New Iridium
Leanne Wittlin
Iridium is a chemical element; it has symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of 22.56 g/cm3 (0.815 lb/cu in)[8] as defined by experimental X-ray crystallography.[a] 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. 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 in the acid-insoluble residues of platinum ores by the English chemist Smithson Tennant. The name iridium, derived from the Greek word iris (rainbow), refers to the various colors of its compounds. Iridium is one of the rarest elements in Earth's crust, with an estimated annual production of only 6,800 kilograms (15,000 lb) in 2023.[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'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 density of iridium is only slightly lower (by about 0.12%) than that of osmium, the 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]
Iridium is the most corrosion-resistant metal known.[25] It is not attacked by acids, including aqua regia, but it can be dissolved in concentrated hydrochloric acid in the presence of sodium perchlorate.[12] 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]
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 Mssbauer effect. This renders it useful for Mssbauer spectroscopy for research in physics, chemistry, biochemistry, metallurgy, and mineralogy.[33]
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.[15] For oxidation states +4 and above, only the 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]
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]
The discovery of iridium is intertwined with that of platinum and the other metals of the platinum group. The first European reference to platinum appears in 1557 in the writings of the Italian humanist Julius Caesar Scaliger as a description of an unknown noble metal found between Darin and Mexico, "which no fire nor any Spanish artifice has yet been able to liquefy".[46] From their first encounters with platinum, the Spanish generally saw the metal as a kind of impurity in gold, and it was treated as such. It was often simply thrown away, and there was an official decree forbidding the adulteration of gold with platinum impurities.[47]
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 1750, after studying the platinum sent to him by Wood, Brownrigg presented a detailed account of the metal to the Royal Society, stating that he had seen no mention of it in any previous accounts of known minerals.[49] Brownrigg also made note of platinum's extremely high melting point and refractory metal-like behaviour toward borax. Other chemists across Europe soon began studying platinum, including Andreas Sigismund Marggraf,[50] Torbern Bergman, Jns 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]
Chemists who studied platinum dissolved it in aqua regia (a mixture of hydrochloric and nitric acids) to create soluble salts. They always observed a small amount of a dark, insoluble residue.[16] Joseph Louis Proust thought that the residue was graphite.[16] The French chemists Victor Collet-Descotils, Antoine Franois, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough for further experiments.[16]
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 has ever been constructed" (at that time).[16] The first to obtain high-purity iridium was Robert 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 nearly immalleable and very hard. The first melting in appreciable quantity was done by 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 Munich, Germany in 1957 Rudolf Mssbauer, in what has been called one of the "landmark experiments in twentieth-century physics",[55] discovered the resonant and recoil-free emission and absorption of gamma rays by atoms in a solid metal sample containing only 191Ir.[56] This phenomenon, known as the Mssbauer 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]
Along with many elements having atomic weights higher than that of iron, iridium is only naturally formed by the r-process (rapid neutron capture) in neutron star mergers and possibly rare types of supernovae.[58][59][60]
Iridium is one of the nine least abundant stable elements in Earth's crust, having an average mass fraction of 0.001 ppm in crustal rock; platinum is 10 times more abundant, gold is 40 times more abundant, silver and mercury are 80 times more abundant.[15] Tellurium is about as abundant as iridium.[15] In contrast to its low abundance in crustal rock, iridium is relatively common in meteorites, with concentrations of 0.5 ppm or more.[62] 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 and siderophilic ("iron-loving") character of iridium, it descended below the crust and into Earth's core when the planet was still molten.[40]