Hafnium is a chemical element; it has symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869, though it was not identified until 1922, by Dirk Coster and George de Hevesy. Hafnium is named after , the Latin name for Copenhagen, where it was discovered. The element is obtained only by separation from zirconium, with most of the world's hafnium production coming from processes that also produce zirconium. These processes make use of heavy mineral sands ore deposits, which include the minerals zircon, rutile, and ilmenite, among others.
Hafnium is most often used in alloys with nickel, and was used in larger quantities to produce the control rods used in nuclear reactors. Hafnium's large neutron capture cross section makes it a good material for neutron absorption in control rods in nuclear power plants, but at the same time requires that it be removed from the neutron-transparent corrosion-resistant zirconium alloys used in nuclear reactors. It is ductile, and is also used in filaments and electrodes. Some semiconductor fabrication processes use its oxide for integrated circuits at and smaller, and superalloys used for special applications can contain hafnium in combination with niobium, titanium, or tungsten.
Pure hafnium is not toxic, but is extremely flammable to the point of being pyrophoric—capable of spontaneous combustion in air. Several industrial processes involved in the production of hafnium have by-products that can be hazardous when released into the environment, and several hafnium compounds have hazards of their own. One nuclear isomer of hafnium, Hf, was the source of a controversy for its potential use as a weapon, but it has never been successfully produced for practical use.
Characteristics
Physical characteristics
Hafnium is a shiny, silvery, ductile metal that is corrosion-resistant and chemically similar to zirconium in that they have the same number of valence electrons and are in the same group. Also, their relativistic effects are similar: The expected expansion of atomic radii from period 5 to 6 is almost exactly canceled out by the lanthanide contraction. Hafnium changes from its alpha form, a hexagonal close-packed lattice, to its beta form, a body-centered cubic lattice, at . 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.
A notable physical difference between these metals is their density, with zirconium having about one-half the density of hafnium. The most notable nuclear properties of hafnium are its high thermal neutron capture cross section, roughly three orders of magnitude greater than that of zirconium, and that the nuclei of several different hafnium isotopes readily absorb two or more neutrons apiece. Because zirconium is practically transparent to thermal neutrons, it is commonly used for the metal components of nuclear reactors—especially the cladding of their nuclear fuel rods.
Chemical characteristics
Hafnium reacts in air to form a protective film of hafnium oxide in the monoclinic phase that inhibits further corrosion. 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. The metal is resistant to concentrated alkalis.
As a consequence of lanthanide contraction, the chemistry of hafnium and zirconium is so similar that the two cannot be separated based on differing chemical reactions. The melting and boiling points of the compounds and the solubility in solvents are the major differences in the chemistry of these twin elements.
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 Hf has a very long half-life of years.
The extinct radionuclide Hf has a half-life of , and is an important tracker isotope for the formation of planetary cores. No other radioisotope has a half-life over 1.87 years.
The longest-lived nuclear isomer Hf (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 Hf, 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
Hafnium is estimated to make up about between 3.0 and 4.8 ppm of the Earth's upper crust by mass. ABUNDANCE OF ELEMENTS IN THE EARTH'S CRUST AND IN THE SEA, CRC Handbook of Chemistry and Physics, 97th edition (2016–2017), p. 14-17 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, , 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
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.
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 or the fractional distillation of the chloride—did not prove suitable for an industrial-scale production. After zirconium was chosen as a material for nuclear reactor programs in the 1940s, a separation method had to be developed. Liquid–liquid extraction processes with a wide variety of solvents were developed and are still used for producing hafnium. Other methods to purify hafnium from zirconium include molten salt extraction and crystallization of fluorozirconates. 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). Consequently, compounds of hafnium(IV) and zirconium(IV) have very similar chemical and physical properties. Hafnium and zirconium tend to occur together in nature and the similarity of their ionic radii makes their chemical separation rather difficult. Hafnium tends to form inorganic compounds in the oxidation state of +4. Halogens react with it to form hafnium tetrahalides. At higher temperatures, hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Some hafnium compounds in lower oxidation states are known.
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. These tetrahalides are precursors to various organohafnium compounds, 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 (), with a melting point of and a boiling point of roughly , is very similar to zirconia, but slightly more basic. Hafnium carbide is the most refractory binary compound known, with a melting point over , and hafnium nitride is the most refractory of all known metal nitrides, with a melting point of . 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
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.