Editing Iridium (section)

=== 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>