Iridium

Overview

Iridium is a chemical element; it has the symbol Ir and atomic number 77. This very hard, brittle, silvery-white transition metal of the platinum group is considered the second-densest naturally occurring metal (after osmium), with a density of 22.56 g/cm3 (0.815 lb/cu in) as defined by experimental X-ray crystallography. 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, even at temperatures as high as 2,000 °C (3,600 °F). Iridium was discovered in 1803 in the acid-insoluble residues of platinum ores by the British 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. The dominant uses of iridium and its alloys are 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. 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 non-avian 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.

Physical properties

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. It is the only metal to maintain good mechanical properties in air at temperatures above 1600 °C (2910 °F). It has the 10th highest boiling point among all elements and becomes a superconductor at temperatures below 0.14 K (−273.01 °C; −459.42 °F). Iridium's modulus of elasticity is the second-highest among the metals, surpassed only by osmium. 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. The measured density of iridium is only slightly lower (by about 0.12 %) than that of osmium, the densest metal known. 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, 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. 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). The Vickers hardness of pure platinum is 56 HV, whereas an alloy of 50 % platinum and iridium can reach over 500 HV.

Chemical properties

Iridium is the most corrosion-resistant metal known. It is not attacked by acids, including aqua regia, but it can be dissolved in concentrated hydrochloric acid in the presence of sodium perchlorate. In the presence of oxygen, it reacts with cyanide salts. Traditional oxidants also react, including the halogens and oxygen at higher temperatures. Iridium also reacts directly with sulfur at atmospheric pressure to yield iridium disulfide.

Isotopes

Iridium has two naturally occurring stable isotopes, 191Ir and 193Ir, with natural abundances of 37.3 % and 62.7 %, respectively. There are also 40 known radioisotopes, ranging in mass number from 164 to 205. 192Ir, which falls between the two stable isotopes, is the most stable radioisotope, has a half-life of 73.82 days, and finds application in brachytherapy 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. Isotopes with masses below 191 decay by some combination of β+ decay, α decay, and (rare) proton emission. Synthetic isotopes heavier than 193 decay by β− decay, and 192Ir has both, but β+ decay (95.24 %) dominates over electron capture (4.76 %). 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, making it more stable than any of iridium's ground-state radioisotopes. 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.

Oxidation states

Iridium forms compounds in oxidation states between −3 and +9, but the most common oxidation states are +1, +2, +3, and +4. Well-characterized compounds containing iridium in the +6 oxidation state include IrF6 and the oxides Sr2MgIrO6 and Sr2CaIrO6. Iridium(VIII) oxide (IrO4) was generated under matrix isolation conditions at 6 K in argon. The highest oxidation state (+9), which is also the highest recorded for any element, is found in gaseous [IrO4]+.

Binary compounds

Iridium forms trihydride at very high pressures. Only one binary oxide is well-characterized: iridium dioxide, IrO2. It is a blue-black solid that adopts the fluorite structure. A sesquioxide, Ir2O3, has been described as a blue-black powder, which is oxidized to IrO2 by HNO3. Other iridium chalcogenides include the sulfides IrS2, Ir2S3 and IrS3, and the selenides IrSe2 and Ir2Se3. Binary trihalides, IrX3, are known for all of the halogens. For oxidation states +4 and above, only the tetrafluoride, pentafluoride and hexafluoride are known. Iridium hexafluoride, IrF6, is a volatile yellow solid, composed of octahedral molecules. It decomposes in water and is reduced to IrF4. Iridium pentafluoride is also a strong oxidant, but it is a tetramer, Ir4F20, formed by four corner-sharing octahedra.

Complexes

Iridium has extensive coordination chemistry. Iridium in its complexes is always low-spin. Ir(III) and Ir(IV) generally form octahedral complexes. Polyhydride complexes are known for the +5 and +3 oxidation states. One example is IrH5(PiPr3)2 (iPr = isopropyl). The ternary hydride Mg6Ir2H11 is believed to contain both the IrH54− and the 18-electron IrH45− anion. Iridium also forms oxyanions with oxidation states +4 and +5. K2IrO3 and KIrO3 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. Just like many elements, iridium forms important chloride complexes. Hexachloroiridic (IV) acid, H2IrCl6, and its ammonium salt are common iridium compounds from both industrial and preparative perspectives. 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 IrCl62− ion has an intense dark brown color, and can be readily reduced to the lighter-colored IrCl63− and vice versa. Iridium trichloride, IrCl3, which can be obtained in anhydrous form from direct oxidation of iridium powder by chlorine at 650 °C, or in hydrated form by dissolving Ir2O3 in hydrochloric acid, is often used as a starting material for the synthesis of other Ir(III) compounds. Another compound used as a starting material is potassium hexachloroiridate(III), K3IrCl6.

Organoiridium chemistry

Organoiridium compounds contain iridium–carbon bonds. Early studies identified the very stable tetrairidium dodecacarbonyl, Ir4(CO)12. In this compound, each of the iridium atoms is bonded to the other three, forming a tetrahedral cluster. The discovery of Vaska's complex (IrCl(CO)[P(C6H5)3]2) opened the door for oxidative addition reactions, a process fundamental to useful reactions. For example, Crabtree's catalyst, a homogeneous catalyst for hydrogenation reactions.

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.

Platinum group

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 Darién and Mexico, "which no fire nor any Spanish artifice has yet been able to liquefy". 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.

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. In 1741, Charles Wood, 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 Royal Society, stating that he had seen no mention of it in any previous accounts of known minerals. 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, Torbern Bergman, 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.

Discovery

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. Joseph Louis Proust thought that the residue was graphite. The French chemists Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough for further experiments. 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 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". 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. He obtained dark red crystals (probably of Na2[IrCl6]·nH2O) by a sequence of reactions with sodium hydroxide and hydrochloric acid. 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. Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.

Metalworking and applications

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). 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 O2 and H2 gas for each 1 kilogram (2.2 lb) of iridium. 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. 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). In Munich, Germany in 1957 Rudolf Mössbauer, in what has been called one of the "landmark experiments in twentieth-century physics", discovered the resonant and recoil-free emission and absorption of gamma rays by atoms in a solid metal sample containing only 191Ir. 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.

Occurrence

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.

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; gold is 4 times more abundant, platinum is 10 times more abundant, silver and mercury are 80 times more abundant. Osmium, tellurium, ruthenium, rhodium and rhenium are about as abundant as iridium. In contrast to its low abundance in crustal rock, iridium is relatively common in meteorites, with concentrations of 0.5 ppm or more. 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. Iridium is found in nature as an uncombined element or in natural alloys, especially the iridium–osmium alloys osmiridium (osmium-rich) and iridosmium (iridium-rich). In nickel and copper deposits, the platinum group metals occur as sulfides, tellurides, antimonides, and arsenides. In all of these compounds, platinum can be exchanged with a small amount of iridium or osmium. As with all of the platinum group metals, iridium can be found naturally in alloys with raw nickel or raw copper. A number of iridium-dominant minerals, with iridium as the species-forming element, are known. They are exceedingly rare and often represent the iridium analogues of the above-given ones. The examples are irarsite and cuproiridsite, to mention some. Within Earth's crust, iridium is found at highest concentrations in three types of geologic structure: igneous deposits (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, (near the largest known impact structure, the Vredefort impact structure) though the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin (also an impact crater) in Canada are also significant sources of iridium. Smaller reserves are found in the United States. Iridium is also found in secondary deposits, combined with platinum and other platinum group metals in alluvial deposits. The alluvial deposits used by 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.