Iridium

Iridium is a chemical element with the symbol Ir and atomic number 77. This silvery-white metal, which has a slight yellowish tint, belongs to the platinum group. It is very hard and brittle, and it ranks as the second-densest naturally occurring metal, trailing only osmium; its density, measured by X-ray crystallography, is 22.56 g/cm³. The element gets its name from the Greek word *iris*, meaning rainbow, a reference to the many colors seen in its compounds.

Only two naturally occurring isotopes exist: 191Ir and 193Ir, both stable, with 193Ir being more abundant. Iridium is exceptionally corrosion-resistant, holding up even at temperatures as high as 2,000 °C. It is one of the rarest elements in Earth’s crust; in 2023, global production was estimated at just 6,800 kilograms.

British chemist Smithson Tennant discovered iridium in 1803, isolating it from the acid-insoluble leftovers of platinum ores. Its main uses include high-performance spark plugs, crucibles for recrystallizing semiconductors at high temperatures, and electrodes for chlorine production in the chloralkali process. Iridium compounds, particularly chlorides and iodides, serve as industrial catalysts, and the element is also a component in some OLEDs.

Iridium is far more abundant in meteorites than in Earth’s crust. This fact led to the Alvarez hypothesis: an unusually high iridium layer at the Cretaceous–Paleogene boundary suggested that a massive extraterrestrial impact—now linked to the Chicxulub crater—caused the extinction of non-avian dinosaurs 66 million years ago. Similarly, an iridium anomaly in Pacific Ocean core samples points to the Eltanin impact roughly 2.5 million years ago.

**Characteristics**

**Physical properties**

As a platinum group metal, iridium looks like platinum but with a faint yellow cast. Its hardness, brittleness, and very high melting point make solid iridium difficult to machine, form, or weld—the heat-affected zone tends to crack. Powder metallurgy is the usual fabrication method. It is the only metal that retains good mechanical properties in air above 1,600 °C. Iridium has the tenth-highest boiling point of all elements and becomes a superconductor below 0.14 K. Its modulus of elasticity is second only to osmium among metals, and its high shear modulus and very low Poisson’s ratio indicate great stiffness and resistance to deformation, which complicates manufacturing. Despite these challenges and its high cost, iridium is used where mechanical strength is needed under extreme conditions. Its measured density is only about 0.12% lower than osmium’s, and for a time there was debate over which metal was denser. With improved measurement accuracy, X-ray data now give 22.56 g/cm³ for iridium and 22.59 g/cm³ for osmium. Iridium is so brittle that welding is difficult, but adding small amounts of titanium and zirconium (0.2% each) improves ductility. For comparison, pure platinum has a Vickers hardness of 56 HV, while a 50% platinum-iridium alloy can exceed 500 HV.

**Chemical properties**

Iridium is the most corrosion-resistant metal known. It resists all acids, including aqua regia, but dissolves in concentrated hydrochloric acid with sodium perchlorate. In the presence of oxygen, it reacts with cyanide salts. Traditional oxidants like halogens and oxygen at high temperatures also attack it, and it reacts directly with sulfur at atmospheric pressure to form iridium disulfide.

**Isotopes**

The two stable isotopes, 191Ir and 193Ir, occur naturally at abundances of 37.3% and 62.7%, respectively. There are 40 known radioisotopes, with mass numbers from 164 to 205. The most stable radioisotope is 192Ir, with a half-life of 73.82 days; it is used in brachytherapy and industrial radiography, especially for nondestructive weld testing in the oil and gas industries. Iridium-192 sources have been involved in several radiological accidents. Three other isotopes—188Ir, 189Ir, and 190Ir—have half-lives of at least a day. Isotopes lighter than 191 decay via β+ decay, α decay, or rare proton emission; those heavier than 193 decay by β− decay. 192Ir decays mostly by β+ decay (95.24%), with electron capture making up the rest. At least 32 metastable isomers, with mass numbers from 164 to 197, have been characterized. The most stable is 192m2Ir, which decays by isomeric transition with a half-life of 241 years—longer than any of iridium’s ground-state radioisotopes. The isotope 191Ir was the first to show a Mössbauer effect, making it valuable for Mössbauer spectroscopy in physics, chemistry, biochemistry, metallurgy, and mineralogy.

**Chemistry**

**Oxidation states**

Iridium forms compounds in oxidation states ranging from −3 to +9, though the most common are +1, +2, +3, and +4. Well-characterized compounds in the +6 state include IrF₆ and others.