Osmium

Osmium is a chemical element; it has symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group. Osmium has the highest density of any stable element and one of the rarest elements in the Earth's crust, with an estimated abundance of 50 parts per trillion. Manufacturers use alloys of osmium with platinum, iridium, and other platinum-group metals for fountain pen nib tipping, electrical contacts, and other applications that require extreme durability and hardness.

Characteristics

Physical properties

Osmium is a hard, brittle, blue-gray metal, and the densest stable element—about twice as dense as lead. The density of osmium is slightly greater than that of iridium; the two are so similar that each was at one time considered to be the densest element. Only in the 1990s were measurements made accurately enough to be certain that osmium is the denser of the two.
Osmium has a blue-gray tint. The reflectivity of single crystals of osmium is complex and strongly direction-dependent, with light in the red and near-infrared wavelengths being more strongly absorbed when polarized parallel to the c crystal axis than when polarized perpendicular to the c axis; the c-parallel polarization is also slightly more reflected in the mid-ultraviolet range. Reflectivity reaches a sharp minimum at around 1.5 eV for the c-parallel polarization and at 2.0 eV for the c-perpendicular polarization, and peaks for both in the visible spectrum at around 3.0 eV.
Osmium is a hard but brittle metal that remains lustrous even at high temperatures. It has a very low compressibility. Correspondingly, its bulk modulus is extremely high, reported between and, which rivals that of diamond. The hardness of osmium is moderately high at. Because of its hardness, brittleness, low vapor pressure, and very high melting point, solid osmium is difficult to machine, form, or work.

Chemical properties

Osmium forms compounds with oxidation states ranging from −2 to +8. The most common oxidation states are +2, +3, +4, and +8. The +8 oxidation state is notable for being the highest attained by any chemical element aside from iridium's +9 and is encountered besides osmium only in xenon, ruthenium, hassium, iridium, and plutonium. Examples of the −1 and −2 oxidation states are and, respectively; these reactive compounds are used to synthesize osmium cluster compounds. Another example of the −1 oxidation state of osmium is.
The most common compound exhibiting the +8 oxidation state, osmium tetroxide, is a volatile, water-soluble solid with a "pronounced and nauseating" smell. Osmium tetroxide forms red perosmates upon reaction with a base. With ammonia, it forms the nitrido-osmates Potassium osmiamate|. The +4 oxide, osmium dioxide, is a darkly-colored, non-volatile, and much less reactive compound.
Osmium pentafluoride is known, but osmium trifluoride has not yet been synthesized. The lower oxidation states are stabilized by the larger halogens, so that the trichloride, tribromide, triiodide, and even diiodide are known. The oxidation state +1 is known only for osmium monoiodide, whereas several carbonyl complexes of osmium, such as triosmium dodecacarbonyl, represent oxidation state 0.
In general, the lower oxidation states of osmium are stabilized by ligands that are good σ-donors and π-acceptors. The higher oxidation states are stabilized by strong σ- and π-donors, such as and.
Despite its broad range of compounds in numerous oxidation states, osmium in bulk form at ordinary temperatures and pressures is stable in air. It resists attack by most acids and bases including aqua regia, but is attacked by and at high temperatures, and by hot concentrated nitric acid to produce. It can be dissolved by molten alkalis fused with an oxidizer such as sodium peroxide or potassium chlorate to give osmates such as potassium osmate|.

Isotopes

Osmium has seven naturally occurring isotopes, five of which are stable:,,,, and . At least 37 artificial radioisotopes and 20 nuclear isomers exist, with mass numbers ranging from 160 to 203; the most stable of these is with a half-life of 6.0 years.
The two primordial radioisotopes and are known to undergo alpha decay with long half-lives: the current best values being and years, approximately times the age of the universe, that for practical purposes it can be considered stable. Alpha decay is predicted for all the other naturally occurring isotopes, but has not been observed, presumably due to very long half-lives; and are also predicted to undergo double beta decay, but this not been observed either.
¹⁸⁹Os has a spin of 3/2 but ¹⁸⁷Os has a nuclear spin 1/2. Its low natural abundance and low nuclear magnetic moment means that it is one of the most difficult natural abundance isotopes for NMR spectroscopy.
is the descendant of and is used extensively in dating terrestrial as well as meteoric rocks. It has also been used to measure the intensity of continental weathering over geologic time and to fix minimum ages for stabilization of the mantle roots of continental cratons. This decay is a reason why rhenium-rich minerals are abnormally rich in.

History

Osmium was discovered in 1803 by Smithson Tennant and William Hyde Wollaston in London, England. The discovery of osmium is intertwined with that of platinum and the other metals of the platinum group. Platinum reached Europe as platina, first encountered in the late 17th century in silver mines around the Chocó Department, in Colombia. The discovery that this metal was not an alloy, but a distinct new element, was published in 1748.
Chemists who studied platinum dissolved it in aqua regia to create soluble salts. They always observed a small amount of a dark, insoluble residue. Joseph Louis Proust thought that the residue was graphite. Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed iridium in the black platinum residue in 1803, but did not obtain enough material for further experiments. Later the two French chemists Fourcroy and Vauquelin identified a metal in a platinum residue they called ptène.
In 1803, Smithson Tennant 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 was of this new metal—which he named ptene, from the Greek word πτηνος for winged. However, Tennant, who had the advantage of a much larger amount of residue, continued his research and identified two previously undiscovered elements in the black residue, iridium and osmium. He obtained a yellow solution by reactions with sodium hydroxide at red heat. After acidification he was able to distill the formed OsO₄. He named it osmium after Greek osme meaning "a smell", because of the chlorine-like and slightly garlic-like smell of the volatile osmium tetroxide. Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.
Uranium and osmium were early successful catalysts in the Haber process, the nitrogen fixation reaction of nitrogen and hydrogen to produce ammonia, giving enough yield to make the process economically successful. At the time, a group at BASF led by Carl Bosch bought most of the world's supply of osmium to use as a catalyst. Shortly thereafter, in 1908, cheaper catalysts based on iron and iron oxides were introduced by the same group for the first pilot plants, removing the need for the expensive and rare osmium.
Osmium is now obtained primarily from the processing of platinum and nickel ores.

Occurrence

Osmium is one of the least abundant stable elements in Earth's crust, with an average mass fraction of 50 parts per trillion in the continental crust.
Osmium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium, and iridosmium. In nickel and copper deposits, the platinum-group metals occur as sulfides, tellurides, antimonides, and arsenides ; in all these compounds platinum is exchanged by a small amount of iridium and osmium. As with all of the platinum-group metals, osmium can be found naturally in alloys with nickel or copper.
Within Earth's crust, osmium, like iridium, is found at highest concentrations in three types of geologic structure: igneous deposits, 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, though the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin in Canada are also significant sources of osmium. Smaller reserves can be found in the United States. 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.

Production

Osmium is obtained commercially as a by-product from nickel and copper mining and processing. During electrorefining of copper and nickel, noble metals such as silver, gold and the platinum-group metals, together with non-metallic elements such as selenium and tellurium, settle to the bottom of the cell as anode mud, which forms the starting material for their extraction. Separating the metals requires that they first be brought into solution. Several methods can achieve this, depending on the separation process and the composition of the mixture. Two representative methods are fusion with sodium peroxide followed by dissolution in aqua regia, and dissolution in a mixture of chlorine with hydrochloric acid. Osmium, ruthenium, rhodium, and iridium can be separated from platinum, gold, and base metals by their insolubility in aqua regia, leaving a solid residue. Rhodium can be separated from the residue by treatment with molten sodium bisulfate. The insoluble residue, containing ruthenium, osmium, and iridium, is treated with sodium oxide, in which Ir is insoluble, producing water-soluble ruthenium and osmium salts. After oxidation to the volatile oxides, is separated from by precipitation of ₃RuCl₆ with ammonium chloride.
After it is dissolved, osmium is separated from the other platinum-group metals by distillation or extraction with organic solvents of the volatile osmium tetroxide. The first method is similar to the procedure used by Tennant and Wollaston. Both methods are suitable for industrial-scale production. Modern methods involve reducing ammonium hexachloroosmate using hydrogen, yielding the metal as a powder or sponge that can be treated using powder metallurgy techniques.
Estimates of annual worldwide osmium production are on the order of several hundred to a few thousand kilograms. Production and consumption figures for osmium are not well reported because demand for the metal is limited and can be fulfilled with the byproducts of other refining processes. To reflect this, statistics often report osmium with other minor platinum group metals such as iridium and ruthenium. US imports of osmium from 2014 to 2021 averaged 155 kg annually.