<ref name="apjl717_2_L92">{{cite journal
 | last1=McDonald | first1=I. | last2=Sloan | first2=G. C.
 | last3=Zijlstra | first3=A. A. | last4=Matsunaga | first4=N.
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 | last7=Bernard-Salas | first7=J. | last8=Markwick | first8=A. J.
 | title=Rusty Old Stars: A Source of the Missing Interstellar Iron?
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 | volume=717 | issue=2 | pages=L92–L97 | date=2010
 | doi=10.1088/2041-8205/717/2/L92 | bibcode=2010ApJ...717L..92M |arxiv = 1005.3489 | s2cid=14437704 }}</ref>

[[File:Ironattenuation.PNG|thumb|upright=1.15|alt=A graph of attenuation coefficient vs. energy between 1 meV and 100 keV for several photon scattering mechanisms.|[[Photon]] [[mass attenuation coefficient]] for iron]]

Although a further tiny energy gain could be extracted by synthesizing [[nickel-62|<sup>62</sup>Ni]], which has a marginally higher binding energy than <sup>56</sup>Fe, conditions in stars are unsuitable for this process. Element production in supernovas greatly favor iron over nickel, and in any case, <sup>56</sup>Fe still has a lower mass per nucleon than <sup>62</sup>Ni due to its higher fraction of lighter protons.<ref>{{cite journal |title=Iron and Nickel Abundances in H~II Regions and Supernova Remnants |date=1995 |bibcode=1995AAS...186.3707B |last1=Bautista |first1= Manuel A. |last2=Pradhan |first2=Anil K. |journal=Bulletin of the American Astronomical Society |volume=27 |page=865}}</ref> Hence, elements heavier than iron require a [[supernova]] for their formation, involving [[r-process|rapid neutron capture]] by starting <sup>56</sup>Fe nuclei.{{sfn|Greenwood|Earnshaw|1997|p=12}}

In the [[timeline of the far future|far future]] of the universe, assuming that [[proton decay]] does not occur, cold [[nuclear fusion|fusion]] occurring via [[quantum tunnelling]] would cause the light nuclei in ordinary matter to fuse into <sup>56</sup>Fe nuclei. Fission and [[Alpha decay|alpha-particle emission]] would then make heavy nuclei decay into iron, converting all stellar-mass objects to cold spheres of pure iron.<ref name="twoe">{{cite journal |title=Time without end: Physics and biology in an open universe |first=Freeman J. |last=Dyson |journal=[[Reviews of Modern Physics]] |volume=51 |issue=3 |year=1979 |pages=447–60 |doi=10.1103/RevModPhys.51.447 |bibcode = 1979RvMP...51..447D }}</ref>