Iridium

Iridium, 77Ir
Pieces of pure iridium
Iridium
Pronunciationm/ (RID-ee-əm)
Appearancesilvery white
Standard atomic weight Ar, std(Ir)192.217(2)[1]
Iridium in the periodic table
HydrogenHelium
LithiumBerylliumBoronCarbonNitrogenOxygenFluorineNeon
SodiumMagnesiumAluminiumSiliconPhosphorusSulfurChlorineArgon
PotassiumCalciumScandiumTitaniumVanadiumChromiumManganeseIronCobaltNickelCopperZincGalliumGermaniumArsenicSeleniumBromineKrypton
RubidiumStrontiumYttriumZirconiumNiobiumMolybdenumTechnetiumRutheniumRhodiumPalladiumSilverCadmiumIndiumTinAntimonyTelluriumIodineXenon
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon
FranciumRadiumActiniumThoriumProtactiniumUraniumNeptuniumPlutoniumAmericiumCuriumBerkeliumCaliforniumEinsteiniumFermiumMendeleviumNobeliumLawrenciumRutherfordiumDubniumSeaborgiumBohriumHassiumMeitneriumDarmstadtiumRoentgeniumCoperniciumNihoniumFleroviumMoscoviumLivermoriumTennessineOganesson
Rh

Ir

Mt
osmiumiridiumplatinum
Atomic number (Z)77
Groupgroup 9
Periodperiod 6
Blockd-block
Element category  Transition metal
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 (near r.t.)22.56 g/cm3
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)1101001 k10 k100 k
at T (K)271329573252361440694659
Atomic properties
Oxidation states−3, −1, 0, +1, +2, +3, +4, +5, +6, +7, +8, +9[2]
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 structureface-centered cubic (fcc)
Face-centered cubic crystal structure for iridium
Speed of sound thin rod4825 m/s (at 20 °C)
Thermal expansion6.4 µm/(m·K)
Thermal conductivity147 W/(m·K)
Electrical resistivity47.1 nΩ·m (at 20 °C)
Magnetic orderingparamagnetic[3]
Magnetic susceptibility+25.6·10−6 cm3/mol (298 K)[4]
Young's modulus528 GPa
Shear modulus210 GPa
Bulk modulus320 GPa
Poisson ratio0.26
Mohs hardness6.5
Vickers hardness1760–2200 MPa
Brinell hardness1670 MPa
CAS Number7439-88-5
History
Discovery and first isolationSmithson Tennant (1803)
Main isotopes of iridium
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
188Irsyn1.73 dε188Os
189Irsyn13.2 dε189Os
190Irsyn11.8 dε190Os
191Ir37.3%stable
192Irsyn73.827 dβ192Pt
ε192Os
192m2Irsyn241 yIT192Ir
193Ir62.7%stable
193mIrsyn10.5 dIT193Ir
194Irsyn19.3 hβ194Pt
194m2Irsyn171 dIT194Ir
| references

Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, iridium is considered to be the second-densest metal (after osmium) with a density of 22.56 g/cm3 as defined by experimental X-ray crystallography. However, at room temperature and standard atmospheric pressure, iridium has been calculated to have a density of 22.65 g/cm3, 0.04 g/cm3 higher than osmium measured the same way[5]. Still, the experimental X-ray crystallography value is considered to be the most accurate, as such iridium is considered to be the second densest element[6]. It is the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.

Iridium was discovered in 1803 among insoluble impurities in natural platinum. Smithson Tennant, the primary discoverer, named iridium for 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 annual production and consumption of only three tonnes. 191Ir and 193Ir are the only two naturally occurring isotopes of iridium, as well as the only stable isotopes; the latter is the more abundant.

The most important iridium compounds in use are the salts and acids it forms with chlorine, though iridium also forms a number of organometallic compounds used in industrial catalysis, and in research. Iridium metal is employed when high corrosion resistance at high temperatures is needed, 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. Iridium radioisotopes are used in some radioisotope thermoelectric generators.

Iridium is found in meteorites in much higher abundance than in the Earth's crust.[7] 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. Similarly, an iridium anomaly in core samples from the Pacific Ocean suggested the Eltanin impact of about 2.5 million years ago.

It is thought that the total amount of iridium in the planet Earth 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

A flattened drop of dark gray substance
One troy ounce (31.1035 g) of arc-melted iridium

A member of the platinum group metals, iridium is white, resembling platinum, but with a slight yellowish cast. Because of its hardness, brittleness, and very high melting point, solid iridium is difficult to machine, form, or work; thus powder metallurgy is commonly employed instead.[8] It is the only metal to maintain good mechanical properties in air at temperatures above 1,600 °C (2,910 °F).[9] It has the 10th highest boiling point among all elements and becomes a superconductor at temperatures below 0.14 K.[10]

Iridium's modulus of elasticity is the second-highest among the metals, only being surpassed by osmium.[9] 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.[9]

The measured density of iridium is only slightly lower (by about 0.12%) than that of osmium, the densest metal known.[11][12] 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,[13] but, with increased accuracy in factors used for calculating density X-ray crystallographic data yielded densities of 22.56 g/cm3 for iridium and 22.59 g/cm3 for osmium.[14]

Chemical properties

Iridium is the most corrosion-resistant metal known:[15] it is not attacked by almost any acid, aqua regia, molten metals, or silicates at high temperatures. It can, however, be attacked by some molten salts, such as sodium cyanide and potassium cyanide,[16] as well as oxygen and the halogens (particularly fluorine)[17] at higher temperatures.[18] Iridium also reacts directly with sulfur at atmospheric pressure to yield iridium disulfide.[19]

Compounds

Oxidation states[note 1]
−3 [Ir(CO)
3
]3−
−1 [Ir(CO)
3
(PPh
3
)]
0 Ir
4
(CO)
12
+1 [Ir(CO)Cl(PPh
3
)
2
]
+2 IrCl
2
+3 IrCl
3
+4 IrO
2
+5 Ir
4
F
20
+6 IrF
6
+7 [(η2
-O
2
)IrO
2
]+
+8 IrO
4
+9 [IrO
4
]+
[2]

Iridium forms compounds in oxidation states between −3 and +9; the most common oxidation states are +3 and +4.[8] Well-characterized examples of the high +6 oxidation state are rare, but include 6 and two mixed oxides Sr
2
MgIrO
6
and Sr
2
CaIrO
6
.[8][20] In addition, it was reported in 2009 that iridium(VIII) oxide (IrO
4
) was prepared under 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.[21] The highest oxidation state (+9), which is also the highest recorded for any element, is only known in one cation, IrO+
4
; it is only known as gas-phase species and is not known to form any salts.[2]

Iridium dioxide, IrO
2
, a blue black solid, is the only well-characterized oxide of iridium.[8] A sesquioxide, Ir
2
O
3
, has been described as a blue-black powder which is oxidized to IrO
2
by HNO
3
.[17] The corresponding disulfides, diselenides, sesquisulfides, and sesquiselenides are known, and IrS
3
has also been reported.[8] Iridium also forms iridates with oxidation states +4 and +5, such as K
2
IrO
3
and KIrO
3
, which can be prepared from the reaction of potassium oxide or potassium superoxide with iridium at high temperatures.[22]

Although no binary hydrides of iridium, Ir
x
H
y
are known, complexes are known that contain IrH4−
5
and IrH3−
6
, where iridium has the +1 and +3 oxidation states, respectively.[23] The ternary hydride Mg
6
Ir
2
H
11
is believed to contain both the IrH4−
5
and the 18-electron IrH5−
4
anion.[24]

No monohalides or dihalides are known, whereas trihalides, IrX
3
, are known for all of the halogens.[8] For oxidation states +4 and above, only the tetrafluoride, pentafluoride and hexafluoride are known.[8] Iridium hexafluoride, IrF
6
, is a volatile and highly reactive yellow solid, composed of octahedral molecules. It decomposes in water and is reduced to 4, a crystalline solid, by iridium black.[8] Iridium pentafluoride has similar properties but it is actually a tetramer, Ir
4
F
20
, formed by four corner-sharing octahedra.[8] Iridium metal dissolves in molten alkali-metal cyanides to produce the Ir(CN)3+
6
(hexacyanoiridate) ion.

Hexachloroiridic(IV) acid, H
2
IrCl
6
, and its ammonium salt are the most important iridium compounds from an industrial perspective.[25] They are involved 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.[25] Iridium trichloride, IrCl
3
, which can be obtained in anhydrous form from direct oxidation of iridium powder by chlorine at 650 °C,[25] or in hydrated form by dissolving Ir
2
O
3
in hydrochloric acid, is often used as a starting material for the synthesis of other Ir(III) compounds.[8] Another compound used as a starting material is ammonium hexachloroiridate(III), (NH
4
)
3
IrCl
6
. Iridium(III) complexes are diamagnetic (low-spin) and generally have an octahedral molecular geometry.[8]

Organoiridium compounds contain iridium–carbon bonds where the metal is usually in lower oxidation states. For example, oxidation state zero is found in tetrairidium dodecacarbonyl, Ir
4
(CO)
12
, which is the most common and stable binary carbonyl of iridium.[8] 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 their discoverers. One is Vaska's complex, IrCl(CO)[P(C
6
H
5
)
3
]
2
, which has the unusual property of binding to the dioxygen molecule, O
2
.[26] Another one is Crabtree's catalyst, a homogeneous catalyst for hydrogenation reactions.[27] These compounds are both square planar, d8 complexes, with a total of 16 valence electrons, which accounts for their reactivity.[28]

An iridium-based organic LED material has been documented, and found to be much brighter than DPA or PPV, so could be the basis for flexible OLED lighting in the future.[29]

Isotopes

Iridium has two naturally occurring, stable isotopes, 191Ir and 193Ir, with natural abundances of 37.3% and 62.7%, respectively.[30] At least 37 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 metastable isomers have been characterized, ranging in mass number from 164 to 197. The most stable of these is 192m2Ir, which decays by 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]