Iridium Wiki

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Iridium 1 Iridium Iridium Appearance silvery white General properties Name, symbol, number iridium, Ir, 77 Pronunciation  / ɪˈrɪdiəm/ i-  rid -ee-  əm Element category transition metal Group, period, block 9, 6, d Standard atomic weight 192.217 g·mol 1 Electron configuration [Xe] 4f 14 5d 7 6s 2 Electrons per shell 2, 8, 18, 32, 15, 2 (Image) Physical properties Phase solid Density (near r.t.) 22.56 [1]  g·cm 3 Liquid density at m.p. 19 g·cm 3 Melting point 2739 K,2466 °C,4471 °F Boiling point 4701 K,4428 °C,8002 °F Heat of fusion 41.12 kJ·mol 1 Heat of vaporization 563 kJ·mol 1 Specific heat capacity (25 °C) 25.10 J·mol 1 ·K 1 Vapor pressure  P  /Pa 1 10 100 1 k 10 k 100 k at T  /K 2713 2957 3252 3614 4069 4659 Atomic properties Oxidation states 3,1, 0, 1, 2, 3, 4, 5, 6 Electronegativity 2.20 (Pauling scale)

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Iridium

Iridium

Appearance

silvery white

General propertiesName, symbol, number iridium, Ir, 77

Pronunciation / ɪˈrɪdiəm/ i- rid -ee- ə m

Element category transition metal

Group, period, block 9, 6, d

Standard atomic weight 192.217 g·mol− 1

Electron configuration [Xe] 4f 145d7 6s2

Electrons per shell 2, 8, 18, 32, 15, 2 (Image)

Physical properties

Phase solid

Density (near r.t.) 22.56[1] g·cm− 3

Liquid density at m.p. 19 g·cm− 3

Melting point 2739 K,2466 °C,4471 °F

Boiling point 4701 K,4428 °C,8002 °F

Heat of fusion 41.12 kJ·mol− 1

Heat of vaporization 563 kJ·mol− 1

Specific heat capacity (25 °C) 25.10 J·mol− 1·K− 1

Vapor pressure

P /Pa 1 10 100 1 k 10 k 100 k

at T /K 2713 2957 3252 3614 4069 4659

Atomic properties

Oxidation states − 3,− 1, 0, 1, 2,3, 4, 5, 6

Electronegativity 2.20 (Pauling scale)

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Ionization energies 1st: 880 kJ·mol− 1

2nd: 1600 kJ·mol− 1

Atomic radius 136 pm

Covalent radius 141±6 pm

Miscellanea

Crystal structure face-centered cubic

Magnetic ordering paramagnetic[2]

Electrical resistivity (20 °C) 47.1 nΩ·m

Thermal conductivity (300 K) 147 W·m− 1·K− 1

Thermal expansion 6.4 µm/(m·K)

Speed of sound (thin rod) (20 °C) 4825 m/s

Young's modulus 528 GPa

Shear modulus 210 GPa

Bulk modulus 320 GPa

Poisson ratio 0.26

Mohs hardness 6.5

Vickers hardness 1760 MPa

Brinell hardness 1670 MPa

CAS registry number 7439-88-5

Most stable isotopes

iso NA half-life DM DE (MeV) DP

188Ir syn 1.73 d ε 1.64 188Os

189Ir syn 13.2 d ε 0.532 189Os

190Ir syn 11.8 d ε 2.000 190Os

191Ir 37.3% 191Ir is stable with 114 neutron

192Ir syn 73.827 d β – 1.460 192Pt

ε 1.046 192Os

192m2Ir syn 241 y IT 0.161 192Ir

193Ir 62.7% 193Ir is stable with 116 neutron

193mIr syn 10.5 d IT 0.080 193Ir

194Ir syn 19.3 h β – 2.247 194Pt

194m2Ir syn 171 d IT ? 194Ir

Iridium ( / ɪˈrɪdiəm/ i- rid -ee- ə m) is the chemical element with atomic number 77, and is represented by thesymbolIr . A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densestelement (after osmium) and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although

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only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much morereactive and can be flammable.Iridium was discovered in 1803 among insoluble impurities in natural platinum. Smithson Tennant, the primarydiscoverer, named the iridium for the goddess Iris, personification of the rainbow, because of the striking and diversecolors of its salts. Iridium is one of the rarest elements in the Earth's crust, with annual production and consumption

of only three tonnes.191

Ir and193

Ir are the only two naturally occurring isotopes of iridium as well as the only stableisotopes; the latter is the more abundant of the two.The most important iridium compounds in use are the salts and acids it forms with chlorine, though iridium alsoforms a number of organometallic compounds used in industrial catalysis, and in research. Iridium metal isemployed when high corrosion resistance at high temperatures is needed, as in high-end spark plugs, crucibles forrecrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in thechloralkali process. Iridium radioisotopes are used in some radioisotope thermoelectric generators.Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. Forthis reason the unusually high abundance of iridium in the clay layer at the K – T geologic boundary gave rise to theAlvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and manyother species 65 million years ago. It is thought that the total amount of iridium in the planet Earth is much higherthan that observed in crustal rocks, but as with other platinum group metals, the high density and tendency of iridiumto bond with iron caused most iridium to descend below the crust when the planet was young and still molten.

Characteristics

Physical properties

1 troy ounce (31 g) of arc-melted iridium

A member of the platinum group metals, iridium is white, resemblingplatinum, but with a slight yellowish cast. Because of its hardness,

brittleness, and very high melting point (the ninth highest of allelements), solid iridium is difficult to machine, form, or work, and thuspowder metallurgy is commonly employed instead.[3] It is the onlymetal to maintain good mechanical properties in air at temperaturesabove 1600 °C.[4]Iridium has a very high boiling point (10th among allelements) and becomes a superconductor at temperatures below0.14 K.[5]

Iridium's modulus of elasticity is the second highest among the metals,only being surpassed by osmium.[4]This, together with a high modulusof rigidity and a very low figure for Poisson's ratio (the relationship of

longitudinal to lateral strain), indicate the high degree of stiffness and resistance to deformation that have renderedits fabrication into useful components a matter of great difficulty. Despite these limitations and iridium's high cost, anumber of applications have developed where mechanical strength is an essential factor in some of the extremelysevere conditions encountered in modern technology.[4]

The measured density of iridium is only slightly lower (by about 0.12%) than that of osmium, the densest elementknown.[6][7]There had been some ambiguity regarding which of the two elements was denser, due to the small sizeof the difference in density and difficulties in measuring it accurately,[8]but, with increased accuracy in factors usedfor calculating density X-ray crystallographic data yielded densities of 22.56 g/cm3 for iridium and 22.59 g/cm3 forosmium.[9]

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Chemical propertiesIridium is the most corrosion-resistant metal known:[10] it is not attacked by almost any acid, aqua regia, moltenmetals or silicates at high temperatures. It can, however, be attacked by some molten salts, such as sodium cyanideand potassium cyanide,[10]as well as oxygen and the halogens (particularly fluorine)[11]at higher temperatures.[12]

Compounds

Oxidation states[13]

− 3 [Ir(CO)3]3−

− 1 [Ir(CO)3(PPh3)]−

0 Ir4(CO)12

+1 [Ir(CO)Cl(PPh3)2]

+2 IrCl2

+3 IrCl3+4 IrO2

+5 Ir4F20

+6 IrF6

Iridium forms compounds in oxidation states between−3 to +6; the most common oxidation states are +3 and +4.[3]

Well-characterized examples of the highest oxidation state are rare, but include IrF6 and two mixed oxidesSr2MgIrO6 and Sr2CaIrO6.[3][14]In addition, it was reported in 2009 that iridium(VIII) tetroxide (IrO4) was preparedunder matrix isolation conditions (6 K in Ar) by UV irradiation of an iridium-peroxo complex. This species,however, is not expected to be stable as a bulk solid at higher temperatures.[15]

Iridium dioxide, IrO2, a brown powder, is the only well-characterized oxide of iridium.[3] A sesquioxide, Ir2O3, hasbeen described as a blue-black powder which is oxidized to IrO2 by HNO3.[11] The corresponding disulfides,diselenides, sesquisulfides and sesquiselenides are known and IrS3 has also been reported.[3] Iridium also formsiridates with oxidation states +4 and +5, such as K2IrO3 and KIrO3, which can be prepared from the reaction of potassium oxide or potassium superoxide with iridium at high temperatures.[16]

While no binary hydrides of iridium, Ir xH

yare known, complexes are known that contain IrH and IrH, where iridium

has the +1 and +3 oxidation states, respectively.[17] The ternary hydride Mg6Ir2H11 is believed to contain both theIrH and the 18-electron IrH anion.[18]

No monohalides or dihalides are known, whereas trihalides, IrX3, are known for all of the halogens.[3]For oxidation

states +4 and above, only the tetrafluoride, pentafluoride and hexafluoride are known.[3] Iridium hexafluoride, IrF6,is a volatile and highly reactive yellow solid, composed of octahedral molecules. It decomposes in water and isreduced to IrF4, a crystalline solid, by iridium black.[3]Iridium pentafluoride has similar properties but it is actually atetramer, Ir4F20, formed by four corner-sharing octahedra.[3]

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Vaska's complex

Hexachloroiridic(IV) acid, H2IrCl6, and its ammonium salt are themost important iridium compounds from an industrial perspective.[19]

They are involved in the purification of iridium and used as precursorsfor most other iridium compounds, as well as in the preparation of anode coatings. The IrCl ion has an intense dark brown color, and can

be readily reduced to the lighter-colored IrCl and vice versa.[19]

Iridium trichloride, IrCl3, which can be obtained in anhydrous formfrom direct oxidation of iridium powder by chlorine at 650 °C,[19]or inhydrated form by dissolving Ir2O3 in hydrochloric acid, is often used as

a starting material for the synthesis of other Ir(III) compounds.[3] Another compound used as a starting material isammonium hexachloroiridate(III), (NH4)3IrCl6. Iridium(III) complexes are diamagnetic (low-spin) and generallyhave an octahedral molecular geometry.[3]

Organoiridium compounds contain iridium – carbon bonds where the metal is usually in lower oxidation states. Forexample, oxidation state zero is found in tetrairidium dodecacarbonyl, Ir4(CO)12, which is the most common andstable binary carbonyl of iridium.[3] In this compound, each of the iridium atoms is bonded to the other three,forming a tetrahedral cluster. Some organometallic Ir(I) compounds are notable enough to be named after theirdiscoverers. One is Vaska's complex, IrCl(CO)[P(C6H5)3]2, which has the unusual property of binding to thedioxygen molecule, O2.

[20] Another one is Crabtree's catalyst, a homogeneous catalyst for hydrogenationreactions.[21] These compounds are both square planar, d8 complexes, with a total of 16 valence electrons, whichaccounts for their reactivity.[22]

IsotopesIridium has two naturally occurring, stable isotopes,191Ir and193Ir, with natural abundances of 37.3% and 62.7%,respectively.[23] At least 34 radioisotopes have also been synthesized, ranging in massnumber from 164 to199.192

Ir, which falls between the two stable isotopes, is the most stable radioisotope, with a half-life of 73.827 days, andfinds application in brachytherapy[24]and in industrial radiography, particularly for non-destructive testing of weldsin steel in the oil and gas industries; iridium-192 sources have been responsible for a number of radiologicalaccidents. Three other isotopes have half-lives of at least a day — 188Ir, 189Ir, 190Ir.[23] Isotopes with masses below191 decay by some combination of β+ decay, α decay, and proton emission, with the exceptions of 189Ir, whichdecays by electron capture, and190Ir, which decays by positron emission. Synthetic isotopes heavier than 191 decayby β− decay, although192Ir also has a minor electron capture decay path.[23] All known isotopes of iridium werediscovered between 1934 and 2001; the most recent is171Ir.[25]

At least 32 metastable isomers have been characterized, ranging in mass number from 164 to 197. The most stable of these is192m2Ir, which decays by isomeric transition with a half-life of 241 years,[23]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 only2 µs.[23]The isotope191Ir was the first one of any element to be shown to present a Mössbauer effect. This renders ituseful for Mössbauer spectroscopy for research in physics, chemistry, biochemistry, metallurgy, and mineralogy.[26]

HistoryThe discovery of iridium is intertwined with that of platinum and the other metals of the platinum group. Nativeplatinum used by ancient Ethiopians[27]and by South American cultures[28]always contained a small amount of theother platinum group metals, including iridium. Platinum reached Europe as platina ("small silver"), found in the17th century by the Spanish conquerors in a region today known as the department of Chocó in Colombia.[29]The

discovery that this metal was not an alloy of known elements, but instead a distinct new element, did not occur until1748.[30]

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Chemists who studied platinum dissolved it in aqua regia (a mixture of hydrochloric and nitric acids) to createsoluble salts. They always observed a small amount of a dark, insoluble residue.[4] Joseph Louis Proust thought thatthe residue was graphite.[4] The French chemists Victor Collet-Descotils, Antoine François, comte de Fourcroy, andLouis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough for furtherexperiments.[4]

In 1803, British scientist Smithson Tennant (1761 – 1815) analyzed the insoluble residue and concluded that it mustcontain a new metal. Vauquelin treated the powder alternately with alkali and acids[10]and obtained a volatile newoxide, which he believed to be of this new metal — which he named ptene , from the Greek word πτηνος (ptènos) forwinged.[31][32]Tennant, who had the advantage of a much greater amount of residue, continued his research andidentified the two previously undiscovered elements in the black residue, iridium and osmium.[4][10]He obtaineddark red crystals (probably of Na2[IrCl6]·nH2O) by a sequence of reactions with sodium hydroxide and hydrochloricacid.[32] He named iridium after Iris (Ιρις), the Greek winged goddess of the rainbow and the messenger of theOlympian gods, because many of the salts he obtained were strongly colored.[33][34]Discovery of the new elementswas documented in a letter to the Royal Society on June 21, 1804.[4][35]

The Greek goddess Iris, after whom iridium wasnamed.

British scientist John George Children was the first to melt a sample of

iridium in 1813 with the aid of "the greatest galvanic battery that hasever been constructed" (at that time).[4] The first to obtain high purityiridium was Robert Hare in 1842. He found that it had a density of around 21.8 g/cm3 and noted that the metal is nearly unmalleable andvery hard. The first melting in appreciable quantity was done by HenriSainte-Claire Deville and Jules Henri Debray in 1860. They requiredburning more than 300 L of pure O2 and H2 for each kilogram of iridium.[4]

These extreme difficulties in melting the metal limited the possibilitiesfor handling iridium. John Isaac Hawkins was looking to obtain a fineand hard point for fountain pen nibs and in 1834 managed to create aniridium-pointed gold pen. In 1880 John Holland and William LoflandDudley were able to melt iridium by adding phosphorus and patentedthe process in the United States; British company Johnson Mattheylater stated that they had been using a similar process since 1837 andhad already presented fused iridium at a number of World Fairs.[4]Thefirst use of an alloy of iridium with ruthenium in thermocouples wasmade by Otto Feussner in 1933. These allowed for the measurement of high temperatures in air up to 2000 °C.[4]

In 1957 Rudolf Mössbauer, in what has been called one of the "landmark experiments in twentieth century

physics",[36] discovered the resonant and recoil-free emission and absorption of gamma rays by atoms in a solidmetal sample containing only191Ir.[37] This phenomenon, known as the Mössbauer effect (which has since beenobserved for other nuclei, such as57Fe), and developed as Mössbauer spectroscopy, has made importantcontributions to research in physics, chemistry, biochemistry, metallurgy, and mineralogy.[26] Mössbauer receivedthe Nobel Prize in Physics in 1961, just three years after he published his discovery.[38]

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Occurrence

The Willamette Meteorite, the sixth-largestmeteorite found in the world, has 4.7 ppm

iridium.[39]

Iridium is one of the least abundant elements in the Earth's crust,having an average mass fraction of 0.001 ppm in crustal rock; gold is40 times more abundant, platinum is 10 times more abundant, andsilver and mercury are 80 times more abundant.[3] Tellurium is aboutas abundant as iridium, and only three naturally occurring elements areless abundant: rhenium, ruthenium, and rhodium, iridium being 10times more abundant than the last two.[3] In contrast to its lowabundance in crustal rock, iridium is relatively common in meteorites,with concentrations of 0.5 ppm or more.[40] It is thought that theoverall concentration of iridium on Earth is much higher than what isobserved in crustal rocks, but because of the density and siderophilic("iron-loving") character of iridium, it descended below the crust andinto the Earth's core when the planet was still molten.[19]

Iridium is found in nature as an uncombined element or in naturalalloys; especially the iridium – osmium alloys, osmiridium (osmiumrich), and iridiosmium (iridium rich).[10] In the nickel and copperdeposits the platinum group metals occur as sulfides (i.e. (Pt,Pd)S),tellurides (i.e. PtBiTe), antimonides (PdSb), and arsenides (i.e. PtAs2).In all of these compounds platinum is exchanged by a small amount of iridium and osmium. As with all of theplatinum group metals, iridium can be found naturally in alloys with raw nickel or raw copper.[41]

Within the Earth's crust, iridium isfound at highest concentrations in three types of geologic structure: igneousdeposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures.The largest known primary reserves are in the Bushveld igneous complex in South Africa,[42] though the largecopper – nickel deposits near Norilsk in Russia, and the Sudbury Basin in Canada are also significant sources of iridium. Smaller reserves are found in the United States.[42] Iridium is also found in secondary deposits, combinedwith platinum and other platinum group metals in alluvial deposits. The alluvial deposits used by pre-Columbianpeople in the Chocó Department of Colombia are still a source for platinum-group metals. As of 2003 the worldreserves had not been estimated.[10]

K –T boundary presence

The red arrow points to the K – T boundary.

The K – T boundary of 65 million years ago, marking the temporalborder between the Cretaceous and Tertiary periods of geological time,

was identified by a thin stratum of iridium-rich clay.[43]

A team led byLuis Alvarez proposed in 1980 an extraterrestrial origin for thisiridium, attributing it to an asteroid or comet impact.[43] Their theory,known as the Alvarez hypothesis, is now widely accepted to explainthe demise of the dinosaurs. A large buried impact crater structure withan estimated age of about 65 million years was later identified underwhat is now the Yucatán Peninsula (the Chicxulub crater).[44][45]

Dewey M. McLean and others argue that the iridium may have been of volcanic origin instead, as the Earth's core is rich in iridium, and active volcanoes such as Piton de la Fournaise, inthe island of Réunion, are still releasing iridium.[46][47]

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Production

Year Price($/ozt)

[48][49][50]

2001 415.25

2002 294.62

2003 93.02

2004 185.33

2005 169.51

2006 349.45

2007 444.43

2008 448.34

2009 420.4

2010 635

Iridium is obtained commercially as a by-product from nickel and copper mining and processing. Duringelectrorefining of copper and nickel, noble metals such as silver, gold and the platinum group metals as well asselenium and tellurium settle to the bottom of the cell asanode mud , which forms the starting point for theirextraction.[48][51]In order to separate the metals, they must first be brought into solution. Several methods areavailable depending on the separation process and the composition of the mixture; two representative methods arefusion with sodium peroxide followed by dissolution in aqua regia, and dissolution in a mixture of chlorine withhydrochloric acid.[19][42]

After it is dissolved, iridium is separated from the other platinum group metals by precipitating (NH4)2IrCl6 or by

extracting IrCl with organic amines.[52]

The first method is similar to the procedure Tennant and Wollaston used fortheir separation. The second method can be planned as continuous liquid – liquid extraction and is therefore moresuitable for industrial scale production. In either case, the product is reduced using hydrogen, yielding the metal as apowder or sponge that can be treated using powder metallurgy techniques.[53][54]

Annual production of iridium circa 2000 was around 3 tonnes or about 100,000 troy ounces (ozt).[55][10]The price of iridium as of 2007 was $440 USD/ozt,[48] but the price fluctuates considerably, as shown in the table. The highvolatility of the prices of the platinum group metals has been attributed to supply, demand, speculation, andhoarding, amplified by the small size of the market and instability in the producing countries.[56]

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ApplicationsThe global demand for iridium in 2007 was 119,000 troy ounces (3,700 kg), out of which 25,000 ozt (780 kg) wereused for electrical applications such as spark plugs; 34,000 ozt (1,100 kg) for electrochemical applications such aselectrodes for the chloralkali process; 24,000 ozt (750 kg) for catalysis; and 36,000 ozt (1,100 kg) for other uses.[57]

Industrial and medical

Molecular structure of Ir(mppy)3

The high melting point, hardness and corrosion resistance of iridium and itsalloys determine most of its applications. Iridium and especiallyiridium – platinum alloys or osmium – iridium alloys have a low wear and areused, for example, for multi-pored spinnerets, through which a plasticpolymer melt is extruded to form fibers, such as rayon.[58]

Osmium – iridium is used for compass bearings and for balances.[10]

Corrosion and heat resistance makes iridium an important alloying agent.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 corrosionresistance.[10]Iridium is also used as a hardening agent in platinum alloys.The Vickers hardness of pure platinum is 56 HV while platinum with 50%of iridium can reach over 500 HV.[59][60]

Devices that must withstand extremely high temperatures are often made from iridium. For example,high-temperature crucibles made of iridium are used in the Czochralski process to produce oxide single-crystals(such as sapphires) for use in computer memory devices and in solid state lasers.[61][62]The crystals, such asgadolinium gallium garnet and yttrium gallium garnet, are grown by melting pre-sintered charges of mixed oxidesunder oxidizing conditions at temperatures up to 2100 °C.[4] Its resistance to arc erosion makes iridium alloys ideal

for electrical contacts for spark plugs.[62][63]

Iridium compounds are used as catalysts in the Cativa process for carbonylation of methanol to produce aceticacid.[64] Iridium itself is used as a catalyst in a type of automobile engine introduced in 1996 called thedirect-ignition engine.[10]

The radioisotope iridium-192 is one of the two most important sources of energy for use in industrial γ-radiographyfor non-destructive testing of metals.[65][66]Additionally,192Ir is used as a source of gamma radiation for thetreatment of cancer using brachytherapy, a form of radiotherapy where a sealed radioactive source is placed inside ornext to the area requiring treatment. Specific treatments include high dose rate prostate brachytherapy, bilary ductbrachytherapy, and intracavitary cervix brachytherapy.[10]

Scientific

International Prototype Meter bar

An alloy of 90% platinum and 10% iridium was used in 1889 toconstruct the International Prototype Meter and kilogram mass, kept bythe International Bureau of Weights and Measures near Paris.[10] Themeter bar was replaced as the definition of the fundamental unit of length in 1960 by a line in the atomic spectrum of krypton,[67][68]butthe kilogram prototype is still the international standard of mass.[69]

Iridium has been used in the radioisotope thermoelectric generators of unmanned spacecraft such as theVoyager , Viking , Pioneer , Cassini ,

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Galileo , and New Horizons . Iridium was chosen to encapsulate the plutonium-238 fuel in the generator because itcan withstand the operating temperatures of up to 2000 °C and for its great strength.[4]

Another use concerns X-ray optics, especially X-ray telescopes.[70] The mirrors of the Chandra X-ray Observatoryare coated with a layer of iridium 60 nm thick. Iridium proved to be the best choice for reflecting X-rays after nickel,gold, and platinum were also tested. The iridium layer, which had to be smooth to within a few atoms, was applied

by depositing iridium vapor under high vacuum on a base layer of chromium.[71]

Iridium is used in particle physics for the production of antiprotons, a form of antimatter. Antiprotons are made byshooting a high-intensity proton beam at aconversion target , which needs to be made from a very high densitymaterial. Although tungsten may be used instead, iridium has the advantage of better stability under the shock wavesinduced by the temperature rise due to the incident beam.[72]

Oxidative addition to hydrocarbons in organoiridium chemistry[73][74]

Carbon – hydrogen bond activation(C – H activation) is an area of researchon reactions that cleavecarbon – hydrogen bonds, which weretraditionally regarded as unreactive.The first reported successes atactivating C – H bonds in saturated

hydrocarbons, published in 1982, used organometallic iridium complexes that undergo an oxidative addition with thehydrocarbon.[73][74]

Iridium complexes are being investigated as catalysts for asymmetric hydrogenation. These catalysts have been usedin the synthesis of natural products and able to hydrogenate certain difficult substrates, such as unfunctionalizedalkenes, enantioselectively (generating only one of the two possible enantiomers).[75][76]

Iridium forms a variety of complexes of fundamental interest in triplet harvesting.[77][78][79]

Historical

Fountain pen nib labeled Iridium Point

Iridium – osmium alloys were used to tip fountain pen nibs. The first major useof iridium was in 1834 in nibs mounted on gold.[4] Since 1944, the famousParker 51 fountain pen was fitted with a nib tipped by a ruthenium andiridium alloy (with 3.8% iridium). The tip material in modern fountain pens isstill conventionally called "iridium," although there is seldom any iridium init; other metals such as tungsten have taken its place.[80]

An iridium – platinum alloy was used for the touch holes or vent pieces of cannon. According to a report of the ParisExhibition of 1867, one of the pieces being exhibited by Johnson and Matthey "has been used in a Withworth gun for

more than 3000 rounds, and scarcely shows signs of wear yet. Those who know the constant trouble and expensewhich are occasioned by the wearing of the vent-pieces of cannon when in active service, will appreciate thisimportant adaptation".[81]

The pigmentiridium black , which consists of very finely divided iridium, is used for painting porcelain an intenseblack; it was said that "all other porcelain black colors appear grey by the side of it".[82]

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PrecautionsIridium in bulk metallic form is not biologically important or hazardous to health due to its lack of reactivity withtissues; there are only about 20 parts per trillion of iridium in human tissue.[10] However, finely divided iridiumpowder can be hazardous to handle, as it is an irritant and may ignite in air.[42]Very little is known about the toxicityof iridium compounds because they are used in very small amounts, but soluble salts, such as the iridium halides,

could be hazardous due to elements other than iridium or due to iridium itself.[24]However, most iridium compoundsare insoluble, which makes absorption into the body difficult.[10]

A radioisotope of iridium,192Ir, is dangerous like other radioactive isotopes. The only reported injuries related toiridium concern accidental exposure to radiation from192Ir used in brachytherapy.[24]High-energy gamma radiationfrom 192Ir can increase the risk of cancer. External exposure can cause burns, radiation poisoning, and death.Ingestion of 192Ir can burn the linings of the stomach and the intestines.[83] 192Ir, 192mIr, and194mIr tend to deposit inthe liver, and can pose health hazards from both gamma and beta radiation.[40]

Notes

[1] J. W. Arblaster: Densities of Osmium and Iridium , in: Platinum Metals Review, 1989, 33, 1, S. 14 –

16; Volltext (http://www.platinummetalsreview. com/pdf/pmr-v33-i1-014-016. pdf).[2] Magnetic susceptibility of the elements and inorganic compounds (http://www-d0. fnal. gov/hardware/cal/lvps_info/engineering/

elementmagn. pdf), in Handbook of Chemistry and Physics 81st edition, CRC press.[3] Greenwood, N. N.; Earnshaw, A. (1997).Chemistry of the Elements (2nd ed.). Oxford: Butterworth – Heinemann. pp. 1113 – 1143, 1294.

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pmr-v39-i4-164-164). Platinum Metals Review 39 (4): 164. .[7] Cotton, Simon (1997).Chemistry of Precious Metals . Springer-Verlag New York, LLC. p. 78. ISBN 9780751404135.[8] Lide, D. R. (1990).CRC Handbook of Chemistry and Physics (70th Edn.) . Boca Raton (FL):CRC Press.[9] Arblaster, J. W. (1989). "Densities of osmium and iridium: recalculations based upon a review of the latest crystallographic data" (http://

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References

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Article Sources and Contributors 15

Article Sources and ContributorsIridium Source : http://en.wikipedia.org/w/index.php?oldid=472181577Contributors : 129.186.219.xxx, 24.15.135.xxx, 63.192.137.xxx, AXRL, Achim1999, Ahoerstemeier, Alansohn,Alchemist-hp, AlimanRuna, All Is One, Anclation, Andonic, Andre Engels, AndrewBuck, Animum, Ansaranbbo, Antandrus, Anyeverybody, Archimerged, Aremith, Ark2120, Armeria, ArtLaPella, Asdfgh1234567, Aswery2341, AtheWeatherman, Axiosaurus, Baccyak4H, Beetstra, Benbest, Bender235, Benjiboi, Betterusername, BillFlis, Bobo The Ninja, Bobo192, Bryan Derksen,Bsadowski1, CWY2190, CYD, CanisRufus, Canthusus, Carnildo, Casliber, Causa sui, Cbdorsett, Chimesmonster, Chrishmt0423, Chuckalex, Cimon Avaro, Clearlynot, Cometstyles, Conversionscript, Corbon, Cryptic C62, Cybercobra, Cyde, DVdm, Davejenk1ns, David Koebel, David Latapie, David.Monniaux, David3565, Dbfirs, Dead-Portalist, Deflective, Deglr6328, Delldot,Deltabeignet, Dendodge, Deor, Discospinster, Divibisan, DocKrin, DocWatson42, Donarreiskoffer, Doulos Christos, Dschwen, Dtreadwell, EPO, EarthStar, Edgar181, EdgeOfEpsilon, EgraS, El

C, Ellude2, Emperorbma, English836, Enon, Epbr123, Eric-Wester, Erwinrossen, Facts707, Femto, Finalius, Fivemack, Foobar, Fractisrob, Frank Lofaro Jr., Freestyle-69, Fresheneesz, Func,Galoubet, Gilliam, Good vibrations, Gopher65, Graham87, Greenhorn1, Grendelkhan, Gtziavelis, Hak-kâ-ngìn, Har ryHenryGebel, Headbomb, Helge Skjeveland, Herbee, Heron, Hongooi,Hoopla-pdx, Hornlitz, IanOfNorwich, Icairns, Ideyal, Iempleh, Intently, Iridescent, Ishikawa Minoru, Itub, Jaan513, Jaraalbe, Jauhienij, JaydenDark, Jeff Muscato, Jeffq, Jericho4.0,Jerryjackson42, Jerzy, Jimp, Joanjoc, Joshua, Julesd, Jzlong, Ka Faraq Gatri, Kbrose, Keilana, Kelovy, Kevinsam, Khukri, Kitichai, Korath, Kotiwalo, Kwamikagami, L0LLz117, Lamro,LarryMorseDCOhio, Lars Washington, Lawrence Cohen, Lazserus, Lelle1987, Lewis R, Locos epraix, Looper5920, Luckas Blade, LyleHoward, Magioladitis, Marc Venot, Martin451,Materialscientist, Matthew Stannard, Mav, Mayumashu, Mejor Los Indios, Mhking, Michael Devore, Michaelbusch, Minesweeper, Misha Vargas, Mr0t1633, MrFish, Mtruch, Munge, Myasuda,NatureA16, NawlinWiki, Neosiber, Nergaal, Newone, Nickkid5, Nihiltres, NuclearWarfare, OlliffeObscurity, Orangemarlin, Otolemur crassicaudatus, Paranoid, Petergans, Piperh, PlatinumX,Plexust, Poolkris, Pras, Prestonmcconkie, RTC, Raywil, Rebroad, RedHillian, Rehoboam, Remember, Reza kalani, Rich Farmbrough, Rjwilmsi, Roberta F., Romanm, Rossnorman, RoyBoy,Ruslik0, Ryan Reich, SEWilco, SJK, SandyGeorgia, Santacruzing, Santăr, Saperaud, Sbharris, Schneelocke, Schutz, Sengkang, Shaddack, Shirt58, Sl, Slicky, Smilesfozwood, Smokefoot,Snoyes, Speedevil, Spencer, Stephenb, Stifynsemons, Stojo71, Stone, Sunil060902, Tablizer, Tacopolice911811, Tagishsimon, Tahlana, Tbhotch, TerraFrost, Tetracube, Thingg, Thumperward,Thunderbrand, Tide rolls, Titoxd, Tondelayo, Travist, Trojancowboy, Turk oğlan, Tuyvan, Ulric1313, Una Smith, UnitedStatesian, VASANTH S.N., Vegaswikian, Vsmith, Vuo, Warren,Wavelength, WereSpielChequers, William Avery, Wimt, Wjfox2005, WojPob, Wolfrock, Xenophon777, Xeworlebi, Yar, Yekrats, Ymwang42, You7742, Yyy, ZS, Zamphuor,ل یق ع ف ش ا 366 ,کanonymous edits

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