Iridium is achemical element; it hassymbolIr andatomic number 77. A very hard, brittle, silvery-whitetransition metal of theplatinum group, it is considered the second-densest naturally occurring metal (afterosmium) with a density of 22.56 g/cm3 (0.815 lb/cu in) as defined by experimentalX-ray crystallography.[a]191Ir and193Ir are the only two naturally occurringisotopes of iridium, as well as the onlystable isotopes; the latter is the more abundant. It is one of the mostcorrosion-resistant metals, even at temperatures as high as 2,000 °C (3,630 °F).
Iridium was discovered in 1803 in the acid-insoluble residues ofplatinum ores by the English chemistSmithson Tennant. The nameiridium, derived from the Greek wordiris (rainbow), refers to the various colors of its compounds. Iridium isone of the rarest elements inEarth's crust, with an estimated annual production of only 6,800 kilograms (15,000 lb) in 2023.
The dominant uses of iridium are the metal itself and its alloys, as in high-performancespark plugs,crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in thechloralkali process. Important compounds of iridium are chlorides and iodides in industrialcatalysis. Iridium is a component of someOLEDs.
A member of theplatinum group metals, iridium is white, resembling platinum, but with a slight yellowish cast. Because of its hardness, brittleness, and very highmelting point, solid iridium is difficult to machine, form, or work; thuspowder metallurgy is commonly employed instead.[12] It is the only metal to maintain good mechanical properties in air at temperatures above 1,600 °C (2,910 °F).[13] It has the 10th highestboiling point among all elements and becomes asuperconductor at temperatures below 0.14 K (−273.010 °C; −459.418 °F).[14]
Iridium'smodulus of elasticity is the second-highest among the metals, being surpassed only byosmium.[13] This, together with a highshear modulus and a very low figure forPoisson's ratio (the relationship of longitudinal to lateralstrain), 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.[13]
The measureddensity of iridium is only slightly lower (by about 0.12%) than that of osmium, thedensest metal known.[15][16] 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,[17] but, with increased accuracy in factors used for calculating density,X-ray crystallographic data yielded densities of 22.56 g/cm3 (0.815 lb/cu in) for iridium and 22.59 g/cm3 (0.816 lb/cu in) for osmium.[18]
Iridium is extremely brittle, to the point of being hard toweld because the heat-affected zone cracks, but it can be made more ductile by addition of small quantities oftitanium andzirconium (0.2% of each apparently works well).[19]
TheVickers hardness of pure platinum is 56 HV, whereas platinum with 50% of iridium can reach over 500 HV.[20][21]
Iridium is the mostcorrosion-resistant metal known.[22] It is not attacked byacids, includingaqua regia, but it can be dissolved in concentrated hydrochloric acid in the presence of sodium perchlorate. In the presence ofoxygen, it reacts withcyanide salts.[23] Traditionaloxidants also react, including thehalogens and oxygen[24] at higher temperatures.[25] Iridium also reacts directly withsulfur at atmospheric pressure to yieldiridium disulfide.[26]
Iridium has two naturally occurring stableisotopes,191Ir and193Ir, withnatural abundances of 37.3% and 62.7%, respectively.[27] At least 37radioisotopes have also been synthesized, ranging inmass number from 164 to 202.192Ir, which falls between the two stable isotopes, is the most stableradioisotope, with ahalf-life of 73.827 days, and finds application inbrachytherapy[28] and in industrialradiography, particularly fornondestructive 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—188Ir,189Ir, and190Ir.[27] Isotopes with masses below 191 decay by some combination ofβ+ decay,α decay, and (rare)proton emission, with the exception of189Ir, which decays byelectron capture. Synthetic isotopes heavier than 191 decay byβ− decay, although192Ir also has a minor electron capture decay path.[27] All known isotopes of iridium were discovered between 1934 and 2008, with the most recent discoveries being200–202Ir.[29]
At least 32metastable isomers have been characterized, ranging in mass number from 164 to 197. The most stable of these is192m2Ir, which decays byisomeric transition with a half-life of 241 years,[27] making it more stable than any of iridium's synthetic isotopes in their ground states. The least stable isomer is190m3Ir with a half-life of only 2 μs.[27] The isotope191Ir was the first one of any element to be shown to present aMössbauer effect. This renders it useful forMössbauer spectroscopy for research in physics, chemistry,biochemistry,metallurgy, andmineralogy.[30]
Iridium forms compounds inoxidation states between −3 and +9, but the most common oxidation states are +1, +2, +3, and +4.[12] Well-characterized compounds containing iridium in the +6 oxidation state includeIrF6 and the oxidesSr2MgIrO6 andSr2CaIrO6.[12][31]iridium(VIII) oxide (IrO4) was generated under matrix isolation conditions at 6 K inargon.[32] The highest oxidation state (+9), which is also the highest recorded forany element, is found in gaseous[IrO4]+.[5]
Binary trihalides,IrX 3, are known for all of the halogens.[12] For oxidation states +4 and above, only thetetrafluoride,pentafluoride andhexafluoride are known.[12] Iridium hexafluoride,IrF 6, is a volatile yellow solid, composed of octahedral molecules. It decomposes in water and is reduced toIrF 4.[12] Iridium pentafluoride is also a strong oxidant, but it is atetramer,Ir 4F 20, formed by four corner-sharing octahedra.[12]
Iridium in its complexes is alwayslow-spin. Ir(III) and Ir(IV) generally formoctahedral complexes.[12] Polyhydride complexes are known for the +5 and +3 oxidation states.[33] One example isIrH5(PiPr3)2 (iPr =isopropyl).[34] The ternary hydrideMg 6Ir 2H 11 is believed to contain both theIrH4− 5 and the 18-electronIrH5− 4 anion.[35]
Iridium also formsoxyanions with oxidation states +4 and +5.K 2IrO 3 andKIrO 3 can be prepared from the reaction ofpotassium oxide orpotassium superoxide with iridium at high temperatures. Such solids are not soluble in conventional solvents.[36]
Just like many elements, iridium forms important chloride complexes. Hexachloroiridic (IV) acid,H 2IrCl 6, and itsammonium salt are common iridium compounds from both industrial and preparative perspectives.[37] They are intermediates in the purification of iridium and used as precursors for most other iridium compounds, as well as in the preparation ofanode coatings. TheIrCl2− 6 ion has an intense dark brown color, and can be readily reduced to the lighter-coloredIrCl3− 6 and vice versa.[37]Iridium trichloride,IrCl 3, which can be obtained inanhydrous form from direct oxidation of iridium powder bychlorine at 650 °C,[37] or in hydrated form by dissolvingIr 2O 3 inhydrochloric acid, is often used as a starting material for the synthesis of other Ir(III) compounds.[12] Another compound used as a starting material is potassium hexachloroiridate(III),K3IrCl6.[38]
The Greek goddessIris, after whom iridium was named.
The discovery of iridium is intertwined with that of platinum and the other metals of theplatinum group. The first European reference to platinum appears in 1557 in the writings of the Italian humanistJulius Caesar Scaliger as a description of an unknown noble metal found betweenDarién and Mexico, "which no fire nor any Spanish artifice has yet been able toliquefy".[44] From their first encounters with platinum, the Spanish generally saw the metal as a kind ofimpurity in gold, and it was treated as such. It was often simply thrown away, and there was an official decree forbidding theadulteration of gold with platinum impurities.[45]
Thisalchemical symbol for platinum was made by joining the symbols of silver (moon) and gold (sun).Antonio de Ulloa is credited in European history with the discovery of platinum.
In 1735,Antonio de Ulloa andJorge Juan y Santacilia saw Native Americans mining platinum while theSpaniards were travelling throughColombia andPeru for eight years. Ulloa and Juan found mines with the whitish metalnuggets and took them home to Spain. Ulloa returned to Spain and established the firstmineralogy lab in Spain and was the first to systematically study platinum, which was in 1748. His historical account of the expedition included a description of platinum as being neitherseparable norcalcinable. Ulloa also anticipated the discovery of platinum mines. After publishing the report in 1748, Ulloa did not continue to investigate the new metal. In 1758, he was sent to superintendmercury mining operations inHuancavelica.[44]
In 1750, after studying the platinum sent to him by Wood, Brownrigg presented a detailed account of the metal to theRoyal Society, stating that he had seen no mention of it in any previous accounts of known minerals.[47] Brownrigg also made note of platinum's extremely high melting point and refractory metal-like behaviour towardborax. Other chemists across Europe soon began studying platinum, includingAndreas Sigismund Marggraf,[48]Torbern Bergman,Jöns Jakob Berzelius,William Lewis, andPierre Macquer. In 1752,Henrik Scheffer published a detailed scientific description of the metal, which he referred to as "white gold", including an account of how he succeeded in fusing platinum ore with the aid ofarsenic. Scheffer described platinum as being lesspliable than gold, but with similar resistance tocorrosion.[44]
In 1803 British scientistSmithson Tennant (1761–1815) analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternately withalkali and acids[22] and obtained a volatile new oxide, which he believed to be of this new metal—which he namedptene, from the Greek wordπτηνόςptēnós, "winged".[49][50] Tennant, who had the advantage of a much greater amount of residue, continued his research and identified the two previously undiscovered elements in the black residue, iridium andosmium.[13][22] He obtained dark red crystals (probably ofNa 2[IrCl 6]·nH 2O) by a sequence of reactions withsodium hydroxide andhydrochloric acid.[50] He named iridium afterIris (Ἶρις), the Greek winged goddess of therainbow and the messenger of theOlympian gods, because many of thesalts he obtained were strongly colored.[c][51] Discovery of the new elements was documented in a letter to theRoyal Society on June 21, 1804.[13][52]
British scientistJohn George Children was the first to melt a sample of iridium in 1813 with the aid of "the greatest galvanic battery that has ever been constructed" (at that time).[13] The first to obtain high-purity iridium wasRobert Hare in 1842. He found it had a density of around 21.8 g/cm3 (0.79 lb/cu in) and noted the metal is nearlyimmalleable and very hard. The first melting in appreciable quantity was done byHenri Sainte-Claire Deville andJules Henri Debray in 1860. They required burning more than 300 litres (79 US gal) of pureO 2 andH 2 gas for each 1 kilogram (2.2 lb) of iridium.[13]
These extreme difficulties in melting the metal limited the possibilities for handling iridium.John Isaac Hawkins was looking to obtain a fine and hard point forfountain pennibs, and in 1834 managed to create an iridium-pointed gold pen. In 1880,John Holland andWilliam Lofland Dudley were able to melt iridium by addingphosphorus and patented the process in the United States; British companyJohnson Matthey later stated they had been using a similar process since 1837 and had already presented fused iridium at a number ofWorld Fairs.[13] The first use of analloy of iridium withruthenium inthermocouples was made by Otto Feussner in 1933. These allowed for the measurement of high temperatures in air up to 2,000 °C (3,630 °F).[13]
InMunich, Germany in 1957Rudolf Mössbauer, in what has been called one of the "landmark experiments in twentieth-century physics",[53] discovered the resonant andrecoil-free emission and absorption ofgamma rays byatoms in a solid metal sample containing only191Ir.[54] This phenomenon, known as theMössbauer effect resulted in the awarding of theNobel Prize in Physics in 1961, at the age 32, just three years after he published his discovery.[55]
Iridium is one of the least abundant elements in Earth's crust.TheWillamette Meteorite, the sixth-largest meteorite found in the world, has 4.7 ppm iridium.[59]
Iridium is one of the nine least abundant stableelements inEarth's crust, having an averagemass fraction of 0.001 ppm in crustal rock;gold is 4 times more abundant,platinum is 10 times more abundant,silver andmercury are 80 times more abundant.[12]Osmium,tellurium,ruthenium,rhodium andrhenium are about as abundant as iridium.[60] In contrast to its low abundance in crustal rock, iridium is relatively common inmeteorites, with concentrations of 0.5 ppm or more.[61] The overall concentration of iridium on Earth is thought to be much higher than what is observed in crustal rocks, but because of the density andsiderophilic ("iron-loving") character of iridium, it descended below the crust and intoEarth's core when the planet was stillmolten.[37]
Iridium is found in nature as an uncombined element or in naturalalloys, especially the iridium–osmium alloysosmiridium (osmium-rich) andiridosmium (iridium-rich).[22] Innickel and copper deposits, theplatinum group metals occur assulfides,tellurides,antimonides, andarsenides. In all of these compounds,platinum can be exchanged with a small amount of iridium or osmium. As with all of the platinum group metals, iridium can be found naturally in alloys with raw nickel orraw copper.[62] A number of iridium-dominantminerals, with iridium as the species-forming element, are known. They are exceedingly rare and often represent the iridium analogues of the above-given ones. The examples are irarsite and cuproiridsite, to mention some.[63][64][65] Within Earth's crust, iridium is found at highest concentrations in three types ofgeologic structure:igneous deposits (crustal intrusions from below),impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in theBushveld igneous complex in South Africa,[66] (near the largest known impact structure, theVredefort impact structure) though the large copper–nickel deposits nearNorilsk in Russia, and theSudbury Basin (also an impact crater) in Canada are also significant sources of iridium. Smaller reserves are found in the United States.[66] Iridium is also found in secondary deposits, combined withplatinum and otherplatinum group metals inalluvial deposits. The alluvial deposits used bypre-Columbian people in theChocó Department ofColombia are still a source for platinum-group metals. As of 2003, world reserves have not been estimated.[22]
Iridium in sediments can come fromcosmic dust, volcanoes,precipitation from seawater, microbial processes, orhydrothermal vents,[69] and its abundance can be strongly indicative of the source.[70][69] It tends to associate with other ferrous metals inmanganese nodules.[67] Iridium is one of the characteristic elements of extraterrestrial rocks, and, along with osmium, can be used as a tracer element for meteoritic material in sediment.[71][72] For example, core samples from the Pacific Ocean with elevated iridium levels suggested theEltanin impact of about 2.5 million years ago.[11]
Worldwide production of iridium was about 7,300 kilograms (16,100 lb) in 2018.[80] The price is high and varying (see table). Illustrative factors that affect the price include oversupply of Ir crucibles[79][81]and changes inLED technology.[82]
Platinum metals occur together as dilute ores. Iridium is one of the rarer platinum metals: for every 190 tonnes of platinum obtained from ores, only 7.5 tonnes of iridium is isolated.[83] To separate the metals, they must first be brought intosolution. Two methods for rendering Ir-containing ores soluble are (i) fusion of the solid withsodium peroxide followed by extraction of the resulting glass inaqua regia and (ii) extraction of the solid with a mixture ofchlorine withhydrochloric acid.[37][66] From soluble extracts, iridium is separated by precipitating solidammonium hexachloroiridate ((NH 4) 2IrCl 6) or by extractingIrCl2− 6 with organic amines.[84] The first method is similar to the procedure Tennant and Wollaston used for their original separation. The second method can be planned as continuousliquid–liquid extraction and is therefore more suitable for industrial scale production. In either case, the product, an iridium chloride salt, is reduced with hydrogen, yielding the metal as a powder orsponge, which is amenable topowder metallurgy techniques.[85][86] Iridium is also obtained commercially as a by-product fromnickel and copper mining and processing. Duringelectrorefining of copper and nickel, noble metals such as silver, gold and theplatinum group metals as well asselenium andtellurium settle to the bottom of the cell asanode mud, which forms the starting point for their extraction.[79]
Due to iridium's resistance to corrosion it has industrial applications. The main areas of use are electrodes for producing chlorine and other corrosive products,OLEDs, crucibles,catalysts (e.g.acetic acid), and ignition tips for spark plugs.[83]
Resistance to heat and corrosion are the bases for several uses of iridium and its alloys.
Owing to its high melting point, hardness, andcorrosion resistance, iridium is used to make crucibles. Suchcrucibles are used in theCzochralski process to produce oxide single-crystals (such assapphires) for use in computer memory devices and in solid state lasers.[88][89] The crystals, such asgadolinium gallium garnet and yttrium gallium garnet, are grown by melting pre-sintered charges of mixed oxides under oxidizing conditions at temperatures up to 2,100 °C (3,810 °F).[13]
Certain long-life aircraft engine parts are made of an iridium alloy, and an iridium–titanium alloy is used for deep-water pipes because of its corrosion resistance.[22] Iridium is used formulti-poredspinnerets, through which a plastic polymer melt is extruded to form fibers, such asrayon.[90] Osmium–iridium is used for compass bearings and for balances.[13]
Because of their resistance to arc erosion, iridium alloys are used by some manufacturers for the centre electrodes ofspark plugs,[88][91] and iridium-based spark plugs are particularly used in aviation.
The radioisotopeiridium-192 is one of the two most important sources of energy for use in industrialγ-radiography fornon-destructive testing of metals.[97][98] Additionally,192 Ir is used as a source ofgamma radiation for the treatment of cancer usingbrachytherapy, a form of radiotherapy where a sealed radioactive source is placed inside or next to the area requiring treatment. Specific treatments include high-dose-rate prostate brachytherapy, biliary duct brachytherapy, and intracavitary cervix brachytherapy.[22]Iridium-192 is normally produced by neutron activation of isotopeiridium-191 in natural-abundance iridium metal.[99]
Iridium–osmium alloys were used infountain pennib tips. The first major use of iridium was in 1834 in nibs mounted on gold.[13] Starting in 1944, theParker 51 fountain pen was fitted with a nib tipped by a ruthenium and iridium alloy (with 3.8% iridium). The tip material in modern fountain pens is still conventionally called "iridium", although there is seldom any iridium in it; other metals such asruthenium,osmium, andtungsten have taken its place.[103]
An iridium–platinum alloy was used for thetouch holes or vent pieces ofcannon. According to a report of theParis Exhibition of 1867, one of the pieces being exhibited byJohnson and Matthey "has been used in a Whitworth gun for more than 3000 rounds, and scarcely shows signs of wear yet. Those who know the constant trouble and expense which are occasioned by the wearing of the vent-pieces of cannon when in active service, will appreciate this important adaptation".[104]
The pigmentiridium black, which consists of very finely divided iridium, is used for paintingporcelain an intense black; it was said that "all other porcelain black colors appear grey by the side of it".[105]
Iridium in bulk metallic form is not biologically important or hazardous to health due to its lack of reactivity with tissues; there are only about 20 parts per trillion of iridium in human tissue.[22] Like most metals, finely divided iridium powder can be hazardous to handle, as it is an irritant and may ignite in air.[66] Iridium is relatively unhazardous otherwise, with the only effect of Iridium ingestion being irritation of thedigestive tract.[106] However, soluble salts, such as the iridium halides, could be hazardous due to elements other than iridium or due to iridium itself.[28] At the same time, most iridium compounds are insoluble, which makes absorption into the body difficult.[22]
A radioisotope of iridium,192 Ir, is dangerous, like other radioactive isotopes. The only reported injuries related to iridium concern accidental exposure to radiation from192 Ir used inbrachytherapy.[28] High-energy gamma radiation from192 Ir can increase the risk of cancer. External exposure can cause burns,radiation poisoning, and death. Ingestion of192Ir can burn the linings of the stomach and the intestines.[107]192Ir,192mIr, and194mIr tend to deposit in theliver, and can pose health hazards from bothgamma andbeta radiation.[61]
^At room temperature and standard atmospheric pressure, iridium has been calculated to have a density of 22.65 g/cm3 (0.818 lb/cu in), 0.04 g/cm3 (0.0014 lb/cu in) higher than osmium measured the same way. Still, the experimental X-ray crystallography value is considered to be the most accurate, and as such iridium is considered to be the second densest element.[9]
^Most common oxidation states of iridium are in bold. The right column lists one representative compound for each oxidation state.
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^General section citations:Recalibration of the U.S. National Prototype Kilogram, R.S.Davis, Journal of Research of the National Bureau of Standards,90, No. 4,July–August 1985 (5.5MB PDFArchived 2017-02-01 at theWayback Machine); andThe Kilogram and Measurements of Mass and Force, Z.J.Jabbouret al., J. Res. Natl. Inst. Stand. Technol.106, 2001,25–46 (3.5MB PDF)
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