Editing Hafnium

{{For|the group that caused the 2021 Microsoft Exchange Server data breach|Hafnium (group)}}
{{Distinguish|text = the compound [[hydrogen fluoride]], formula HF}}
{{Infobox hafnium}}

'''Hafnium''' is a [[chemical element]]; it has [[Symbol (chemistry)|symbol]] '''Hf''' and [[atomic number]] 72. A [[lustre (mineralogy)|lustrous]], silvery gray, [[tetravalence|tetravalent]] [[transition metal]], hafnium chemically resembles [[zirconium]] and is found in many zirconium [[mineral]]s. Its existence was [[Mendeleev's predicted elements|predicted by Dmitri Mendeleev]] in 1869, though it was not identified until 1922, by [[Dirk Coster]] and [[George de Hevesy]]. Hafnium is named after {{lang|la|Hafnia}}, the [[Latin]] name for [[Copenhagen#Etymology|Copenhagen]], where it was discovered.

Hafnium is used in filaments and electrodes.  Some [[semiconductor]] fabrication processes use its oxide for [[integrated circuit]]s at 45 nanometers and smaller feature lengths. Some [[superalloy]]s used for special applications contain hafnium in combination with [[niobium]], [[titanium]], or [[tungsten]].

Hafnium's large [[neutron capture]] [[cross section (physics)|cross section]] makes it a good material for [[neutron]] absorption in [[control rod]]s in [[nuclear power plant]]s, but at the same time requires that it be removed from the neutron-transparent corrosion-resistant [[zirconium alloy]]s used in [[nuclear reactor]]s.

==Characteristics==

===Physical characteristics===
[[File:Hafnium bits.jpg|thumb|left|Pieces of hafnium]]
<section begin=properties />
Hafnium is a shiny, silvery, [[ductility|ductile]] [[metal]] that is [[corrosion]]-resistant and chemically similar to zirconium<ref name="ASTM" /> in that they have the same number of [[valence electron]]s and are in the same group. Also, their [[relativistic quantum chemistry|relativistic effects]] are similar: The expected expansion of atomic radii from period 5 to 6 is almost exactly canceled out by the [[lanthanide contraction]]. Hafnium changes from its alpha form, a hexagonal close-packed lattice, to its beta form, a body-centered cubic lattice, at {{convert|2388|K}}.<ref>{{cite journal |last1=O'Hara |first1=Andrew |last2=Demkov |first2=Alexander A. |title=Oxygen and nitrogen diffusion in α-hafnium from first principles |journal=[[Applied Physics Letters]] |date=2014 |volume=104 |issue=21 |page=211909 |doi=10.1063/1.4880657 |bibcode=2014ApPhL.104u1909O }}</ref>  The physical properties of hafnium metal samples are markedly affected by zirconium impurities, especially the nuclear properties, as these two elements are among the most difficult to separate because of their chemical similarity.<ref name="ASTM" /><section end=properties />

A notable physical difference between these metals is their [[density]], with zirconium having about one-half the density of hafnium. The most notable [[nuclear physics|nuclear]] properties of hafnium are its high [[thermal neutron|thermal]] [[neutron capture cross section]] and that the nuclei of several different hafnium isotopes readily absorb two or more [[neutron]]s apiece.<ref name="ASTM" /> In contrast with this, zirconium is practically transparent to thermal neutrons, and it is commonly used for the metal components of nuclear reactors—especially the cladding of their [[nuclear fuel rod]]s.

===Chemical characteristics===
{{See also|Hafnium#Chemical compounds|label1=&sect;&nbsp;Chemical compounds}}
[[File:Hafnium(IV) oxide.jpg|thumb|left|[[Hafnium(IV) oxide|Hafnium dioxide]] (HfO<sub>2</sub>)]]
Hafnium reacts in air to form a [[Passivation (chemistry)|protective film]] that inhibits further [[corrosion]].  Despite this, the metal is attacked by hydrofluoric acid and concentrated sulfuric acid, and can be oxidized with [[halogen]]s or burnt in air.  Like its sister metal zirconium, finely divided hafnium can ignite spontaneously in air. The metal is resistant to concentrated [[alkali]]s.

As a consequence of [[lanthanide contraction]], the chemistry of hafnium and zirconium is so similar that the two cannot be separated based on differing chemical reactions. The melting and boiling points of the compounds and the [[solubility]] in solvents are the major differences in the chemistry of these twin elements.<ref name="Holl">{{cite book|publisher = [[Walter de Gruyter]]|date = 1985|edition = 91–100|pages=1056–1057|isbn = 978-3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|first = Arnold F.|last = Holleman |author2 = Wiberg, Egon |author3=Wiberg, Nils |language = de|doi=10.1515/9783110206845|author-link2=Egon Wiberg}}</ref>

===Isotopes===
{{Main|Isotopes of hafnium}}
At least 40 isotopes of hafnium have been observed, ranging in [[mass number]] from 153 to 192.<ref name="EC">{{cite web|title = Periodic Table of Elements: Hf – Hafnium|url = http://environmentalchemistry.com/yogi/periodic/Hf-pg2.html#Nuclides|publisher = J.K. Barbalace Inc.|access-date=2021-11-12|first=Kenneth L.|last=Barbalace|website=environmentalchemistry.com}}</ref><ref name="Audi">{{NUBASE 2016}}</ref><ref name=PRC108>{{cite journal |first1=K. |last1=Haak |first2=O. B. |last2=Tarasov |first3=P. |last3=Chowdhury |display-authors=et al. |title=Production and discovery of neutron-rich isotopes by fragmentation of <sup>198</sup>Pt |date=2023 |journal=Physical Review C |volume=108 |number=34608 |page=034608 |doi=10.1103/PhysRevC.108.034608|bibcode=2023PhRvC.108c4608H |s2cid=261649436 }}</ref> The five stable isotopes have mass numbers ranging from 176 to 180 inclusive. The radioactive isotopes' [[half-life|half-lives]] range from 400&nbsp;[[SI prefix|ms]] for <sup>153</sup>Hf<ref name="Audi" /> to {{val|7.0|e=16}} years for the most stable one, the [[primordial radionuclide|primordial]] <sup>174</sup>Hf.<ref name="EC" /><ref name=174Hf2020>{{cite journal |title=Search for α decay of naturally occurring Hf-nuclides using a Cs<sub>2</sub>HfCl<sub>6</sub> scintillator |year=2020 |first1=V. |last1=Caracciolo |first2=S. |last2=Nagorny |first3=P. |last3=Belli |first4=R. |last4=Bernabei |first5=F. |last5=Cappella |first6=R. |last6=Cerulli |first7=A. |last7=Incicchitti |first8=M. |last8=Laubenstein |first9=V |last9=Merlo |first10=S. |last10=Nisi |first11=P. |last11=Wang |display-authors=3 |journal=Nuclear Physics A |volume=1002 |number=121941 |page=121941 |doi=10.1016/j.nuclphysa.2020.121941|arxiv=2005.01373 |bibcode=2020NuPhA100221941C |s2cid=218487451 }}</ref>

The [[extinct radionuclide]] <sup>182</sup>Hf has a half-life of {{val|8.9|0.1|u=million years}}, and is an [[Hafnium–tungsten dating|important tracker isotope]] for the formation of [[planetary core]]s.<ref name=":0">{{cite journal | vauthors = Kleine T, Walker RJ | title = Tungsten Isotopes in Planets | journal = Annual Review of Earth and Planetary Sciences | volume = 45 | issue = 1 | pages = 389–417 | date = August 2017 | pmid = 30842690 | pmc = 6398955 | doi = 10.1146/annurev-earth-063016-020037 | bibcode = 2017AREPS..45..389K }}</ref> The [[nuclear isomer]] <sup>178m2</sup>Hf was at the [[Hafnium controversy|center of a controversy]] for several years regarding its potential use as a weapon.

===Occurrence===
[[File:Zircão.jpeg|thumb|left|Zircon crystal (2×2 cm) from [[Tocantins]], [[Brazil]]]]

Hafnium is estimated to make up about between 3.0 and 4.8 [[Parts per million|ppm]] of the [[Earth]]'s upper [[crust (geology)|crust]] by mass.<ref>{{Cite book |last1=Haygarth |first1=John C. |url=https://onlinelibrary.wiley.com/doi/10.1002/9781118788417.ch1 |title=Zirconium and Hafnium |last2=Graham |first2=Ronald A. |date=2013-09-30 |publisher=John Wiley & Sons, Inc. |isbn=978-1-118-78841-7 |editor-last=Mishra |editor-first=Brajendra |location=Hoboken, NJ, USA |pages=1–71 |language=en |doi=10.1002/9781118788417.ch1}}</ref>{{rp|5}} <ref name=CRC>ABUNDANCE OF ELEMENTS IN THE EARTH’S CRUST AND IN THE SEA, ''CRC Handbook of Chemistry and Physics,'' 97th edition (2016–2017), p. 14-17</ref> It does not exist as a free element on Earth, but is found combined in [[solid solution]] with zirconium in natural [[zirconium]] compounds such as [[zircon]], ZrSiO<sub>4</sub>, which usually has about 1–4% of the Zr replaced by Hf. Rarely, the Hf/Zr ratio increases during crystallization to give the isostructural mineral [[hafnon]] {{chem2|(Hf,Zr)SiO4}}, with atomic Hf > Zr.<ref>{{cite book|title = The Rock-Forming Minerals: Orthosilicates|first1 = William Alexander|last1 = Deer |author-link1=William Alexander Deer|last2 = Howie|first2= Robert Andrew |author-link2=Robert A. Howie|last3=Zussmann|first3=Jack|isbn=978-0-582-46526-8|date = 1982|publisher = [[Longman|Longman Group Limited]]|pages=418–442|volume=1A|url=https://books.google.com/books?id=Yi0SAQAAMAAJ&q=9780582465268}}</ref> An obsolete name for a variety of zircon containing unusually high Hf content is ''alvite''.<ref>{{cite journal|title = The Mineralogy of Hafnium|first = O. Ivan|last = Lee|journal = [[Chemical Reviews]]|date = 1928|volume = 5|issue=1|pages=17–37|doi = 10.1021/cr60017a002|url=https://archive.org/details/in.ernet.dli.2015.27353/page/n23/mode/2up}}</ref>

A major source of zircon (and hence hafnium) ores is [[heavy mineral sands ore deposits]], [[pegmatite]]s, particularly in [[Brazil]] and [[Malawi]], and [[carbonatite]] intrusions, particularly the Crown Polymetallic Deposit at [[Mount Weld]], [[Western Australia]]. A potential source of hafnium is [[Trachyte|trachyte tuffs]] containing rare zircon-hafnium silicates [[eudialyte]] or [[armstrongite]], at [[Dubbo]] in [[New South Wales]], Australia.<ref>{{cite web|url = http://www.alkane.com.au/projects/nsw/dubbo/DZP%20Summary%20June07.pdf|title = The Dubbo Zirconia Project |last=Chalmers|first=Ian|date = June 2007|publisher = Alkane Resources Limited|access-date = 2008-09-10|archive-url = https://web.archive.org/web/20080228054038/http://www.alkane.com.au/projects/nsw/dubbo/DZP%20Summary%20June07.pdf|archive-date = 2008-02-28}}</ref>

==Production==
[[File:Hafnium ebeam remelted.jpg|thumb|left|Melted tip of a hafnium consumable electrode used in an [[Electron-beam additive manufacturing|electron beam]] [[Electron-beam furnace|remelting furnace]], a 1&nbsp;cm cube, and an oxidized hafnium electron beam-remelted ingot (left to right)]]

The heavy mineral sands ore deposits of the [[titanium]] ores [[ilmenite]] and [[rutile]] yield most of the mined zirconium, and therefore also most of the hafnium.<ref>{{cite web|title = 2008 Minerals Yearbook: Zirconium and Hafnium|first = Joseph|last = Gambogi|publisher=[[United States Geological Survey]]|date=2010|access-date=2021-11-11|url = https://www.usgs.gov/centers/nmic/zirconium-and-hafnium-statistics-and-information}}</ref>

Zirconium is a good nuclear fuel-rod cladding metal, with the desirable properties of a very low neutron capture cross section and good chemical stability at high temperatures. However, because of hafnium's neutron-absorbing properties, hafnium impurities in zirconium would cause it to be far less useful for nuclear reactor applications. Thus, a nearly complete separation of zirconium and hafnium is necessary for their use in nuclear power. The production of hafnium-free zirconium is the main source of hafnium.<ref name="ASTM">{{cite book|url = https://books.google.com/books?id=dI_LssydVeYC|title = ASTM Manual on Zirconium and Hafnium|first = J. H.|last = Schemel|publisher = [[ASTM]]|date = 1977|isbn=978-0-8031-0505-8|pages=1–5|location =Philadelphia|volume=STP 639}}</ref>
[[File:Hafnium pellets with a thin oxide layer.jpg|thumb|right|Hafnium oxidized ingots which exhibit [[thin-film optics|thin-film optical]] effects]]

The chemical properties of hafnium and zirconium are nearly identical, which makes the two difficult to separate.<ref name="Larsen">{{cite journal|title = Concentration of Hafnium. Preparation of Hafnium-Free Zirconia|first1 = Edwin M.|last1 = Larsen|last2 = Fernelius |first2=W. Conard |last3=Quill|first3=Laurence |journal = [[Ind. Eng. Chem. Anal. Ed.]]|date=1943|volume=15|pages=512–515|doi =10.1021/i560120a015|issue = 8|url=https://docecity.com/concentration-of-hafnium-preparation-of-hafnium-free-zirconi-5f1098025158d.html}}</ref> The methods first used—[[Fractional crystallization (chemistry)|fractional crystallization]] of ammonium fluoride salts<ref name="Ark1924a" /> or the fractional distillation of the chloride<ref name="Ark1924b" />—have not proven suitable for an industrial-scale production. After zirconium was chosen as a material for nuclear reactor programs in the 1940s, a separation method had to be developed. [[Liquid–liquid extraction]] processes with a wide variety of solvents were developed and are still used for producing hafnium.<ref name="Hend" /> About half of all hafnium metal manufactured is produced as a by-product of zirconium refinement. The end product of the separation is [[Hafnium tetrachloride|hafnium(IV) chloride]].<ref name="USGS1952">{{cite book|publisher = The first production plants Bureau of Mines|title = Minerals yearbook metals and minerals (except fuels)|date = 1952|chapter-url = http://digicoll.library.wisc.edu/cgi-bin/EcoNatRes/EcoNatRes-idx?type=turn&entity=EcoNatRes.MinYB1952v1.p1172&isize=M|last = Griffith|first = Robert F.|chapter =Zirconium and hafnium|pages=1162–1171}}</ref> The purified hafnium(IV) chloride is converted to the metal by reduction  with [[magnesium]] or [[sodium]], as in the [[Kroll process]].<ref name="Gilb">{{cite journal|title = Preliminary Investigation of Hafnium Metal by the Kroll Process|first = H. L.|last = Gilbert|author2=Barr, M. M.|journal = Journal of the Electrochemical Society|date =1955|volume =102|page=243|doi = 10.1149/1.2430037|issue = 5}}</ref>
: <chem>HfCl4{} + 2 Mg ->[1100~^\circ\text{C}] Hf{} + 2 MgCl2</chem>

Further purification is effected by a [[chemical transport reaction]] developed by [[Crystal bar process|Arkel and de Boer]]: In a closed vessel, hafnium reacts with [[iodine]] at temperatures of {{convert|500|°C|sigfig=1}}, forming [[hafnium(IV) iodide]]; at a tungsten filament of {{convert|1700|°C|sigfig=2}} the reverse reaction happens preferentially, and the chemically bound iodine and hafnium dissociate into the native elements. The hafnium forms a solid coating at the tungsten filament, and the iodine can react with additional hafnium, resulting in a steady iodine turnover and ensuring the [[chemical equilibrium]] remains in favor of hafnium production.<ref name="Holl" /><ref name="Ark1925" />
: <chem>Hf{} + 2 I2 ->[500~^\circ\text{C}] HfI4</chem>
: <chem>HfI4 ->[1700~^\circ\text{C}] Hf{} + 2 I2</chem>

==Chemical compounds==
{{Main|Hafnium compounds}}
Due to the [[lanthanide contraction]], the [[ionic radius]] of hafnium(IV) (0.78&nbsp;ångström) is almost the same  as that of  [[zirconium]](IV) (0.79&nbsp;[[angstrom]]s).<ref name="lanl72" /> Consequently, compounds of hafnium(IV) and zirconium(IV) have very similar chemical and physical properties.<ref name="lanl72" /> Hafnium and zirconium tend to occur together in nature and the similarity of their ionic radii makes their chemical separation rather difficult. Hafnium tends to form [[inorganic chemistry|inorganic compounds]] in the oxidation state of +4. [[Halogen]]s react with it to form hafnium tetrahalides.<ref name="lanl72" /> At higher temperatures, hafnium reacts with [[oxygen]], [[nitrogen]], [[carbon]], [[boron]], [[sulfur]], and [[silicon]].<ref name="lanl72" /> Some hafnium compounds in lower oxidation states are known.<ref>{{Greenwood&Earnshaw2nd|pages=971–975}}</ref>

[[Hafnium(IV) chloride]] and hafnium(IV) iodide have some applications in the production and purification of hafnium metal.  They are volatile solids with polymeric structures.<ref name="Holl" /> These tetrachlorides are precursors to various [[Organozirconium chemistry|organohafnium compounds]] such as hafnocene dichloride and tetrabenzylhafnium.

The white [[hafnium oxide]] (HfO<sub>2</sub>), with a melting point of {{convert|2,812|C|K F}} and a boiling point of roughly {{convert|5,100|C|K F|sigfig=2}}, is very similar to [[zirconia]], but slightly more basic.<ref name="Holl" /> [[Hafnium carbide]] is the most [[refractory]] [[binary compound]] known, with a melting point over {{convert|3,890|C|K F|0}}, and hafnium nitride is the most refractory of all known metal nitrides, with a melting point of {{convert|3,310|C|K F|0}}.<ref name="lanl72">{{cite web|url = http://periodic.lanl.gov/72.shtml |title = Los Alamos National Laboratory – Hafnium|access-date=2008-09-10}}</ref> This has led to proposals that hafnium or its carbides might be useful as construction materials that are subjected to very high temperatures. The mixed carbide [[tantalum hafnium carbide]] ({{chem|Ta|4|HfC|5}}) possesses the highest melting point of any currently known compound, {{cvt|4263|K|C F}}.<ref>{{cite journal|title=Researches on Systems with Carbides at High Melting Point and Contributions to the Problem of Carbon Fusion|journal=Z. Tech. Phys.|author1=Agte, C. |author2=Alterthum, H. |name-list-style=amp | volume= 11|year= 1930|pages=182–191}}</ref> Recent supercomputer simulations suggest a hafnium alloy with a melting point of {{convert|4,400|K|C F|0}}.<ref name="hong2015">{{cite journal|last1=Hong|first1=Qi-Jun|last2=van de Walle|first2=Axel|title=Prediction of the material with highest known melting point from ab initio molecular dynamics calculations|journal=Phys. Rev. B|date=2015|volume=92|issue=2|page=020104|doi=10.1103/PhysRevB.92.020104|bibcode=2015PhRvB..92b0104H|doi-access=free}}</ref>

==History==
[[File:Moseley step ladder.jpg|upright=1.2|thumb|left|Photographic recording of the characteristic [[X-ray]] emission lines of some elements]]
Hafnium's existence was [[Mendeleev's predicted elements|predicted by Dmitri Mendeleev]] in 1869.
In his report on ''The Periodic Law of the Chemical Elements'', in 1869, [[Dmitri Mendeleev]] had implicitly [[Mendeleev's predicted elements|predicted the existence]] of a heavier analog of titanium and zirconium. At the time of his formulation in 1871, Mendeleev believed that the elements were ordered by their [[atomic mass]]es and placed [[lanthanum]] (element 57) in the spot below zirconium. The exact placement of the elements and the location of missing elements was done by determining the specific weight of the elements and comparing the chemical and physical properties.<ref>{{cite journal|first=Masanori|last=Kaji|title=D. I. Mendeleev's concept of chemical elements and ''The Principles of Chemistry''|journal=[[Bulletin for the History of Chemistry]]|volume=27|page=4|date=2002|doi=10.70359/bhc2002v027p004 |url=http://www.scs.uiuc.edu/~mainzv/HIST/awards/OPA%20Papers/2005-Kaji.pdf|access-date=2008-08-20|archive-url=https://web.archive.org/web/20081217080509/http://www.scs.uiuc.edu/~mainzv/HIST/awards/OPA%20Papers/2005-Kaji.pdf|archive-date=2008-12-17}}</ref>

The [[X-ray spectroscopy]] done by [[Henry Moseley]] in 1914 showed a direct dependency between [[spectral line]] and [[effective nuclear charge]]. This led to the nuclear charge, or [[atomic number]] of an element, being used to ascertain its place within the periodic table. With this method, Moseley determined the number of [[lanthanides]] and showed the gaps in the atomic number sequence at numbers 43, 61, 72, and 75.<ref>{{cite journal|last = Heilbron|title=The Work of H. G. J. Moseley|first = John L.|volume=57|page=336|date=1966|journal=Isis|doi=10.1086/350143|issue = 3|s2cid=144765815}}</ref>

The discovery of the gaps led to an extensive search for the missing elements. In 1914, several people claimed the discovery after Henry Moseley predicted the gap in the periodic table for the then-undiscovered element 72.<ref>{{cite journal|last = Heimann|first = P. M.|date = 1967|title = Moseley and celtium: The search for a missing element|journal = [[Annals of Science]]|volume = 23|pages=249–260|doi = 10.1080/00033796700203306|issue = 4}}</ref> [[Georges Urbain]] asserted that he found element 72 in the [[rare earth element]]s in 1907 and published his results on ''celtium'' in 1911.<ref>{{cite journal|last = Urbain|first = M. G.|title = Sur un nouvel élément qui accompagne le lutécium et le scandium dans les terres de la gadolinite: le celtium (On a new element that accompanies lutetium and scandium in gadolinite: celtium)|journal = Comptes Rendus|page=141|url = http://gallica.bnf.fr/ark:/12148/bpt6k3105c/f141.table|access-date=2008-09-10|date = 1911|language = fr}}</ref> Neither the spectra nor the chemical behavior he claimed matched with the element found later, and therefore his claim was turned down after a long-standing controversy.<ref name="Mel">{{cite journal|journal = Centaurus|volume = 26|pages =317–322|title = Some Details in the Prehistory of the Discovery of Element 72|first = V. P.|last = Mel'nikov|doi = 10.1111/j.1600-0498.1982.tb00667.x|date = 1982|bibcode = 1982Cent...26..317M|issue = 3 }}</ref> The controversy was partly because the chemists favored the chemical techniques which led to the discovery of ''celtium'', while the physicists relied on the use of the new X-ray spectroscopy method that proved that the substances discovered by Urbain did not contain element 72.<ref name="Mel" /> In 1921, [[Charles Rugeley Bury|Charles R. Bury]]<ref name="doi.org">Kragh, Helge. "Niels Bohr's Second Atomic Theory." Historical Studies in the Physical Sciences, vol. 10, University of California Press, 1979, pp. 123–186, https://doi.org/10.2307/27757389.</ref><ref>{{cite journal|journal = J. Am. Chem. Soc.|title = Langmuir's Theory of the Arrangement of Electrons in Atoms and Molecules|first = Charles R.|last = Bury|volume = 43|date = 1921|pages=1602–1609|doi = 10.1021/ja01440a023|issue = 7| bibcode=1921JAChS..43.1602B |url = https://zenodo.org/record/1428812}}</ref> suggested that element 72 should resemble zirconium and therefore was not part of the rare earth elements group. By early 1923, [[Niels Bohr]] and others agreed with Bury.<ref>{{cite book|title = The Theory of Spectra and Atomic Constitution: Three Essays|url = https://archive.org/details/TheTheoryOfSpectraAndAtomicConstitution|first = Niels|last = Bohr|date = June 2008|page=[https://archive.org/details/TheTheoryOfSpectraAndAtomicConstitution/page/n123 114]| publisher=Kessinger |isbn = 978-1-4365-0368-6}}</ref><ref>{{cite web|title=Nobel Lecture: The Structure of the Atom | author=Niels Bohr | date=11 December 1922| access-date=25 March 2021| url=https://www.nobelprize.org/uploads/2018/06/bohr-lecture.pdf}}</ref> These suggestions were based on Bohr's theories of the atom which were identical to chemist Charles Bury,<ref name="doi.org"/> the X-ray spectroscopy of Moseley, and the chemical arguments of [[Friedrich Paneth]].<ref>{{cite book|first = F. A.|last = Paneth|chapter = Das periodische System (The periodic system)|title = Ergebnisse der Exakten Naturwissenschaften 1|date =1922|page=362|language = de}}</ref><ref>{{cite journal|title = Hafnium|url = http://www.jce.divched.org/Journal/Issues/1982/Mar/jceSubscriber/JCE1982p0242.pdf|journal = Journal of Chemical Education|last = Fernelius|first = W. C.|date = 1982|page = 242|doi = 10.1021/ed059p242|volume = 59|issue = 3|bibcode = 1982JChEd..59..242F|access-date = 2009-09-03|archive-date = 2020-03-15|archive-url = https://web.archive.org/web/20200315031648/http://www.jce.divched.org/Journal/Issues/1982/Mar/jceSubscriber/JCE1982p0242.pdf}}</ref>

Encouraged by these suggestions and by the reappearance in 1922 of Urbain's claims that element 72 was a rare earth element discovered in 1911, [[Dirk Coster]] and [[Georg von Hevesy]] were motivated to search for the new element in zirconium ores.<ref>{{cite journal|volume = 174|date = 1922|last = Urbain|first = M. G.|title = Sur les séries L du lutécium et de l'ytterbium et sur l'identification d'un celtium avec l'élément de nombre atomique 72 |trans-title=The L series from lutetium to ytterbium and the identification of element 72 celtium|journal = Comptes Rendus|page=1347|url = http://gallica.bnf.fr/ark:/12148/bpt6k3127j/f1348.table|access-date=2008-10-30|language = fr}}</ref> Hafnium was discovered by the two in 1923 in Copenhagen, Denmark, validating the original 1869 prediction of Mendeleev.<ref>"Two Danes Discover New Element, Hafnium{{snd}}Detect It by Means of Spectrum Analysis of Ore Containing Zirconium", ''The New York Times'', January 20, 1923, p. 4</ref><ref name="CosterHevesy1923">{{cite journal|journal = Nature|volume = 111|page=79|date=1923|doi = 10.1038/111079a0|title = On the Missing Element of Atomic Number 72|first = D.|last = Coster|author2=Hevesy, G.|issue=2777|bibcode=1923Natur.111...79C|doi-access = free}}</ref><!--follow up publications of Urbain's claim that celtium and hafnium are identical {{doi|10.1038/111218a0}}{{doi|10.1038/111252a0}}--><ref>{{cite journal|title = The Discovery and Properties of Hafnium|first = G.|last = Hevesy|journal = Chemical Reviews|date = 1925|volume = 2|pages=1–41|doi = 10.1021/cr60005a001}}</ref> It was ultimately found in [[zircon]] in Norway through X-ray spectroscopy analysis.<ref name="hev1503">{{cite journal|title = Über die Auffindung des Hafniums und den gegenwärtigen Stand unserer Kenntnisse von diesem Element|first = Georg|last = von Hevesy|doi = 10.1002/cber.19230560702|journal = Berichte der Deutschen Chemischen Gesellschaft (A and B Series)|volume = 56|pages=1503–1516|date = 1923|issue = 7| s2cid=96017606 }}</ref> The place where the discovery took place led to the element being named for the Latin name for "Copenhagen", ''Hafnia'', the home town of [[Niels Bohr]].<ref name ="Scerri">{{cite journal|title = Prediction of the nature of hafnium from chemistry, Bohr's theory and quantum theory|first = Eric R.|last = Scerri|journal = Annals of Science|date = 1994|volume = 51|pages= 137–150|doi =10.1080/00033799400200161|issue = 2}}</ref><ref name="Authier2013">{{cite book |author-first=André |author-last=Authier |title=Early Days of X-ray Crystallography |url=https://books.google.com/books?id=ej9oAgAAQBAJ&pg=PA153 |date= 2013 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-163501-4 |page=153}}</ref><ref>{{cite book
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 |page= 10
 |isbn=0-7172-5677-4}}
</ref> Today, the [[University of Copenhagen Faculty of Science|Faculty of Science]] of the [[University of Copenhagen]] uses in its [[Seal (emblem)|seal]] a stylized image of the hafnium atom.<ref>{{cite web|publisher = University of Copenghagen|access-date=2016-11-19|title = University Life 2005|url = http://universitetshistorie.ku.dk/filer/aarsberetning/universitetsliv_2005_uk.pdf/ |format=pdf|page=43}}</ref>

Hafnium was separated from zirconium through repeated recrystallization of the double [[ammonium]] or [[potassium]] fluorides by [[Valdemar Thal Jantzen]] and von Hevesey.<ref name="Ark1924a">{{cite journal|title = Die Trennung von Zirkonium und Hafnium durch Kristallisation ihrer Ammoniumdoppelfluoride (The separation of zirconium and hafnium by crystallization of their double ammonium fluorides)|journal = [[Zeitschrift für Anorganische und Allgemeine Chemie]]|volume = 141|date = 1924|pages= 284–288|first1 = A. E.|last1 = van Arkel|last2 = de Boer|first2=J. H.|doi = 10.1002/zaac.19241410117|language = de |author-link1=Anton Eduard van Arkel |author-link2=Jan Hendrik de Boer |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015006985249}}</ref> [[Anton Eduard van Arkel]] and [[Jan Hendrik de Boer]] were the first to prepare metallic hafnium by passing hafnium tetraiodide vapor over a heated [[tungsten]] filament in 1924.<ref name="Ark1924b">{{cite journal|title = Die Trennung des Zirkoniums von anderen Metallen, einschließlich Hafnium, durch fraktionierte Distillation|trans-title=The separation of zirconium from other metals, including hafnium, by fractional distillation| journal = [[Zeitschrift für Anorganische und Allgemeine Chemie]]|volume = 141|issue=1|date = 1924-12-23|pages= 289–296|first1 = A. E.|last1 = van Arkel|author-link1=Anton Eduard van Arkel|last2 = de Boer|first2=J. H.|author-link2=Jan Hendrik de Boer|doi = 10.1002/zaac.19241410118|language = de|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015006985249}}</ref><ref name="Ark1925">{{cite journal|title = Darstellung von reinem Titanium-, Zirkonium-, Hafnium- und Thoriummetall (Production of pure titanium, zirconium, hafnium and Thorium metal)|journal = Zeitschrift für Anorganische und Allgemeine Chemie|volume = 148|date = 1925|pages= 345–350|first = A. E.|last = van Arkel|author2 = de Boer, J. H.|doi = 10.1002/zaac.19251480133|language = de}}</ref> This process for differential purification of zirconium and hafnium is still in use today.<ref name="ASTM" />

Hafnium was one of the last two [[Stable isotope|stable]] elements to be discovered. The element [[rhenium]] was found in 1908 by [[Masataka Ogawa]], though its atomic number was misidentified at the time, and it was not generally recognised by the scientific community until its rediscovery by [[Walter Noddack]], [[Ida Noddack]], and [[Otto Berg (scientist)|Otto Berg]] in 1925. This makes it somewhat difficult to say if hafnium or rhenium was discovered last.<ref name=nipponium2022>{{cite journal |last1=Hisamatsu |first1=Yoji |last2=Egashira |first2=Kazuhiro |first3=Yoshiteru |last3=Maeno |date=2022 |title=Ogawa's nipponium and its re-assignment to rhenium |journal=Foundations of Chemistry |volume=24 |issue= |pages=15–57 |doi=10.1007/s10698-021-09410-x |doi-access=free }}</ref>

In 1923, six predicted elements were still missing from the periodic table: 43 ([[technetium]]), 61 ([[promethium]]), 85 ([[astatine]]), and 87 ([[francium]]) are radioactive elements and are only present in trace amounts in the environment,<ref>{{cite journal|doi = 10.1016/S0016-7037(98)00282-8|title = Nature's uncommon elements: plutonium and technetium|first = David|last = Curtis |author2 = Fabryka-Martin, June |author3=Dixon, Pauland |author4=Cramer, Jan |journal = Geochimica et Cosmochimica Acta|volume = 63|date =1999|pages= 275–285|bibcode=1999GeCoA..63..275C|issue = 2|url = https://digital.library.unt.edu/ark:/67531/metadc704244/}}</ref> thus making elements 75 ([[rhenium]]) and 72 (hafnium) the last two unknown non-radioactive elements.

==Applications==
Most of the hafnium produced is used in the manufacture of [[control rod]]s for [[nuclear reactor]]s.<ref name="Hend">{{cite web|title = Hafnium|first = James B.|last = Hedrick|url = http://minerals.er.usgs.gov/minerals/pubs/commodity/zirconium/731798.pdf|publisher = United States Geological Survey|access-date=2008-09-10}}</ref>

Hafnium has limited technical applications due to a few factors. First, it's very similar to zirconium, a more abundant element that can be used in most cases. Second, pure hafnium wasn't widely available until the late 1950s, when it became a byproduct of the nuclear industry's need for hafnium-free zirconium. Additionally, hafnium is rare and difficult to separate from other elements, making it expensive. After the Fukushima disaster reduced the demand for hafnium-free zirconium, the price of hafnium increased significantly from around $500–$600/kg ($227-$272/lb) in 2014 to around $1000/kg ($454/lb) in 2015.<ref>{{cite web|last1=Albrecht|first1=Bodo|title=Weak Zirconium Demand Depleting Hafnium Stock Piles|url=http://www.kitco.com/ind/Albrecht/2015-03-11-Weak-Zirconium-Demand-Depleting-Hafnium-Stock-Piles.html|website=Tech Metals Insider|publisher=KITCO|access-date=4 March 2018|date=2015-03-11|archive-date=2021-04-28|archive-url=https://web.archive.org/web/20210428094638/https://www.kitco.com/ind/Albrecht/2015-03-11-Weak-Zirconium-Demand-Depleting-Hafnium-Stock-Piles.html|url-status=dead}}</ref>

===Nuclear reactors===
The nuclei of several hafnium isotopes can each absorb multiple neutrons. This makes hafnium a good material for nuclear reactors' control rods. Its neutron capture cross section (Capture Resonance Integral I<sub>o</sub> ≈ 2000 barns)<ref>{{Cite web |last1=Noguère |first1=G. |last2=Courcelle |first2=A |last3=Palau |first3=J.M. |last4=Siegler |first4=P. |date=2005 |title="Low-neutron-energy cross sections of the hafnium isotopes". |url=https://www.oecd-nea.org/dbdata/nds_jefreports/jefreport-23/supp/jefdoc/jefdoc-1077.pdf}}</ref> is about 600 times that of zirconium (other elements that are good neutron-absorbers for control rods are [[cadmium]] and [[boron]]). Excellent mechanical properties and exceptional corrosion-resistance properties allow its use in the harsh environment of [[pressurized water reactor]]s.<ref name="Hend" /> The German research reactor [[Forschungsreaktor München II|FRM II]] uses hafnium as a neutron absorber.<ref name="FRMII">{{cite web|publisher = Strahlenschutzkommission|url = http://www.ssk.de/werke/volltext/1995/ssk9512.pdf|title = Forschungsreaktor München II (FRM-II): Standort und Sicherheitskonzept|access-date=2008-09-22|date=1996-02-07|archive-url = https://web.archive.org/web/20071020064013/http://www.ssk.de/werke/volltext/1995/ssk9512.pdf |archive-date = October 20, 2007}}</ref> It is also common in military reactors, particularly in US naval submarine reactors, to slow reactor rates that are too high.<ref name="Schemel1977">{{cite book|author=J. H. Schemel|title=ASTM Manual on Zirconium and Hafnium|date=1977|publisher=ASTM International|isbn=978-0-8031-0505-8|page=21}}</ref><ref>{{Cite book |title=[[World Book]] |publisher=[[Berkshire Hathaway]] |year=2020 |isbn=978-0-7166-0120-3 |edition=2020 |location=[[Chicago]] |page=5 |language=English}}</ref> It is seldom found in civilian reactors, the first core of the [[Shippingport Atomic Power Station]] (a conversion of a naval reactor) being a notable exception.<ref name="YanHino2011">{{cite book|editor=Xing L. Yan|editor2=Ryutaro Hino|title=Nuclear Hydrogen Production Handbook|date=2011|publisher=CRC Press|isbn=978-1-4398-1084-2|page=192|author=C.W. Forsberg|author2=K. Takase|author3=N. Nakatsuka|name-list-style=amp|chapter=Water Reactor}}</ref>

===Alloys===
[[File:Apollo AS11-40-5866.jpg|thumb|right|Hafnium-containing rocket nozzle of the [[Apollo Lunar Module]] in the lower right corner]]
Hafnium is used in [[alloy]]s with [[iron]], [[titanium]], [[niobium]], [[tantalum]], and other metals. An alloy used for [[liquid-propellant rocket|liquid-rocket]] thruster nozzles, for example the main engine of the [[Apollo Lunar Module]]s, is C103 which consists of 89% niobium, 10% hafnium and 1% titanium.<ref name="hightemp">{{cite web|url = https://www.cbmm.com/portug/sources/techlib/science_techno/table_content/sub_3/images/pdfs/016.pdf|title = Niobium alloys and high Temperature Applications|first = John|last = Hebda|publisher = CBMM|date = 2001|access-date = 2008-09-04|archive-url = https://web.archive.org/web/20081217080513/http://www.cbmm.com.br/portug/sources/techlib/science_techno/table_content/sub_3/images/pdfs/016.pdf|archive-date = 2008-12-17}}</ref>

Small additions of hafnium increase the adherence of protective oxide scales on nickel-based alloys. It thereby improves the [[corrosion]] resistance, especially under cyclic temperature conditions that tend to break oxide scales, by inducing thermal stresses between the bulk material and the oxide layer.<ref>{{cite journal|title = Effect of hafnium on the structure and properties of nickel alloys|first = S. B.|last = Maslenkov |author2 = Burova, N. N. |author3=Khangulov, V. V. |journal = Metal Science and Heat Treatment|volume = 22|date = 1980|doi=10.1007/BF00779883|pages=283–285|issue = 4|bibcode = 1980MSHT...22..283M|s2cid = 135595958}}</ref><ref>{{cite journal|first = V. M.|last = Beglov |author2 = Pisarev, B. K. |author3=Reznikova, G. G. |title = Effect of boron and hafnium on the corrosion resistance of high-temperature nickel alloys|journal = Metal Science and Heat Treatment|volume = 34|date = 1992|doi = 10.1007/BF00702544|pages=251–254|issue = 4|bibcode = 1992MSHT...34..251B |s2cid = 135844921 }}</ref><ref>{{cite journal|first = R. F.|last = Voitovich|author2=Golovko, É. I.|title = Oxidation of hafnium alloys with nickel|journal = Metal Science and Heat Treatment|volume = 17|date = 1975|doi = 10.1007/BF00663680|pages=207–209|issue = 3|bibcode = 1975MSHT...17..207V|s2cid = 137073174}}</ref>

===Microprocessors===
Hafnium-based compounds are employed in [[gate (transistor)|gates]] of transistors as insulators in the 45&nbsp;nm (and below) generation of [[integrated circuits]] from [[Intel]], [[IBM]] and others.<ref>{{Cite patent|country=US|number=6013553|assign=[[Texas Instruments Inc.]]
|inventor1-last=Wallace |inventor1-first=Robert M.
|inventor2-last=Stoltz|inventor2-first=Richard A.
|inventor3-last=Wilk |inventor3-first=Glen D.
|title=Zirconium and/or hafnium oxynitride gate dielectric
|pubdate=2000-01-11 
}}</ref><ref>{{cite news|url = https://www.nytimes.com/2007/01/27/technology/27chip.html|title = Intel Says Chips Will Run Faster, Using Less Power|first = John|last = Markoff|newspaper = New York Times|date=2007-01-27|access-date=2008-09-10}}</ref> Hafnium oxide-based compounds are practical [[high-k dielectric]]s, allowing reduction of the gate leakage current which improves performance at such scales.<ref>{{cite news|last = Fulton III|first = Scott M.|title = Intel Reinvents the Transistor|publisher = BetaNews|date=January 27, 2007|url =http://www.betanews.com/article/Intel_Reinvents_the_Transistor/1169872301|access-date=2007-01-27}}</ref><ref>{{cite news|last = Robertson|first = Jordan|title = Intel, IBM reveal transistor overhaul|publisher = The Associated Press|date=January 27, 2007|url = https://www.washingtonpost.com/wp-dyn/content/article/2007/01/27/AR2007012700152.html|access-date=2008-09-10}}</ref><ref>{{Cite web |title=Atomic Layer Deposition (ALD) |url=https://semiengineering.com/knowledge_centers/manufacturing/process/deposition/atomic-layer-deposition/ |access-date=2023-04-30 |website=Semiconductor Engineering |language=en-US}}</ref>

===Isotope geochemistry===
Isotopes of hafnium and [[lutetium]] (along with [[ytterbium]]) are also used in [[isotope geochemistry]] and [[geochronology|geochronological]] applications, in [[lutetium-hafnium dating]]. It is often used as a tracer of isotopic evolution of [[Mantle (geology)|Earth's mantle]] through time.<ref>{{cite journal|last1=Patchett|first1=P. Jonathan|title=Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution|journal=Geochimica et Cosmochimica Acta|date=January 1983|volume=47|issue=1|pages=81–91|doi=10.1016/0016-7037(83)90092-3|bibcode=1983GeCoA..47...81P}}</ref> This is because <sup>176</sup>Lu decays to <sup>176</sup>Hf with a [[half-life]] of approximately 37 billion years.<ref>{{cite journal|last1=Söderlund|first1=Ulf|last2=Patchett|first2=P. Jonathan|last3=Vervoort|first3=Jeffrey D.|last4=Isachsen|first4=Clark E.|title=The 176Lu decay constant determined by Lu–Hf and U–Pb isotope systematics of Precambrian mafic intrusions|journal=Earth and Planetary Science Letters|date=March 2004|volume=219|issue=3–4|pages=311–324|doi=10.1016/S0012-821X(04)00012-3|bibcode=2004E&PSL.219..311S}}</ref><ref>{{cite journal|last1=Blichert-Toft|first1=Janne|author-link=Janne Blichert-Toft|last2=Albarède|first2=Francis|title=The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crust system|journal=Earth and Planetary Science Letters|date=April 1997|volume=148|issue=1–2|pages=243–258|doi=10.1016/S0012-821X(97)00040-X|bibcode=1997E&PSL.148..243B}}</ref><ref>{{cite journal|last1=Patchett|first1=P. J.|last2=Tatsumoto|first2=M.|title=Lu–Hf total-rock isochron for the eucrite meteorites|journal=Nature|date=11 December 1980|volume=288|issue=5791|pages=571–574|doi=10.1038/288571a0|bibcode=1980Natur.288..571P|s2cid=4284487}}</ref>

In most geologic materials, [[zircon]] is the dominant host of hafnium (>10,000 ppm) and is often the focus of hafnium studies in [[geology]].<ref>{{cite journal|last1=Kinny|first1=P. D.|title=Lu-Hf and Sm-Nd isotope systems in zircon|journal=Reviews in Mineralogy and Geochemistry|date=1 January 2003|volume=53|issue=1|pages=327–341|doi=10.2113/0530327|bibcode=2003RvMG...53..327K}}</ref> Hafnium is readily substituted into the zircon [[Crystal structure|crystal lattice]], and is therefore very resistant to hafnium mobility and contamination. Zircon also has an extremely low Lu/Hf ratio, making any correction for initial lutetium minimal. Although the Lu/Hf system can be used to calculate a "[[Nd model ages|model age]]", i.e. the time at which it was derived from a given isotopic reservoir such as the [[depleted-mantle model|depleted mantle]], these "ages" do not carry the same geologic significance as do other geochronological techniques as the results often yield isotopic mixtures and thus provide an average age of the material from which it was derived.

[[Garnet]] is another mineral that contains appreciable amounts of hafnium to act as a geochronometer. The high and variable Lu/Hf ratios found in garnet make it useful for dating [[metamorphism|metamorphic]] events.<ref>{{cite journal|last1=Albarède|first1=F.|last2=Duchêne|first2=S.|last3=Blichert-Toft|first3=J.|last4=Luais|first4=B.|last5=Télouk|first5=P.|last6=Lardeaux|first6=J.-M.|journal=Nature|title=The Lu–Hf dating of garnets and the ages of the Alpine high-pressure metamorphism|date=5 June 1997|volume=387|issue=6633|pages=586–589|doi=10.1038/42446|bibcode=1997Natur.387..586D|s2cid=4260388}}</ref>

===Other uses===
Due to its heat resistance and its affinity to oxygen and nitrogen, hafnium is a good scavenger for oxygen and nitrogen in gas-filled and [[incandescent lamp]]s. Hafnium is also used as the electrode in [[plasma cutting]] because of its ability to shed electrons into the air.<ref>{{cite journal|journal = Journal of Physics D: Applied Physics|volume = 30|date = 1997|pages=636–644|title = Properties of electric arc plasma for metal cutting|first = S.|last = Ramakrishnany|author2=Rogozinski, M. W.|doi = 10.1088/0022-3727/30/4/019|bibcode = 1997JPhD...30..636R|issue = 4 | s2cid=250746818 }}</ref>

The high energy content of <sup>178m2</sup>Hf was the concern of a [[DARPA]]-funded program in the US. This program eventually concluded that using the above-mentioned <sup>178m2</sup>Hf [[nuclear isomer]] of hafnium  to construct high-yield weapons with X-ray triggering mechanisms—an application of [[induced gamma emission]]—was infeasible because of its expense. See ''[[hafnium controversy]]''.

Hafnium [[metallocene]] compounds can be prepared from [[hafnium tetrachloride]] and various [[cyclopentadiene]]-type [[ligand]] species. Perhaps the simplest hafnium metallocene is hafnocene dichloride. Hafnium metallocenes are part of a large collection of Group 4 [[transition metal]] metallocene catalysts <ref>{{cite journal|journal = Chem. Soc. Rev.|date=1998|volume=27|issue=5|pages=323–329|title=Fluorenyl complexes of zirconium and hafnium as catalysts for olefin polymerization|doi=10.1039/a827323z|last1=g. Alt|first1=Helmut|last2=Samuel|first2=Edmond}}</ref> that are used worldwide in the production of [[polyolefin]] resins like [[polyethylene]] and [[polypropylene]].

A pyridyl-amidohafnium catalyst can be used for the controlled iso-selective polymerization of propylene which can then be combined with polyethylene to make a much tougher recycled plastic.<ref>{{cite journal |last=Eagan |first=James |date=24 Feb 2017 |title=Combining polyethylene and polypropylene: Enhanced performance with PE/iPP multiblock polymers |journal=Science |volume=355 |issue=6327 |pages=814–816 |doi=10.1126/science.aah5744|pmid=28232574 |bibcode=2017Sci...355..814E |s2cid=206652330 |url=https://zenodo.org/record/891450 |doi-access=free }}</ref>

[[Hafnium diselenide]] is studied in [[spintronics]] thanks to its [[charge density wave]] and [[superconductivity]].<ref>{{cite journal|url=https://phys.org/news/2022-09-road-spin-polarized-currents.amp|publisher=[[Phys.org]]|title=A new road towards spin-polarized currents|date=September 7, 2022|author=[[Helmholtz Association|Helmholtz Association of German Research Centres]]|journal=Nature Communications|volume=13|issue=1|page=4147|doi=10.1038/s41467-022-31539-2|pmid=35842436|pmc=9288546|access-date=September 8, 2023|archive-date=September 9, 2022|archive-url=https://archive.today/20220909224109/https://phys.org/news/2022-09-road-spin-polarized-currents.amp|url-status=bot: unknown}}</ref>

==Precautions==
Care needs to be taken when [[machining]] hafnium because it is [[pyrophoric]]—fine particles can spontaneously combust when exposed to air. Compounds that contain this metal are rarely encountered by most people. The pure metal is not considered toxic, but hafnium compounds should be handled as if they were toxic because the ionic forms of metals are normally at greatest risk for toxicity, and limited animal testing has been done for hafnium compounds.<ref>{{cite web|url = https://www.osha.gov/SLTC/healthguidelines/hafnium/index.html|title = Occupational Safety & Health Administration: Hafnium|publisher = U.S. Department of Labor|access-date = 2008-09-10|archive-url = https://web.archive.org/web/20080313003040/https://www.osha.gov/SLTC/healthguidelines/hafnium/index.html|archive-date = 2008-03-13}}</ref>

People can be exposed to hafnium in the workplace by breathing, swallowing, skin, and eye contact. The [[Occupational Safety and Health Administration]] (OSHA) has set the legal limit ([[permissible exposure limit]]) for exposure to hafnium and hafnium compounds in the workplace as TWA 0.5&nbsp;mg/m<sup>3</sup> over an 8-hour workday. The [[National Institute for Occupational Safety and Health]] (NIOSH) has set the same [[recommended exposure limit]] (REL). At levels of 50&nbsp;mg/m<sup>3</sup>, hafnium is [[IDLH|immediately dangerous to life and health]].<ref>{{Cite web|title = CDC – NIOSH Pocket Guide to Chemical Hazards – Hafnium|url = https://www.cdc.gov/niosh/npg/npgd0309.html|website = www.cdc.gov|access-date = 2015-11-03}}</ref>

==References==
{{Reflist|30em}}

==Further reading==
* {{cite book |last=Scerri |first=E.R. |author-link=Eric Scerri |date=2013 |title=A tale of seven elements |url= |location=Oxford |publisher=Oxford University Press |page= |isbn=978-0-19-539131-2}}

==External links==
{{Commons}}
{{Wiktionary}}
* [http://periodic.lanl.gov/72.shtml Hafnium] at [[Los Alamos National Laboratory]]'s [http://periodic.lanl.gov/index.shtml periodic table of the elements]
* [http://www.periodicvideos.com/videos/072.htm Hafnium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [http://www.americanelements.com/hf.htm Hafnium Technical & Safety Data]
* [http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+hafnium,+elemental NLM Hazardous Substances Databank – Hafnium, elemental]
* Don Clark: [https://www.wsj.com/articles/SB119481053795589302 Intel Shifts from Silicon to Lift Chip Performance] – WSJ, 2007
* [https://web.archive.org/*/www.intel.com/technology/45nm/index.htm?iid=homepage+marquee_45nm Hafnium-based Intel 45nm Process Technology]
* [https://www.cdc.gov/niosh/npg/npgd0309.html CDC – NIOSH Pocket Guide to Chemical Hazards]

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