Hafnium

Hafnium
Atomic number	72
Symbol	Hf
Group	4 (titanium group)
Boiling point	4603 °C
Electron configuration	[Xe] 4f14 5d2 6s2
Density	13.31 g/cm^3
Period	6
Cas number	7440-58-6
Melting point	2233 °C
Oxidation states	+4, +3, +2
Phase STP	Solid
Wikidata	Q1119

Hafnium (symbol Hf, atomic number 72) is a lustrous, silvery-gray transition metal in group 4 of the periodic table. Chemically very similar to zirconium, it normally forms tetravalent (+4) compounds. Hafnium has a very high melting point and is outstandingly corrosion resistant. Its combination of properties makes it useful in high-temperature superalloys (jet engines and rocket nozzles), neutron-absorbing control rods in nuclear reactors, and semiconductor microchips (via its oxide HfO₂ as a high-permittivity dielectric). Hafnium is a solid metal at standard conditions with a density around 13.3 g/cm³ and an atomic weight of about 178.49.

Overview and Key Facts

Hafnium is a transition metal in group 4 (period 6) of the periodic table, sitting below zirconium (Zr) and above the transactinide rutherfordium. Like its lighter congeners (titanium, zirconium), hafnium is classified as a refractory metal with high melting point and strong corrosion resistance. At standard conditions it is a solid metal with a lustrous silver-gray appearance and a density around 13.3 g/cm³ (significantly heavier than zirconium). Its most common oxidation state is +4 (forming covalent/ionic Hf(IV) compounds); compounds in other oxidation states (chiefly +3) are rare. Hafnium has 72 protons and electrons, giving it an atomic weight of about 178.49 u. It has four valence electrons (configuration [Xe]4f^14 5d^2 6s^2) and no net charge in the elemental form. As with most metals, it conducts heat and electricity.

Atomic Structure and Electron Configuration

Hafnium atoms contain 72 electrons, filling the shells up to xenon (the argon core plus filled 4f^14 subshell). The electron configuration is [Xe]4f^14 5d^2 6s^2. The four outer electrons (two 6s and two 5d) reside in the valence shell and determine hafnium’s chemistry. Because of its filled 4f subshell, hafnium’s atomic radius is surprisingly small for its period: it is only slightly larger than zirconium. In fact, the ionic radius of Hf^4+ (≈78 pm in sixfold coordination) closely matches that of Zr^4+ (≈79 pm), a consequence of the “lanthanide contraction.”

Hafnium’s first ionization energy is about 6.8 electronvolts (≈660 kJ/mol), and its Pauling electronegativity is around 1.3 (similar to Zr). Overall, Hf behaves like a typical early transition metal. It is fairly electropositive and readily forms Hf^4+ ions (mostly in +4 compounds), following the trends of its group and block.

Isotopes and Nuclear Properties

Hafnium has a rich isotopic composition. Thirty-four isotopes are known (mass numbers 153–186), but only five occur naturally and are effectively stable: ^176Hf, ^177Hf, ^178Hf, ^179Hf and ^180Hf. (A very long-lived radioisotope, ^174Hf, also exists with a half-life ≈2×10^15 years, which for most purposes can be treated as stable.) In nature these isotopes occur in roughly 5:18:27:14:36 percent abundance respectively (from ^176Hf through ^180Hf).

In addition, several radioactive isotopes of hafnium have been synthesized. Of special note is ^182Hf (half-life ~8.9 million years), which was present in the early Solar System and whose decay to ^182W is used to date geological processes. Another unusual nuclear state is ^178m2Hf, a metastable isomer with very high spin and energy; it attracted interest (and controversy) for potential controlled energy release, though no practical application emerged.

Hafnium’s nuclei have a very large thermal neutron-capture cross section (on the order of 100 barns for natural Hf), far higher than zirconium’s. This makes Hf useful as a neutron-absorber (for control rods and shielding). (Conversely, nuclear reactor materials based on zirconium must be essentially hafnium-free.) None of the naturally occurring hafnium isotopes is directly radioactive under normal conditions, but several radioisotopes of Hf (and its decay products) are used in physics research and have been considered for industrial uses (e.g. Hf-181 in radiation).

Allotropes and Typical Compounds

Hafnium has no allotropes in the sense of distinct molecular or polymeric forms; the metal exists in a single form that is hexagonal close-packed (hcp, “α-Hf”) at room temperature and converts to a body-centered cubic (bcc, “β-Hf”) structure on heating (around 2388 K, ~2115 °C). In compounds, hafnium nearly always appears in the +4 oxidation state (Hf(IV)). The most important Hf compound is hafnium dioxide (HfO₂, called hafnia), a very high-melting, chemically stable ceramic. Hafnia is widely used as an electrical insulator (high-κ dielectric) in microelectronics and in high-temperature refractory applications.

Hafnium forms volatile tetrahalides (e.g. HfCl₄, HfBr₄, HfI₄) analogous to those of titanium and zirconium; these metal halides are typically moisture-sensitive solids or sublimable liquids. Hf reacts with hydrogen to yield hydrides (such as HfH₂), and with carbon, nitrogen, and boron at high temperatures to form extremely refractory compounds (carbides HfC, nitrides HfN, borides HfB₂, etc.). For example, hafnium carbide has a melting point near 3890 °C (the highest of any binary compound), and hafnium nitride melts around 3310 °C. Mixed compounds like tantalum hafnium carbide (Ta₄HfC₅) have even higher melting points (≈4215 K). Hf also forms silicides and oxysalts and a range of complex inorganic and organometallic compounds (for instance, hafnocene dichloride [Hf(Cp)₂Cl₂] is used in polymerization catalysts).

Physical Properties

At standard conditions hafnium metal is a hard, ductile solid with hexagonal close-packed (HCP) crystal structure (α-Hf). It has a density of about 13.31 g/cm³ (20 °C), making it one of the densest light metals. The metal melts at roughly 2227 °C and boils near 4600 °C, values among the highest of all elements. Its heat capacity is ~24 J·mol^−1·K^−1 at room temperature. Hafnium is a metallic conductor: its electrical resistivity is relatively high (≈3.3×10^−5 Ω·cm at 20 °C), and its thermal conductivity is modest (~23 W·m^−1·K^−1), lower than copper or iron. It has a thermal expansion coefficient around 5.2 µm·m^−1·K^−1 (room temperature).

Most physical properties of hafnium are similar to zirconium’s due to their chemical affinity, but hafnium’s higher atomic mass gives it higher density and heat capacity. Impurities (especially residual Zr) can affect measurements. Hafnium is paramagnetic with a weak temperature-independent susceptibility. In spectroscopy, hafnium emits and absorbs mainly in the ultraviolet; it has no prominent lines in the visible spectrum. Overall, hafnium behaves like a characteristic heavy transition metal with outstanding thermal stability and moderate electrical conductivity.

Chemical Reactivity and Trends

Hafnium metal is relatively inert under ordinary conditions. It develops a protective oxide film in air, so bulk Hf does not oxidize quickly at room temperature. Finely divided or hot Hf will ignite and burn to HfO₂, similar to titanium. The metal is not attacked by most acids (its oxide prevents corrosion), but it reacts with halogens: chlorine gas converts Hf to HfCl₄, and fluorine to HfF₄. Concentrated hydrofluoric acid [HF] dissolves hafnium by forming soluble fluoro-complexes [HfF₆]²⁻. Like zirconium, hafnium resists concentrated alkalis. In general Hf^0 is readily oxidized to Hf^4+; its +3 or +2 states are rare and unstable in aqueous media.

Hafnium’s chemistry closely parallels zirconium’s. Because Hf^4+ is a high-charge, small ion, its compounds tend to be strongly polar or covalent. Hf(IV) salts (halides, sulfates, etc.) hydrolyze in water to form insoluble oxide or hydroxide polymers – e.g., HfCl₄ + H₂O → HfO₂·xH₂O + HCl. The Hf(IV) ion can form coordination complexes with hard ligands (F⁻, O²⁻, OH⁻, citrate, etc.). Hafnium does not form strong acids or bases itself, but HfO₂ is amphoteric: it dissolves in very strong bases (forming [Hf(OH)₆]²⁻) as well as in HF. In the reactivity series, Hf is more reactive than many late transition metals but acts much like its group-4 neighbors: it is a moderately strong reducing metal (easily giving up electrons to form Hf^4+), and its compounds are generally stable and refractory.

Occurrence and Production

Hafnium is not found free in nature but always accompanies zirconium. It makes up about 5.8 parts per million of Earth’s crust by mass. The element occurs in zircon minerals (ZrSiO₄, typically 1–5% Hf by mass) and in the rare Hf-rich mineral hafnon ((Hf,Zr)SiO₄). Because zirconium’s chemistry is almost identical, Hf is distributed similarly in ores: main sources include heavy-mineral sands (ilmenite, rutile deposits) and rare phosphate or silicate rocks. Commercially, most of the world’s hafnium comes as a by-product of zirconium extraction. For example, titanium ore processing (ilmenite, rutile) produces zircon, which is then refined. Nuclear applications require nearly hafnium-free zirconium; the separation leaves behind Hf compounds (tetrachloride or oxide). These are reduced to Hf metal by processes analogous to the Kroll method (using Mg or Na on HfCl₄) and often purified by the van Arkel–de Boer transport method (iodide).

Major sources of zircon (and thus hafnium) are heavy-sand deposits in Australia, South Africa, India, Brazil, and the USA. Leading zirconium (and hafnium) refineries are located in these countries and also in Russia and China. Because demand for hafnium is smaller, it is usually produced only where zirconium is processed. World hafnium reserves track those of zirconium (about 4–5% of zircon resources by weight). The current world production of hafnium metal is on the order of only a few hundred tonnes per year, limited by the niche markets in nuclear and aerospace applications.

Applications and Technology

The largest technological use of hafnium is in nuclear reactors. Hafnium metal and alloys serve as neutron absorbers in control rods and burnable poisons. Its high thermal-neutron capture cross section (~100 barns) allows a small rod of Hf to shut down a reactor quickly. Naval reactors (submarines) also use Hf rods. For these reasons, reactor-grade zirconium alloys are carefully separated to remove nearly all hafnium, whereas the hafnium byproduct is collected for use.

Hafnium is valued in superalloys and special metals for high-temperature applications. Adding a few percent of Hf to nickel-base superalloys (used in jet engines and gas turbines) or to niobium alloys (rocket thruster liners) improves creep resistance and strength at high temperatures. Hafnium’s high melting point also makes it useful in plasma arc electrodes and torch tips, as Hf inserts or coatings. In aerospace engineering, for example, a Nb–10%Hf–1%Ti alloy was used in the Apollo lunar module engine nozzle.

In electronics, hafnium’s oxide (HfO₂) is a key material. HfO₂ is a dielectric (insulator) with a high dielectric constant (~25, compared to ~3.9 for SiO₂), and it replaced silicon dioxide as the gate insulator in advanced CMOS microprocessors (around the 45 nm technology generation). Hafnium oxide and related compounds now appear in capacitors and next-generation memory devices, as well as high-κ gate stacks. Hafnium nitride (HfN) and hafnium silicate layers also serve as diffusion barriers and contact metallizations in microchip fabrication.

Other applications exploit hafnium’s durability and high melting point. Hafnium-containing alloys serve as filaments and electrodes (for example in vacuum tubes or gas discharge lamps). Specialized spark plugs with hafnium electrodes have been developed for their longevity. Thin films of HfO₂ are used as hard, protective optical coatings (for example, heat-resistant mirrors or scratch-resistant glass). Hafnium tetrachloride and other Hf salts are used as precursors in manufacturing metal foams and advanced ceramics, and as catalysts in polymer chemistry (e.g. Ziegler–Natta catalysts for olefin polymerization).

Biology, Environment, and Safety

Hafnium has no known biological role in organisms, and its compounds are of low toxicity. Animal studies indicate very high tolerance: for example, the oral LD₅₀ in rats is >2000 mg/kg for Hf metal, implying that a large dose produces no acute effect. In practice, hafnium metal and its oxide (HfO₂) are chemically inert and poorly soluble, so they are not readily absorbed by the body. Zirconium compounds have been used medically (e.g. to treat plutonium contamination) with few side effects, and hafnium behaves similarly. Specialists take standard precautions against heavy metal dust: inhaling fine hafnium or hafnium-oxide powder should be avoided, and metal powder is pyrophoric (flammable on contact with air).

Environmental fate: Hafnium released to the environment is expected to remain largely in mineral form. Hf oxides and hydroxides are insoluble in water, so Hf does not easily leach into groundwater or accumulate in living organisms. There are no natural biological cycles for hafnium. In use, most hafnium ends up alloyed in high-performance materials or immobilized as oxide in devices. Any nuclear waste containing activated Hf must consider its neutron-absorbing properties, but ordinary hafnium poses negligible radiochemical hazard.

Safety limits: No specific occupational exposure limits are established for hafnium metal, but standard industrial hygiene applies (avoid inhaling dust/fumes, wear gloves and eye protection). Reactive hafnium compounds (especially those with fluorides or chlorides) require the usual chemical safeguards. Overall, hafnium is regarded as having low hazard to health and environment under normal handling, although care is taken to avoid metal dust inhalation and to prevent ignition of fine powder.

History and Etymology

Hafnium’s existence had been predicted by Mendeleev in 1869, but early searches failed. In 1911 Georges Urbain announced a new element (which he called "celtium") from rare-earth minerals, but this was later shown to be incorrect. The true element 72 was discovered in 1922–1923 by Dutch physicist Dirk Coster and Hungarian chemist Georg von Hevesy at the University of Copenhagen. They detected it spectroscopically in zirconium ore (zircon) from Norway, and named it hafnium after Hafnia, the Latin name for Copenhagen. The difficulty in separating hafnium from chemically similar zirconium explains why it was not found earlier; zirconium minerals typically contain a few percent of Hf, which had led to slight errors in early atomic-weight measurements.

The first pure hafnium metal was obtained in 1925 by the Arkel–de Boer “iodide process”: hafnium(IV) iodide was thermally decomposed on a hot tungsten filament to deposit crystals of Hf. The identification of hafnium completed the set of naturally occurring d-block elements. Its name (symbol Hf) reflects the place of isolation, just as zirconium was named for the mineral (zircon) in which hafnium was found.

Data Table

Property	Value
Symbol	Hf
Atomic number	72
Element category	Transition metal
Group, period, block	Group 4, period 6, d-block
Standard atomic weight	178.49 u
Electron configuration	[Xe] 4f^14 5d^2 6s^2
Common oxidation states	+4 (other states: +3, +2)
Phase at STP	Solid (metal)
Density (20 °C)	13.31 g·cm⁻³
Melting point	2227 °C (2500 K)
Boiling point	4600 °C (4900 K)
Heat of fusion	25.1 kJ·mol⁻¹
Heat of vaporization	575 kJ·mol⁻¹
Specific heat (20 °C)	0.14 J·g⁻¹·K⁻¹ (24 J·mol⁻¹·K⁻¹)
Thermal conductivity (25 °C)	23 W·m⁻¹·K⁻¹
Electrical resistivity (20 °C)	~33 μΩ·cm
Electronegativity (Pauling)	1.3
First ionization energy	6.8 eV (660 kJ·mol⁻¹)
Covalent radius	≈144 pm
Crystal structure	Hexagonal close-packed (α-Hf)
Melting phases	α (hcp) → β (bcc) at ≈2388 K
Stable isotopes	^176Hf, ^177Hf, ^178Hf, ^179Hf, ^180Hf
Thermal neutron capture	~100 barns (natural mixture)