Editing Iridium (section)

== Characteristics ==

=== Physical properties ===
[[File:iridium2.jpg|left|thumb|{{convert|1|ozt|g|4|spell=In|abbr=off|lk=on}} of arc-melted iridium|alt=A flattened drop of dark gray substance]]
A member of the [[platinum group]] metals, iridium is white, resembling platinum, but with a slight yellowish cast. Because of its hardness, brittleness, and very high [[melting point]], solid iridium is difficult to machine, form, or work; thus [[powder metallurgy]] is commonly employed instead.<ref name="greenwood" /> It is the only metal to maintain good mechanical properties in air at temperatures above {{convert|1600|C|F}}.<ref name="hunt">{{cite journal |title=A History of Iridium |first=L. B. |last=Hunt |journal=Platinum Metals Review |volume=31 |issue=1 |date=1987 |pages=32–41 |doi=10.1595/003214087X3113241 |s2cid=267552692 |url=https://technology.matthey.com/documents/496120/626258/pmr-v31-i1-032-041.pdf/ |access-date=2022-09-29 |archive-date=2022-09-29 |archive-url=https://web.archive.org/web/20220929092320/https://technology.matthey.com/documents/496120/626258/pmr-v31-i1-032-041.pdf/ |url-status=dead }}</ref> It has the 10th highest [[List of elements by boiling point|boiling point among all elements]] and becomes a [[superconductor]] at temperatures below {{convert|0.14|K|°C °F|lk=in}}.<ref>{{cite book |last=Kittel |first=C.|title=[[Introduction to Solid State Physics]] |edition=7th |publisher=Wiley-India |date=2004 |isbn=978-81-265-1045-0}}</ref>

Iridium's [[modulus of elasticity]] is the second-highest among the metals, being surpassed only by [[osmium]].<ref name="hunt" /> This, together with a high [[shear modulus]] and a very low figure for [[Poisson's ratio]] (the relationship of longitudinal to lateral [[strain (chemistry)|strain]]), indicate the high degree of stiffness and resistance to deformation that have rendered its fabrication into useful components a matter of great difficulty. Despite these limitations and iridium's high cost, a number of applications have developed where mechanical strength is an essential factor in some of the extremely severe conditions encountered in modern technology.<ref name="hunt" />

The measured [[density]] of iridium is only slightly lower (by about 0.12%) than that of osmium, the [[List of elements by density|densest metal]] known.<ref>{{cite journal|title=Osmium, the Densest Metal Known |author=Arblaster, J. W. |journal=Platinum Metals Review |volume=39 |issue=4 |date=1995 |page=164 |doi=10.1595/003214095X394164164 |s2cid=267393021 |url=http://www.platinummetalsreview.com/dynamic/article/view/pmr-v39-i4-164-164 |access-date=2008-10-02 |archive-url=https://web.archive.org/web/20110927045236/http://www.platinummetalsreview.com/dynamic/article/view/pmr-v39-i4-164-164 |archive-date=2011-09-27 |url-status=dead}}</ref><ref>{{cite book |last=Cotton |first=Simon |title=Chemistry of Precious Metals |page=78 |publisher=Springer-Verlag New York, LLC |date=1997 |isbn=978-0-7514-0413-5}}</ref> Some ambiguity occurred regarding which of the two elements was denser, due to the small size of the difference in density and difficulties in measuring it accurately,<ref name="crc">{{cite book |author=Lide, D. R. |title=CRC Handbook of Chemistry and Physics. |url=https://archive.org/details/crchandbookofche00lide |url-access=registration |edition=70th |publisher=Boca Raton (FL):CRC Press |date=1990 |isbn=9780849304712}}</ref> but, with increased accuracy in factors used for calculating density, [[X-ray crystallography|X-ray crystallographic]] data yielded densities of {{cvt|22.56|g/cm3}} for iridium and {{cvt|22.59|g/cm3}} for osmium.<ref>{{cite journal|url=https://technology.matthey.com/article/33/1/14-16/|title=Densities of osmium and iridium: recalculations based upon a review of the latest crystallographic data|author=Arblaster, J. W.|journal=Platinum Metals Review|volume=33|issue=1|date=1989|pages=14–16|doi=10.1595/003214089X3311416 |s2cid=267570193 |access-date=2008-09-17|archive-date=2012-02-07|archive-url=https://web.archive.org/web/20120207064113/http://www.platinummetalsreview.com/pdf/pmr-v33-i1-014-016.pdf|url-status=dead}}</ref>

Iridium is extremely brittle, to the point of being hard to [[Welding|weld]] because the heat-affected zone cracks, but it can be made more ductile by addition of small quantities of [[titanium]] and [[zirconium]] (0.2% of each apparently works well).<ref>{{cite patent|country=US |number=3293031A|invent1=Cresswell, Peter|invent2=Rhys, David|pridate=23/12/1963|fdate=27/11/1964|pubdate=20/12/1966}}</ref>

The [[Vickers hardness]] of pure platinum is 56&nbsp;HV, whereas platinum with 50% of iridium can reach over 500&nbsp;HV.<ref>{{cite journal| url = https://technology.matthey.com/article/4/1/18-26/| journal = Platinum Metals Review| title = Iridium Platinum Alloys – A Critical Review Of Their Constitution And Properties| first = A. S.|last = Darling| date = 1960| volume =4| issue = 1| pages = 18–26| doi = 10.1595/003214060X411826| s2cid = 267392937}} Reviewed in {{Cite journal|s2cid=4211238 | doi = 10.1038/186211a0| bibcode = 1960Natur.186Q.211.| title = Iridium–Platinum Alloys| journal = Nature| year = 1960| volume = 186| issue = 4720| page = 211| doi-access = free}}</ref><ref>{{cite journal|doi = 10.1595/147106705X24409| title = The Hardening of Platinum Alloys for Potential Jewellery Application| first = T.|last = Biggs| author2=Taylor, S. S.| author3=van der Lingen, E.| journal = Platinum Metals Review| date = 2005| volume = 49| issue = 1| pages = 2–15| doi-access = free}}</ref>

=== Chemical properties ===
Iridium is the most [[corrosion-resistant]] metal known.<ref name="Emsley" /> It is not attacked by [[acid]]s, including [[aqua regia]], but it can be dissolved in concentrated hydrochloric acid in the presence of sodium perchlorate. In the presence of [[oxygen]], it reacts with [[cyanide]] salts.<ref name="emsley">{{cite book|last=Emsley|first=John|title=Nature's Building Blocks: An A–Z Guide to the Elements|edition=New|year=2011 |publisher=Oxford University Press|location=New York |isbn=978-0-19-960563-7}}</ref> Traditional [[Oxidizing agent|oxidants]] also react, including the [[halogen]]s and oxygen<ref name="perry">{{cite book|title=Handbook of Inorganic Compounds| author=Perry, D. L.| pages=203–204| date=1995| isbn=978-1439814611| publisher=CRC Press}}</ref> at higher temperatures.<ref name="lagowski">{{cite book| title=Chemistry Foundations and Applications| volume=2| editor=Lagowski, J. J.| pages=[https://archive.org/details/chemistryfoundat0000unse/page/250 250–251]| date=2004| isbn=978-0028657233| publisher=Thomson Gale| url=https://archive.org/details/chemistryfoundat0000unse/page/250}}</ref> Iridium also reacts directly with [[sulfur]] at atmospheric pressure to yield [[iridium disulfide]].<ref name = Munson-1968>{{cite journal
 |url          = https://htracyhall.org/ocr/HTH-Archives/Cabinet%208/Drawer%203%20(MATI%20-%20MOZ)/(Munson,%20R.A.)%20(Muntoni,%20C.)%20(Murase,%20K.)%20(linked)/(Munson,%20R.A.)%20(Muntoni,%20C.)%20(Murase,%20K.)-237_OCR.pdf
 |author-last  = Munson
 |author-first = Ronald A.
 |date         = February 1968
 |title        = The Synthesis of Iridium Disulfide and Nickel diarsenide having the Pyrite Structure
 |journal      = Inorganic Chemistry
 |volume       = 7
 |number       = 2
 |pages        = 389–390
 |doi          = 10.1021/ic50060a047
 |access-date  = 2019-01-19
 |archive-date = 2019-04-12
 |archive-url  = https://web.archive.org/web/20190412090001/https://htracyhall.org/ocr/HTH-Archives/Cabinet%208/Drawer%203%20(MATI%20-%20MOZ)/(Munson,%20R.A.)%20(Muntoni,%20C.)%20(Murase,%20K.)%20(linked)/(Munson,%20R.A.)%20(Muntoni,%20C.)%20(Murase,%20K.)-237_OCR.pdf
 |url-status   = dead
}}</ref>

=== Isotopes ===
{{Main|Isotopes of iridium}}
Iridium has two naturally occurring stable [[isotope]]s, <sup>191</sup>Ir and <sup>193</sup>Ir, with [[natural abundance]]s of 37.3% and 62.7%, respectively.<ref name="nubase" /> At least 37 [[radioisotope]]s have also been synthesized, ranging in [[mass number]] from 164 to 202. [[iridium-192|<sup>192</sup>Ir]], which falls between the two stable isotopes, is the most stable [[Radionuclide|radioisotope]], with a [[half-life]] of 73.827&nbsp;days, and finds application in [[brachytherapy]]<ref name="mager" /> and in industrial [[radiography]], particularly for [[nondestructive testing]] of welds in steel in the oil and gas industries; iridium-192 sources have been involved in a number of radiological accidents. Three other isotopes have half-lives of at least a day—<sup>188</sup>Ir, <sup>189</sup>Ir, and <sup>190</sup>Ir.<ref name="nubase" /> Isotopes with masses below 191 decay by some combination of [[Beta decay#β+ decay|β<sup>+</sup> decay]], [[alpha decay|α decay]], and (rare) [[proton emission]], with the exception of <sup>189</sup>Ir, which decays by [[electron capture]]. Synthetic isotopes heavier than 191 decay by [[Beta decay#β− decay|β<sup>−</sup> decay]], although <sup>192</sup>Ir also has a minor electron capture decay path.<ref name="nubase">{{NUBASE 2003}}</ref> All known isotopes of iridium were discovered between 1934 and 2008, with the most recent discoveries being <sup>200–202</sup>Ir.<ref>{{cite journal |title=Discovery of tantalum, rhenium, osmium, and iridium isotopes |last1=Robinson |first1=R. |last2=Thoennessen |first2=M. |journal=Atomic Data and Nuclear Data Tables |volume=98 |issue=5 |date=2012 |pages=911–932 |arxiv=1109.0526 |doi=10.1016/j.adt.2011.09.003 |bibcode=2012ADNDT..98..911R |s2cid=53992437}}</ref>

At least 32 [[nuclear isomer|metastable isomers]] have been characterized, ranging in mass number from 164 to 197. The most stable of these is <sup>192m2</sup>Ir, which decays by [[isomeric transition]] with a half-life of 241&nbsp;years,<ref name="nubase" /> making it more stable than any of iridium's synthetic isotopes in their ground states. The least stable isomer is <sup>190m3</sup>Ir with a half-life of only 2&nbsp;μs.<ref name="nubase" /> The isotope <sup>191</sup>Ir was the first one of any element to be shown to present a [[Mössbauer effect]]. This renders it useful for [[Mössbauer spectroscopy]] for research in physics, chemistry, [[biochemistry]], [[metallurgy]], and [[mineralogy]].<ref name="ir-191">{{cite book |title=Handbook of Ceramics and Composites |author=Chereminisoff, N. P. |publisher=CRC Press |date=1990 |isbn=978-0-8247-8006-7 |page=424}}</ref>