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This article is about the chemical element. For the satellite phone service, see Iridium (satellite).
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
Atomic number (Z)77
Groupgroup 9
Periodperiod 6
Block  d-block
Electron configuration[Xe] 4f14 5d7 6s2
Electrons per shell2, 8, 18, 32, 15, 2
Physical properties
Phase at STPsolid
Melting point2719 K ​(2446 °C, ​4435 °F)
Boiling point4403 K ​(4130 °C, ​7466 °F)
Density (at 20° C)22.562 g/cm3 [3]
when liquid (at m.p.)19 g/cm3
Heat of fusion41.12 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
Oxidation states−3, –2, −1, 0, +1, +2, +3, +4, +5, +6, +7, +8, +9[4]
ElectronegativityPauling scale: 2.20
Ionization energies
  • 1st: 880 kJ/mol
  • 2nd: 1600 kJ/mol
Atomic radiusempirical: 136 pm
Covalent radius141±6 pm
Color lines in a spectral range
Spectral lines of iridium
Other properties
Natural occurrenceprimordial
Crystal structure ​face-centered cubic (fcc) (cF4)
Lattice constant
Face-centered cubic crystal structure for iridium
a = 383.92 pm (at 20 °C)[3]
Thermal expansion6.47×10−6/K (at 20 °C)[3]
Thermal conductivity147 W/(m⋅K)
Electrical resistivity47.1 nΩ⋅m (at 20 °C)
Magnetic orderingparamagnetic[5]
Molar magnetic susceptibility+25.6 × 10−6 cm3/mol (298 K)[6]
Young's modulus528 GPa
Shear modulus210 GPa
Bulk modulus320 GPa
Speed of sound thin rod4825 m/s (at 20 °C)
Poisson ratio0.26
Mohs hardness6.5
Vickers hardness1760–2200 MPa
Brinell hardness1670 MPa
CAS Number7439-88-5
History
Discovery and first isolationSmithson 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 (Template:PronEng) is a chemical element that has the symbol Ir and atomic number 77. A dense, very hard, brittle, silvery-white transition metal of the platinum family, iridium is used in high-strength alloys that can withstand high temperatures and occurs in natural alloys with platinum or osmium. Iridium is notable for being the most corrosion-resistant metal known and for its significance in the determination of the probable cause of the extinction, by an asteroid impact, of the dinosaurs and many other organisms. It is used in high-temperature apparatuses, electrical contacts, jewellery, and as a hardening agent for platinum.

Characteristics

A platinum group metal, iridium is white, resembling platinum, but with a slight yellowish cast. Due to its extreme hardness, brittleness, and very high melting point, iridium is difficult to machine, form, or work, and powder metallurgy is commonly used. Iridium is the most corrosion-resistant metal known: it cannot be attacked by any acid or by aqua regia. It can, however, be attacked by some molten salts, such as NaCl and NaCN.[8]

The measured density of iridium is only slightly lower than that of osmium, the densest element known. There has been some ambiguity regarding which element is the densest due to the small size of the difference in density and the difficulty in measuring it accurately. The best available calculations of density from X-ray crystallographic data give densities of 22.59 g/cm3 for iridium and 22.56 g/cm3 for osmium.[9]

Applications

The global demand for iridium in 2007 was 119,000 troy ounces (3,700 kg), out of which 25,000 oz (780 kg) were used for electrical applications such as spark plugs, 34,000 oz (1,100 kg) for electrochemical applications such as electrodes for the chloralkali process, 24,000 oz (750 kg) for catalysis, and 36,000 oz (1,100 kg) for other uses.[10]

The high melting point, the hardness and low wear of iridium and its alloys determin most of the applications of iridium. Iridium and especially iridium platinum alloys or osmium iridium alloys have a low wear and are used for examle for multi-pored spinnerets, through which a plastic polymer melt is extruded to form fibers, for example rayon.[11] The Osmium-iridium is used for compass bearings or for balances.[12]

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.[13][14]

High temperature Crucibles for example for the Czochralski process and devices that must withstand extremely high temperatures are made from iridium.[15][16] The resistance to arc erosion makes iridium alloys ideal for electrical contacts and the use as spark plugs).[16][17]

Other use

Historical uses

Fountain pen nib labeled: Iridium Point
  • As an alloy with platinum, in bushing the vents of heavy ordnance
  • In a finely powdered condition (iridium black), for painting porcelain black
  • Iridium-osmium alloys were used to tip early-twentieth-century fountain pen nibs. The tip material in modern fountain pens is still conventionally called "iridium," although there is seldom any iridium in it; other metals such as tungsten take its place.[22]

History

Iridium was discovered in 1803 by British scientist Smithson Tennant in London, England along with osmium in the dark-coloured residue of dissolving crude platinum in aqua regia (a mixture of hydrochloric and nitric acid). The element was named after the Greek word for rainbow (ίρις, iris; iridium means "of rainbows") because many of its salts are strongly coloured. Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.[23][24]

International Prototype Meter bar

An alloy of 90% platinum and 10% iridium was used in 1889 to construct the standard metre bar and kilogramme mass, kept by the International Bureau of Weights and Measures near Paris. The metre bar was replaced as the definition of the fundamental unit of length in 1960 (see krypton), but the kilogram prototype is still the international standard of mass.[25]

K-T boundary

Main article: Cretaceous–Tertiary extinction event
Main article: K–T boundary

The K–T boundary of 65 million years ago, marking the temporal border between the Cretaceous and Tertiary periods of geological time, was identified by a thin stratum of iridium-rich clay. A team led by Luis Alvarez proposed in 1980 an extraterrestrial origin for this iridium, attributing it to an asteroid or comet impact.[26] Their theory is now widely accepted to explain the demise of the dinosaurs. A large buried impact crater structure with an estimated age of about 65 million years was later identified near what is now Yucatán Peninsula (the Chicxulub crater).[27][28] Dewey M. McLean and others argue that the iridium may have been of volcanic origin instead. The Earth's core is rich in iridium, and Piton de la Fournaise on Réunion, for example, is still releasing iridium today.

Occurrence and production

Iridium is found uncombined in secondary deposits with platinum and other platinum group metals in alluvial deposits. The alluvial deposits used by pre-Columbian people in the Chocó Department Colombia are still a source for platinum group metals. The second large alluvial deposit was found in the Ural mountains Russia, which is still mined. In this placer deposits iridium occures as alloy include osmiridium and iridiosmium, both of which are mixtures of iridium and osmium.

Iridium is recovered commercially as a by-product from nickel and copper mining and processing. In the nickel and copper deposits the platinum group metal occure as sulphides (i.e. (Pt,Pd)S)), tellurides (i. e. PtBiTe), antimonides (PdSb) and arsenides (i.e. PtAs2) and end alloy with the raw nickle or raw copper.[29] During electrorefining more noble metals such as silver, gold and the platinum group metals as well as selenium and tellurium settle to the bottom of the cell as anode mud. From this the extraction of the platinum group metals starts.[30][31]

After ruthenium and osmium have been removed, iridium is separated by precipitating (NH4)2IrCl6 or by extracting [IrCl6]2− with organic amines. The first method is similar to the procedure Tennant and Wollastone used for their separation. The second method cna be planed as continous liquid liquid extraction and is therfore mor suitable for industial scale production. In either case, the product is reduced using hydrogen, yielding the metal as a powder or sponge that can be treated using powder metallurgy techniques.[32][33] In 1996, only 3.8 tonnes of iridium were produced in the Western world (compared to 14.2 tonnes of rhodium and 20,000 tonnes of cobalt).[34] The largest known reserves are in the Bushveld complex in South Africa, with smaller reserves in the United States and Canada.[35] The russian production shifted from the alluvial deposits in the ural montains to the copper nickel deposits near Norilsk, while the colombian alluvial deposits in the Choco Department are still mined. A considerable part of the produced iridium comes from scrap recycling.

According to the CRC Handbook, ruthenium and rhodium are the only non-radioactive elements rarer than iridium.[36] Although iridium is one of the least common elements in Earth's crust, it is relatively common in meteorites. It is thought that the concentration of iridium in meteorites matches the concentration of iridium in the Earth as a whole;[citation needed] because of the density and siderophile nature of iridium, it is believed that it descended below the Earth's crust and toward the core at a time when the Earth was young and still molten.[37]

Isotopes

Main article: isotopes of iridium

There are two natural isotopes of iridium, 191Ir and 193Ir, and 34 known radioisotopes, the most stable radioisotope being Ir-192 with a half-life of 73.83 days. Ir-192 beta decays into platinum-192, while most of the other radioisotopes decay into osmium.[38][39]

Compounds

See also: Category:Iridium compounds
Crabtree's catalyst
Vaska's complex

Iridium forms compounds in all oxidation states ranging from −1 to +6, but the most common oxidation states are +3 and +4. The highest oxidation states are only observed for the fluorides. Iridium hexafluoride, IrF6 is a volatile and highly reactive yellow solid, composed of an octahedral molecules. The pentafluoride has similar properties but it is actually a tetramer, Ir4F20, formed by four corner-sharing octahedra. The tetrafluoride and trifluoride are also known. Iridium dioxide, IrO2, is the only well-characterized oxide of iridium.[34]

Iridium(III) chloride IrCl3 is often used as a starting material for the synthesis of other Ir(III) compounds. Another compound used for this purpose is the ammonium salt, (NH4)3IrCl6. Iridium(III) complexes are diamagnetic (low-spin) and generally have an octahedral molecular geometry.[34]

Iridium also forms organometallic compounds with lower oxidation states. Tetrairidium dodecacarbonyl, Ir4(CO)12, the most common and stable binary carbonyl of iridium, has iridium in the formal oxidation state zero. In this compound, each of the iridium atoms is bonded to the other three, forming a tetrahedral cluster.[34]

Some organometallic Ir(I) compounds are notable enough to be named after their discoverers. One is Vaska's complex, IrCl(CO)[P(C6H5)3]2, which has the unusual property of binding to dioxygen molecule, O2.[40] The other is Crabtree's catalyst, a homogeneous catalyst for hydrogenation reactions.[41] These compounds are both square planar, d8 complexes, with a total of 16 valence electrons, which accounts for their reactivity.[42]

Precautions

Iridium in bulk metallic form is not hazardous to health due to its lack of reactivity. However, finely divided iridium can be hazardous to handle, and may ignite in air.[35]

References

  1. ^ "Standard Atomic Weights: Iridium". CIAAW. 2017.
  2. ^ Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (2022-05-04). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
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  8. ^ Los Alamos National Laboratory: Iridium
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  28. ^ Frankel, Charles (1999). The End of the Dinosaurs: Chicxulub Crater and Mass Extinctions. Cambridge University Press. ISBN 0521474477.
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