Hafnium

Physical characteristics Hafnium is a shiny, silvery, ductile metal that is corrosion-resistant and chemically similar to zirconium The physical properties of hafnium metal samples are markedly affected by zirconium impurities, especially the nuclear properties, as these two elements are among the most difficult to separate because of their chemical similarity. Despite this, the metal is attacked by hydrofluoric acid and concentrated sulfuric acid, and can be oxidized with halogens or burnt in air. Like its sister metal zirconium, finely divided hafnium can ignite spontaneously in air. Isotopes At least 40 isotopes of hafnium have been observed, ranging in mass number from 153 to 192. The five stable isotopes have mass numbers from 176 to 180 inclusive; the primordial 174Hf has a very long half-life of years. No other radioisotope has a half-life over 1.87 years. The longest-lived nuclear isomer 178m2Hf (31 years) was at the center of a controversy for several years regarding its potential use as a weapon. Because of its high energy compared to the ground state 178Hf, the isomer was put under scrutiny as being capable of induced gamma emission, which could be weaponized to produce large amounts of gamma radiation all at once. Applications of the isomer have been frustrated due to the difficulty of producing it without the product being immediately destroyed as well as its extremely high cost. Occurrence , Brazil Hafnium is estimated to make up about between 3.0 and 4.8 ppm of the Earth's upper crust by mass. It does not exist as a free element on Earth, but is found combined in solid solution with zirconium in natural zirconium compounds such as zircon, ZrSiO4, which usually has about 1–4% of the Zr replaced by Hf. Rarely, the Hf/Zr ratio increases during crystallization to give the isostructural mineral hafnon , with atomic Hf > Zr. An obsolete name for a variety of zircon containing unusually high Hf content is alvite. A major source of zircon (and hence hafnium) ores is heavy mineral sands ore deposits, pegmatites, particularly in Brazil and Malawi, and carbonatite intrusions, particularly the Crown Polymetallic Deposit at Mount Weld, Western Australia. A potential source of hafnium is trachyte tuffs containing rare zircon-hafnium silicates eudialyte or armstrongite, at Dubbo in New South Wales, Australia. ==Production==

remelting furnace, a 1 cm cube, and an oxidized hafnium electron beam-remelted ingot (left to right) The heavy mineral sands ore deposits of the titanium ores ilmenite and rutile yield most of the mined zirconium, and therefore also most of the hafnium. Zirconium is a good nuclear fuel-rod cladding metal, with the desirable properties of a very low neutron capture cross section and good chemical stability at high temperatures. However, because of hafnium's neutron-absorbing properties, hafnium impurities in zirconium would cause it to be far less useful for nuclear reactor applications. Thus, a nearly complete separation of zirconium and hafnium is necessary for their use in nuclear power. The production of hafnium-free zirconium is the main source of hafnium. effects The chemical properties of hafnium and zirconium are nearly identical, which makes the two difficult to separate. The methods first used—fractional crystallization of ammonium fluoride salts About half of all hafnium metal manufactured is produced as a by-product of zirconium refinement. The end product of the separation is hafnium(IV) chloride. The purified hafnium(IV) chloride is converted to the metal by reduction with magnesium or sodium, as in the Kroll process. : HfCl4{} + 2 Mg ->[1100~^\circ\text{C}] Hf{} + 2 MgCl2 Further purification is effected by a chemical transport reaction developed by Arkel and de Boer: In a closed vessel, hafnium reacts with iodine at temperatures of , forming hafnium(IV) iodide; at a tungsten filament of the reverse reaction happens preferentially, and the chemically bound iodine and hafnium dissociate into the native elements. The hafnium forms a solid coating at the tungsten filament, and the iodine can react with additional hafnium, resulting in a steady iodine turnover and ensuring the chemical equilibrium remains in favor of hafnium production. : Hf{} + 2 I2 ->[500~^\circ\text{C}] HfI4 : HfI4 ->[1700~^\circ\text{C}] Hf{} + 2 I2 ==Chemical compounds==

Due to the lanthanide contraction, the ionic radius of hafnium(IV) (0.78 ångström) is almost the same as that of zirconium(IV) (0.79 angstroms). Hafnium(IV) chloride and hafnium(IV) iodide have some applications in the production and purification of hafnium metal. They are volatile solids with polymeric structures. and hafnium(IV) chloride in particular is used in microelectronics manufacturing as a source of hafnium oxide in atomic layer deposition, much in the same way as zirconium(IV) chloride. The white hafnium oxide (HfO2), with a melting point of and a boiling point of roughly , is very similar to zirconia, but slightly more basic. Hafnium carbonitride has the highest known melting point for any material, which is confirmed to be above by experiment, while calculations predict its melting point to be . ==History==

emission lines of some elements Hafnium's existence was predicted by Dmitri Mendeleev in 1869. In his report on The Periodic Law of the Chemical Elements, in 1869, Dmitri Mendeleev had implicitly predicted the existence of a heavier analog of titanium and zirconium. At the time of his formulation in 1871, Mendeleev believed that the elements were ordered by their atomic masses and placed lanthanum (element 57) in the spot below zirconium. The exact placement of the elements and the location of missing elements was done by determining the specific weight of the elements and comparing the chemical and physical properties. The X-ray spectroscopy done by Henry Moseley in 1914 showed a direct dependency between spectral line and effective nuclear charge. This led to the nuclear charge, or atomic number of an element, being used to ascertain its place within the periodic table. With this method, Moseley determined the number of lanthanides and showed the gaps in the atomic number sequence at numbers 43, 61, 72, and 75. The discovery of the gaps led to an extensive search for the missing elements. In 1914, several people claimed the discovery after Henry Moseley predicted the gap in the periodic table for the then-undiscovered element 72. Georges Urbain asserted that he found element 72 in the rare earth elements in 1907 and published his results on celtium in 1911. Neither the spectra nor the chemical behavior he claimed matched with the element found later, and therefore his claim was turned down after a long-standing controversy. The controversy was partly because the chemists favored the chemical techniques which led to the discovery of celtium, while the physicists relied on the use of the new X-ray spectroscopy method that proved that the substances discovered by Urbain did not contain element 72. suggested that element 72 should resemble zirconium and therefore was not part of the rare earth elements group. By early 1923, Niels Bohr and others agreed with Bury. These suggestions were based on Bohr's theories of the atom which were identical to chemist Charles Bury, Encouraged by these suggestions and by the reappearance in 1922 of Urbain's claims that element 72 was a rare earth element discovered in 1911, Dirk Coster and Georg von Hevesy were motivated to search for the new element in zirconium ores. Hafnium was discovered by the two in 1923 in Copenhagen, Denmark, validating the original 1869 prediction of Mendeleev. It was ultimately found in zircon in Norway through X-ray spectroscopy analysis. The place where the discovery took place led to the element being named for the Latin name for "Copenhagen", Hafnia, the home town of Niels Bohr. Today, the Faculty of Science of the University of Copenhagen uses in its seal a stylized image of the hafnium atom. Hafnium was separated from zirconium through repeated recrystallization of the double ammonium or potassium fluorides by Valdemar Thal Jantzen and von Hevesey. Anton Eduard van Arkel and Jan Hendrik de Boer were the first to prepare metallic hafnium by passing hafnium tetraiodide vapor over a heated tungsten filament in 1924. This process for differential purification of zirconium and hafnium is still in use today. thus making elements 75 (rhenium) and 72 (hafnium) the last two stable elements to be discovered. The element rhenium was found in 1908 by Masataka Ogawa, though its atomic number was misidentified at the time, and it was not generally recognised by the scientific community until its rediscovery by Walter Noddack, Ida Noddack, and Otto Berg in 1925. This makes it somewhat difficult to say if hafnium or rhenium was discovered last. ==Applications==

Much of the hafnium produced is used in the manufacture of control rods for nuclear reactors and as an additive in nickel alloys to increase their heat resistance. Hafnium products, such as tubes and sheets of the metal, could be purchased at /kg($170/lb) in 2009. is about 600 times that of zirconium (other elements that are good neutron-absorbers for control rods are cadmium and boron). Excellent mechanical properties and exceptional corrosion-resistance properties allow its use in the harsh environment of pressurized water reactors. It is also common in military reactors, particularly in US naval submarine reactors, to slow reactor rates that are too high. It is seldom found in civilian reactors, the first core of the Shippingport Atomic Power Station (a conversion of a naval reactor) being a notable exception. Alloys in the lower right corner Hafnium is used in alloys with iron, titanium, niobium, tantalum, and other metals. An alloy used for liquid-rocket thruster nozzles, for example the main engine of the Apollo Lunar Modules, is C103 which consists of 89% niobium, 10% hafnium and 1% titanium. Small additions of hafnium increase the adherence of protective oxide scales on nickel-based alloys. It thereby improves the corrosion resistance, especially under cyclic temperature conditions that tend to break oxide scales, by inducing thermal stresses between the bulk material and the oxide layer. An alloy that includes as little as 1% hafnium can withstand temperatures that are higher than the same alloy without hafnium. Hafnium oxide-based compounds are practical high-κ dielectrics, allowing reduction of the gate leakage current which improves performance at such scales. Isotope geochemistry Isotopes of hafnium and lutetium are also used in isotope geochemistry and geochronological applications, in lutetium-hafnium dating. It is often used as a tracer of isotopic evolution of Earth's mantle through time. This is because 176Lu decays to 176Hf with a half-life of approximately 37 billion years. In most geologic materials, zircon is the dominant host of hafnium (>10,000 ppm) and is often the focus of hafnium studies in geology. Hafnium is readily substituted into the zircon crystal lattice, and is therefore very resistant to hafnium mobility and contamination. Zircon also has an extremely low Lu/Hf ratio, making any correction for initial lutetium minimal. Although the Lu/Hf system can be used to calculate a "model age", i.e. the time at which it was derived from a given isotopic reservoir such as the depleted mantle, these "ages" do not carry the same geologic significance as do other geochronological techniques as the results often yield isotopic mixtures and thus provide an average age of the material from which it was derived. Garnet is another mineral that contains appreciable amounts of hafnium to act as a geochronometer. The high and variable Lu/Hf ratios found in garnet make it useful for dating metamorphic events. Mass spectrometry also makes use of these ratios to date garnet formed through igneous events. Other uses Due to its heat resistance and its affinity to oxygen and nitrogen, hafnium is a good scavenger for oxygen and nitrogen in gas-filled and incandescent lamps. Hafnium is also used as the electrode in plasma cutting because of its ability to shed electrons into the air. Hafnium metallocene compounds can be prepared from hafnium tetrachloride and various cyclopentadiene-type ligand species. Perhaps the simplest hafnium metallocene is hafnocene dichloride. Hafnium metallocenes are part of a large collection of Group 4 transition metal metallocene catalysts that are used worldwide in the production of polyolefin resins like polyethylene and polypropylene. A pyridyl-amidohafnium catalyst can be used for the controlled iso-selective polymerization of propylene, which can then be combined with polyethylene to make a tougher recycled plastic. The high energy content of 178m2Hf was the concern of a DARPA-funded program in the US. This program eventually concluded that using the 178m2Hf nuclear isomer of hafnium to construct high-yield weapons with X-ray triggering mechanisms—an application of induced gamma emission—was infeasible because of its expense and difficulty to manufacture. ==Toxicity and safety==

}} Hafnium is a pyrophoric material, and as such fine particles can spontaneously combust upon exposure to air. Hafnium powder is often wetted with at least 25% water by weight to be considered safe - the metal is insoluble in water. Machining hafnium is particularly hazardous because of the potential for fine particles of the metal to be produced and immediately introduced to frictional force. Compounds that contain this metal are rarely encountered by most people. The pure metal is not considered toxic, though it has been observed to accumulate in the liver when injected into rats. Because the mineral zircon is often associated with traces of the radioactive elements uranium and thorium, the chemically destructive processes used to separate zirconium from hafnium have potential to release these radioactive elements and their decay products into the environment along with other reaction wastes. Additionally, synthesis pathways that involve liquid-liquid extraction introduce ammonium chloride and sulfate into reaction mixtures, which as effluent can reduce available oxygen in water sources or produce cyanides if it comes into contact with thiocyanate-containing compounds. ==References==