Iridium

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Not to be confused with Indium.
This article is about the chemical element. For other uses, see Iridium (disambiguation).
Iridium, 77Ir
Pieces of pure iridium
Iridium
Pronunciation/ɪˈrɪdiəm/ ​(i-RID-ee-əm)
AppearanceSilvery white
Standard atomic weight Ar°(Ir)
Iridium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Rh
↑
Ir
↓
Mt
osmium ← iridium → platinum
kJ/mol
Heat of vaporization564 kJ/mol
Molar heat capacity25.10 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 2713 2957 3252 3614 4069 4659
Atomic properties
Discovery and first isolation
Smithson Tennant (1803)
Isotopes of iridium
Main isotopes[7] Decay
abun­dance half-life (t1/2) mode pro­duct
191Ir 37.3%
stable
192Ir synth 73.827 d
β−
192Pt
ε
192Os
192m2Ir synth 241 y
IT
 		192Ir
193Ir 62.7% stable
 Category: Iridium
| references

Iridium is a

stable isotopes; the latter is the more abundant. It is one of the most corrosion-resistant metals,[11]
even at temperatures as high as 2,000 °C (3,630 °F).

Iridium was discovered in 1803 among insoluble impurities in natural platinum. Smithson Tennant, the primary discoverer, named it after the Greek goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is one of the rarest elements in Earth's crust, with estimated annual production and consumption of only 7.3 tonnes (16 thousand pounds) in 2018.[12]

The dominant uses of iridium are the metal itself and its alloys, as in high-performance spark plugs, crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the chloralkali process. Important compounds of iridium are chlorides and iodides in industrial catalysis. Iridium is a component of some OLEDs.

Iridium is found in meteorites in much higher abundance than in the Earth's crust.[13] For this reason, the unusually high abundance of iridium in the clay layer at the Cretaceous–Paleogene boundary gave rise to the Alvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 66 million years ago, now known to be produced by the impact that formed the Chicxulub crater. Similarly, an iridium anomaly in core samples from the Pacific Ocean suggested the Eltanin impact of about 2.5 million years ago.[14]

It is thought that the total amount of iridium in the planet is much higher than that observed in crustal rocks, but as with other platinum-group metals, the high density and

tendency
of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.

Characteristics

Physical properties

troy ounce (31.1035 grams
) of arc-melted iridium

A member of the

superconductor at temperatures below 0.14 K (−273.010 °C; −459.418 °F).[17]

Iridium's

modulus of elasticity is the second-highest among the metals, being surpassed only by osmium.[16] This, together with a high shear modulus 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 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.[16]

The measured

densest metal known.[18][19] 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,[20] 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.[21]

Iridium is extremely brittle, to the point of being hard to 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).[22]

The

Vickers hardness of pure platinum is 56 HV, whereas platinum with 50% of iridium can reach over 500 HV.[23][24]

Chemical properties

Iridium is the most

corrosion-resistant metal known.[25] It is not attacked by acids, including aqua regia. In the presence of oxygen, it reacts with cyanide salts.[26] Traditional oxidants also react, including the halogens and oxygen[27] at higher temperatures.[28] Iridium also reacts directly with sulfur at atmospheric pressure to yield iridium disulfide.[29]

Isotopes

Main article: Isotopes of iridium

Iridium has two naturally occurring stable

radioisotopes have also been synthesized, ranging in mass number from 164 to 202. 192Ir, which falls between the two stable isotopes, is the most stable radioisotope, with a half-life of 73.827 days, and finds application in brachytherapy[31] 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—188Ir, 189Ir, and 190Ir.[30] Isotopes with masses below 191 decay by some combination of β+ decay, α decay, and (rare) proton emission, with the exception of 189Ir, which decays by electron capture. Synthetic isotopes heavier than 191 decay by β− decay, although 192Ir also has a minor electron capture decay path.[30] All known isotopes of iridium were discovered between 1934 and 2008, with the most recent discoveries being 200–202Ir.[32]

At least 32

isomeric transition with a half-life of 241 years,[30] making it more stable than any of iridium's synthetic isotopes in their ground states. The least stable isomer is 190m3Ir with a half-life of only 2 μs.[30] The isotope 191Ir 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.[33]

Chemistry

See also: Iridium compounds
Oxidation states[b]
−3 [Ir(CO)
3]3−
−1 [Ir(CO)3(PPh3)]1−
0 Ir4(CO)12
+1 [IrCl(CO)(PPh3)2]
+2 Ir(C5H5)2
+3 IrCl3
+4 IrO2
+5 Ir4F20
+6 IrF
6
+7 [Ir(O2)O2]+
+8 IrO4
+9 [IrO4]+[4]

Oxidation states

Iridium forms compounds in

iridium(VIII) oxide (IrO4) was generated under matrix isolation conditions at 6 K in argon.[35] The highest oxidation state (+9), which is also the highest recorded for any element, is found in gaseous [IrO4]+.[4]

Binary compounds

Iridium does not form

binary hydrides. Only one binary oxide is well-characterized: iridium dioxide, IrO
2. It is a blue black solid that adopts the fluorite structure.[15] A sesquioxide, Ir
2O
3, has been described as a blue-black powder, which is oxidized to IrO
2 by HNO
3.[27] The corresponding disulfides, diselenides, sesquisulfides, and sesquiselenides are known, as well as IrS
3.[15]

Binary trihalides, IrX
3, are known for all of the halogens.

tetrafluoride, pentafluoride and hexafluoride are known.[15] Iridium hexafluoride, IrF
6, is a volatile yellow solid, composed of octahedral molecules. It decomposes in water and is reduced to IrF
4.[15] Iridium pentafluoride is also a strong oxidant, but it is a tetramer, Ir
4F
20, formed by four corner-sharing octahedra.[15]

Complexes

iridium trichloride
, a common salt of iridium.

Iridium has extensive

coordination chemistry
.

Iridium in its complexes is always

low-spin. Ir(III) and Ir(IV) generally form octahedral complexes.[15] Polyhydride complexes are known for the +5 and +3 oxidation states.[36] One example is IrH5(PiPr3)2.[37] The ternary hydride Mg
6Ir
2H
11 is believed to contain both the IrH4−
5 and the 18-electron IrH5−
4 anion.[38]

Iridium also forms oxyanions with oxidation states +4 and +5. K
2IrO
3 and KIrO
3 can be prepared from the reaction of potassium oxide or potassium superoxide with iridium at high temperatures. Such solids are not soluble in conventional solvents.[39]

Just like many elements, iridium forms important chloride complexes. Hexachloroiridic (IV) acid, H
2IrCl
6, and its ammonium salt are the most common iridium compounds from both industrial and preparative perspectives.[40] They are intermediates in the purification of iridium and used as precursors for most other iridium compounds, as well as in the preparation of anode coatings. The IrCl2−
6 ion has an intense dark brown color, and can be readily reduced to the lighter-colored IrCl3−
6 and vice versa.[40] Iridium trichloride, IrCl
3, which can be obtained in anhydrous form from direct oxidation of iridium powder by chlorine at 650 °C,[40] or in hydrated form by dissolving Ir
2O
3 in hydrochloric acid, is often used as a starting material for the synthesis of other Ir(III) compounds.[15] Another compound used as a starting material is ammonium hexachloroiridate(III), (NH
4)
3IrCl
6.[citation needed]

In the presence of air, iridium metal dissolves in molten alkali-metal cyanides to produce the Ir(CN)3−
6 (hexacyanoiridate) ion and upon oxidation produces the most stable oxide.[citation needed]

Organoiridium chemistry

Cyclooctadiene iridium chloride dimer is a common complex of Ir(I).

homogeneous catalyst for hydrogenation reactions.[41][42]

Skeletal formula presentation of a chemical transformation. The initial compounds have a C5H5 ring on their top and an iridium atom in the center, which is bonded to two hydrogen atoms and a P-PH3 group or to two C-O groups. Reaction with alkane under UV light alters those groups.
Oxidative addition to hydrocarbons in organoiridium chemistry[43][44]

Iridium complexes played a pivotal role in the development of

Carbon–hydrogen bond activation (C–H activation), which promises to allow functionalization of hydrocarbons, which are traditionally regarded as unreactive.[45]

History

Platinum group

Photo of part of a black vase with brown picture on it: A woman with wings on her back hold an arrow with right hand and gives a jar to a man. A small deer is standing in front of the woman.
The Greek goddess Iris, after whom iridium was named.

The discovery of iridium is intertwined with that of platinum and the other metals of the

adulteration of gold with platinum impurities.[47]

A left-pointing crescent, tangent on its right to a circle containing at its center a solid circular dot
This alchemical 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 and Jorge Juan y Santacilia saw Native Americans mining platinum while the Spaniards were travelling through Colombia and Peru for eight years. Ulloa and Juan found mines with the whitish metal nuggets and took them home to Spain. Ulloa returned to Spain and established the first mineralogy 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 neither separable nor calcinable. 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 superintend mercury mining operations in Huancavelica.[46]

In 1741, Charles Wood,[48] a British metallurgist, found various samples of Colombian platinum in Jamaica, which he sent to William Brownrigg for further investigation.

In 1750, after studying the platinum sent to him by Wood, Brownrigg presented a detailed account of the metal to the

Jöns Jakob Berzelius, William Lewis, and Pierre 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 of arsenic. Scheffer described platinum as being less pliable than gold, but with similar resistance to corrosion.[46]

Discovery

Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough for further experiments.[16]

In 1803 British scientist Smithson Tennant (1761–1815) analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternately with alkali and acids[25] and obtained a volatile new oxide, which he believed to be of this new metal—which he named ptene, from the Greek word πτηνός ptēnós, "winged".[51][52] 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 and osmium.[16][25] He obtained dark red crystals (probably of Na
2[IrCl
6]·nH
2O) by a sequence of reactions with sodium hydroxide and hydrochloric acid.[52] He named iridium after Iris (Ἶρις), the Greek winged goddess of the rainbow and the messenger of the Olympian gods, because many of the salts he obtained were strongly colored.[c][53] Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.[16][54]

Metalworking and applications

British scientist

Henri Sainte-Claire Deville and Jules Henri Debray in 1860. They required burning more than 300 litres (79 US gal) of pure O
2 and H
2 gas for each 1 kilogram (2.2 lb) of iridium.[16]

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 for fountain pen nibs, and in 1834 managed to create an iridium-pointed gold pen. In 1880, John Holland and William Lofland Dudley were able to melt iridium by adding phosphorus and patented the process in the United States; British company Johnson Matthey later stated they had been using a similar process since 1837 and had already presented fused iridium at a number of World Fairs.[16] The first use of an alloy of iridium with ruthenium in thermocouples 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).[16]

In

recoil-free emission and absorption of gamma rays by atoms in a solid metal sample containing only 191Ir.[56] This phenomenon, known as the Mössbauer effect resulted in the awarding of the Nobel Prize in Physics in 1961, at the age 32, just three years after he published his discovery.[57]

Occurrence

Along with all elements having

supernovae and neutron star mergers.[58][59]

Graph sowing on the x axis the elements by atomic number and on y-axis the amount in earth's crust compared to Si abundance. There is a green area with high abundance for the lighter elements between oxygen and iron. The yellow area with lowest abundant elements includes the heavier platinum group metals, tellurium and gold. The lowest abundance is clearly iridium.
Iridium is one of the least abundant elements in Earth's crust.
A large black egg-shaped boulder of porous structure standing on its top, tilted
The Willamette Meteorite, the sixth-largest meteorite found in the world, has 4.7 ppm iridium.[60]

Iridium is one of the nine least abundant stable

Earth's core when the planet was still molten.[40]

Iridium is found in nature as an uncombined element or in natural

pre-Columbian people in the Chocó Department of Colombia are still a source for platinum-group metals. As of 2003, world reserves have not been estimated.[25]

Marine oceanography

Iridium is found within marine organisms,

marine oxygenation, seawater temperature, and various geological and biological processes.[69]

Iridium in sediments can come from cosmic dust, volcanoes, precipitation from seawater, microbial processes, or hydrothermal vents,[69] and its abundance can be strongly indicative of the source.[70][69] It tends to associate with other ferrous metals in manganese 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 the Eltanin impact of about 2.5 million years ago.[14]

Some of the

asteroid impacts.[73]

Cretaceous–Paleogene boundary presence

A cliff with pronounced layered structure: yellow, gray, white, gray. A red arrow points between the yellow and gray layers.
The red arrow points to the Cretaceous–Paleogene boundary.
Main article: Cretaceous–Paleogene extinction event

The Cretaceous–Paleogene boundary of 66 million years ago, marking the temporal border between the Cretaceous and Paleogene periods of geological time, was identified by a thin stratum of iridium-rich clay.[74] A team led by Luis Alvarez proposed in 1980 an extraterrestrial origin for this iridium, attributing it to an asteroid or comet impact.[74] Their theory, known as the Alvarez hypothesis, is now widely accepted to explain the extinction of the non-avian dinosaurs. A large buried impact crater structure with an estimated age of about 66 million years was later identified under what is now the Yucatán Peninsula (the Chicxulub crater).[75][76] Dewey M. McLean and others argue that the iridium may have been of volcanic origin instead, because Earth's core is rich in iridium, and active volcanoes such as Piton de la Fournaise, in the island of Réunion, are still releasing iridium.[77][78]

Production

Year Consumption
(tonnes)		 Price (US$)[79]
2001 2.6 $415.25/
ozt
($13.351/g)
2002 2.5 $294.62/
ozt
($9.472/g)
2003 3.3 $93.02/
ozt
($2.991/g)
2004 3.60 $185.33/
ozt
($5.958/g)
2005 3.86 $169.51/
ozt
($5.450/g)
2006 4.08 $349.45/
ozt
($11.235/g)
2007 3.70 $444.43/
ozt
($14.289/g)
2008 3.10 $448.34/
ozt
($14.414/g)
2009 2.52 $420.4/
ozt
($13.52/g)
2010 10.40 $642.15/
ozt
($20.646/g)
2011 9.36 $1,035.87/
ozt
($33.304/g)
2012 5.54 $1,066.23/
ozt
($34.280/g)
2013 6.16 $826.45/
ozt
($26.571/g)
2014 6.1 $556.19/
ozt
($17.882/g)
2015 7.81 $544/
ozt
($17.5/g)
2016 7.71 $586.90/
ozt
($18.869/g)
2017 n.d. $908.35/
ozt
($29.204/g)
2018 n.d. $1,293.27/
ozt
($41.580/g)
2019 n.d. $1,485.80/
ozt
($47.770/g)
2020 n.d. $1,633.51/
ozt
($52.519/g)
2021 n.d. $5,400.00/
ozt
($173.614/g)
2022 n.d. $3,980.00/
ozt
($127.960/g)
2023 n.d. $4,652.38/
ozt
($149.577/g)
2024 n.d. $5,000.00/
ozt
($160.754/g)

Worldwide production of iridium was about 7,300 kilograms (16,100 lb) in 2018.[12] The price is high and varying (see table). Illustrative factors that affect the price include oversupply of Ir crucibles[79][80] and changes in

LED technology.[81]

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.

platinum group metals as well as selenium and tellurium settle to the bottom of the cell as anode mud, which forms the starting point for their extraction.[79]

Leading iridium-producing countries (kg)[86]
Country 2016 2017 2018 2019 2020
 World 7,720 7,180 7,540 7,910 8,170
 
South Africa *
 6,624 6,057 6,357 6,464 6,786
 Zimbabwe 598 619 586 845 836
 
Canada *
 300 200 400 300 300
 
Russia *
 200 300 200 300 250

Applications

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.[82]

Ir metal and alloys

Resistance to heat and corrosion are the bases for several uses of iridium and its alloys.

Owing to its high melting point, hardness, and

Czochralski process to produce oxide single-crystals (such as sapphires) for use in computer memory devices and in solid state lasers.[87][88] The crystals, such as gadolinium 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).[16]

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.[25] Iridium is used for multi-pored spinnerets, through which a plastic polymer melt is extruded to form fibers, such as rayon.[89] Osmium–iridium is used for compass bearings and for balances.[16]

Because of their resistance to arc erosion, iridium alloys are used by some manufacturers for the centre electrodes of spark plugs,[87][90] and iridium-based spark plugs are particularly used in aviation.

Catalysis

Iridium compounds are used as catalysts in the Cativa process for carbonylation of methanol to produce acetic acid.[91][92]

Iridium complexes are often active for

chiral herbicide (S)-metolachlor. As practiced by Syngenta on the scale of 10,000 tons/year, the complex [[ [Ir(COD)Cl]2 in the presence of Josiphos ligands.[95]

Medical imaging

The radioisotope

iridium-191 in natural-abundance iridium metal.[98]

Photocatalysis and OLEDs

Iridium complexes are key components of white OLEDs. Similar complexes are used in photocatalysis.[99]

Scientific

International Prototype Meter
bar

An alloy of 90% platinum and 10% iridium was used in 1889 to construct the

until 20 May 2019, when the kilogram was redefined in terms of the Planck constant.[101]

Historical

Fountain pen nib labelled Iridium Point

Iridium–osmium alloys were used in fountain pen nib tips. The first major use of iridium was in 1834 in nibs mounted on gold.[16] Since 1944, the famous Parker 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 as ruthenium, osmium, and tungsten have taken its place.[102]

An iridium–platinum alloy was used for the

Johnson 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".[103]

The pigment iridium black, which consists of very finely divided iridium, is used for painting porcelain an intense black; it was said that "all other porcelain black colors appear grey by the side of it".[104]

Precautions

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.[25] Like most metals, finely divided iridium powder can be hazardous to handle, as it is an irritant and may ignite in air.[66] By 2015 very little is known about the toxicity of iridium compounds,[105] primarily because it is used so rarely that few people come in contact with it and those who do only with very small amounts. However, soluble salts, such as the iridium halides, could be hazardous due to elements other than iridium or due to iridium itself.[31] At the same time, most iridium compounds are insoluble, which makes absorption into the body difficult.[25]

A radioisotope of iridium, 192
Ir, is dangerous, like other radioactive isotopes. The only reported injuries related to iridium concern accidental exposure to radiation from 192
Ir used in

gamma and beta radiation.[61]

Notes

  1. ^ 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.[9] 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.[10]
  2. ^ Most common oxidation states of iridium are in bold. The right column lists one representative compound for each oxidation state.
  3. ^ Iridium literally means "of rainbows".
  4. ^ The definition of the meter was changed again in 1983. The meter is currently defined as the distance traveled by light in a vacuum during a time interval of 1⁄299,792,458 of a second.

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