Hafnium is a
chemical element
A chemical element is a chemical substance whose atoms all have the same number of protons. The number of protons is called the atomic number of that element. For example, oxygen has an atomic number of 8: each oxygen atom has 8 protons in its ...
; it has
symbol
A symbol is a mark, Sign (semiotics), sign, or word that indicates, signifies, or is understood as representing an idea, physical object, object, or wikt:relationship, relationship. Symbols allow people to go beyond what is known or seen by cr ...
Hf and
atomic number 72. A
lustrous, silvery gray,
tetravalent transition metal, hafnium chemically resembles
zirconium and is found in many zirconium
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...
s. 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
Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
name for
Copenhagen
Copenhagen ( ) is the capital and most populous city of Denmark, with a population of 1.4 million in the Urban area of Copenhagen, urban area. The city is situated on the islands of Zealand and Amager, separated from Malmö, Sweden, by the ...
, where it was discovered.
Hafnium is used in filaments and electrodes. Some
semiconductor
A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
fabrication processes use its oxide for
integrated circuit
An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...
s at 45 nanometers and smaller feature lengths. Some
superalloys used for special applications contain hafnium in combination with
niobium,
titanium, or
tungsten.
Hafnium's large
neutron capture cross section makes it a good material for
neutron
The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
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.
Characteristics
Physical characteristics
Hafnium is a shiny, silvery,
ductile metal
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
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 and that the nuclei of several different hafnium isotopes readily absorb two or more
neutron
The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
s apiece.
In contrast with this, zirconium is practically transparent to thermal neutrons, and 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 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 chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...
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 ranging from 176 to 180 inclusive. The radioactive isotopes'
half-lives range from 400
ms for
153Hf
to years for the most stable one, the
primordial 174Hf.
The
extinct radionuclide 182Hf has a half-life of , and is an
important tracker isotope for the formation of
planetary cores.
The
nuclear isomer 178m2Hf was at the
center of a controversy for several years regarding its potential use as a weapon.
Occurrence
Hafnium is estimated to make up about between 3.0 and 4.8
ppm of the
Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
'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, ZrSiO
4, 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
Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...
and
Malawi, and
carbonatite
Carbonatite () is a type of intrusive rock, intrusive or extrusive rock, extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geoche ...
intrusions, particularly the Crown Polymetallic Deposit at
Mount Weld,
Western Australia
Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...
. A potential source of hafnium is
trachyte tuffs containing rare zircon-hafnium silicates
eudialyte or
armstrongite, at
Dubbo in
New South Wales
New South Wales (commonly abbreviated as NSW) is a States and territories of Australia, state on the Eastern states of Australia, east coast of :Australia. It borders Queensland to the north, Victoria (state), Victoria to the south, and South ...
, 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
—have not proven 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.
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
Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...
or
sodium
Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
, as in the
Kroll process.
:
HfCl4 + 2 Mg -> 100~^\circ\textHf + 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 -> 00~^\circ\textHfI4
:
HfI4 -> 700~^\circ\textHf + 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
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
,
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 tetrachlorides are precursors to various
organohafnium compounds such as hafnocene dichloride and tetrabenzylhafnium.
The white
hafnium oxide (HfO
2), 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 .
This has led to proposals that hafnium or its carbides might be useful as construction materials that are subjected to very high temperatures. The mixed carbide
tantalum hafnium carbide () possesses the highest melting point of any currently known compound, . Recent supercomputer simulations suggest a hafnium alloy with a melting point of .
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.
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.
In 1921,
Charles R. Bury[Kragh, Helge. "Niels Bohr's Second Atomic Theory." Historical Studies in the Physical Sciences, vol. 10, University of California Press, 1979, pp. 123–186, https://doi.org/10.2307/27757389.] 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,
the X-ray spectroscopy of Moseley, and the chemical arguments of
Friedrich Paneth.
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
The University of Copenhagen (, KU) is a public university, public research university in Copenhagen, Copenhagen, Denmark. Founded in 1479, the University of Copenhagen is the second-oldest university in Scandinavia, after Uppsala University.
...
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.
Hafnium was one of 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.
In 1923, six predicted elements were still missing from the periodic table: 43 (
technetium), 61 (
promethium), 85 (
astatine), and 87 (
francium) are radioactive elements and are only present in trace amounts in the environment, thus making elements 75 (
rhenium) and 72 (hafnium) the last two unknown non-radioactive elements.
Applications
Most of the hafnium produced is used in the manufacture of
control rods for
nuclear reactors.
Hafnium has limited technical applications due to a few factors. First, it's very similar to zirconium, a more abundant element that can be used in most cases. Second, pure hafnium wasn't widely available until the late 1950s, when it became a byproduct of the nuclear industry's need for hafnium-free zirconium. Additionally, hafnium is rare and difficult to separate from other elements, making it expensive. After the Fukushima disaster reduced the demand for hafnium-free zirconium, the price of hafnium increased significantly from around $500–$600/kg ($227-$272/lb) in 2014 to around $1000/kg ($454/lb) in 2015.
Nuclear reactors
The nuclei of several hafnium isotopes can each absorb multiple neutrons. This makes hafnium a good material for nuclear reactors' control rods. Its neutron capture cross section (Capture Resonance Integral I
o ≈ 2000 barns) 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.
The German research reactor
FRM II uses hafnium as a neutron absorber.
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

Hafnium is used in
alloy
An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...
s with
iron,
titanium,
niobium,
tantalum
Tantalum is a chemical element; it has Symbol (chemistry), symbol Ta and atomic number 73. It is named after Tantalus, a figure in Greek mythology. Tantalum is a very hard, ductility, ductile, lustre (mineralogy), lustrous, blue-gray transition ...
, 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.
Microprocessors
Hafnium-based compounds are employed in
gates of transistors as insulators in the 45 nm (and below) generation of
integrated circuits from
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
,
IBM
International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...
and others. Hafnium oxide-based compounds are practical
high-k dielectrics, allowing reduction of the gate leakage current which improves performance at such scales.
Isotope geochemistry
Isotopes of hafnium and
lutetium (along with
ytterbium) 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 Half-life is a mathematical and scientific description of exponential or gradual decay.
Half-life, half life or halflife may also refer to:
Film
* Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang
* ''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
Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...
. 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
Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...
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.
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.
The high energy content of
178m2Hf was the concern of a
DARPA-funded program in the US. This program eventually concluded that using the above-mentioned
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. See ''
hafnium controversy''.
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
A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More speciali ...
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 much tougher recycled plastic.
Hafnium diselenide is studied in
spintronics thanks to its
charge density wave and
superconductivity.
Precautions
Care needs to be taken when
machining
Machining is a manufacturing process where a desired shape or part is created using the controlled removal of material, most often metal, from a larger piece of raw material by cutting. Machining is a form of subtractive manufacturing, which util ...
hafnium because it is
pyrophoric—fine particles can spontaneously combust when exposed to air. Compounds that contain this metal are rarely encountered by most people. The pure metal is not considered toxic, but hafnium compounds should be handled as if they were toxic because the ionic forms of metals are normally at greatest risk for toxicity, and limited animal testing has been done for hafnium compounds.
People can be exposed to hafnium in the workplace by breathing, swallowing, skin, and eye contact. The
Occupational Safety and Health Administration (OSHA) has set the legal limit (
permissible exposure limit) for exposure to hafnium and hafnium compounds in the workplace as TWA 0.5 mg/m
3 over an 8-hour workday. The
National Institute for Occupational Safety and Health (NIOSH) has set the same
recommended exposure limit (REL). At levels of 50 mg/m
3, hafnium is
immediately dangerous to life and health.
References
Further reading
*
External links
Hafniumat
Los Alamos National Laboratory'
periodic table of the elementsat ''
The Periodic Table of Videos'' (University of Nottingham)
Hafnium Technical & Safety DataNLM Hazardous Substances Databank – Hafnium, elemental* Don Clark
Intel Shifts from Silicon to Lift Chip Performance– WSJ, 2007
*
ttps://www.cdc.gov/niosh/npg/npgd0309.html CDC – NIOSH Pocket Guide to Chemical Hazards
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Chemical elements
Transition metals
Neutron poisons
1923 in science
Chemical elements with hexagonal close-packed structure