HAFNIUM: DISCOVERY, STRUCTURE, PROPERTIES, USES, RISKS - CHEMISTRY - 2025

The hafnium is a transition metal whose chemical symbol is Hf and has an atomic number of 72. It is the third element of group 4 of the periodic table, being congener titanium and zirconium. With the latter it shares many chemical properties, being located together in minerals of the earth's crust.

Looking for hafnium is looking to where the zirconium is, as it is a by-product of its extraction. The name of this metal comes from the Latin word 'hafnia', whose meaning comes to be the name of Copenhagen, a city where it was discovered in zircon minerals and the controversy regarding its true chemical nature ended.

Metallic hafnium sample. Source: Hi-Res Images of Chemical Elements

Hafnium is a metal that goes unnoticed in the general intellect, in fact few people have even heard of it before. Even among some chemicals it is an uncommon element, due in part to its high production cost, since in most of its applications zirconium can substitute it without any problem.

This metal carries the distinction of being the last of the most stable elements discovered here on Earth; In other words, the other discoveries have constituted a series of ultra-heavy, radioactive elements and / or artificial isotopes.

Hafnium compounds are analogous to those of titanium and zirconium, the oxidation number of +4 predominant in them, such as HfCl ₄, HfO ₂, HfI ₄ and HfBr ₄. Some of them top the list of the most refractory materials ever created, as well as alloys of great thermal resistance and that also act as excellent absorbers of neutrons.

For this reason hafnium has a lot of participation in nuclear chemistry, especially with regard to pressurized water reactors.

Discovery

Transition or rare earth metal

The discovery of hafnium was surrounded by controversy, despite the fact that its existence had already been predicted since 1869 thanks to Mendeleev's periodic table.

The problem was that it was positioned below the zirconium, but it coincided in the same period of the rare earth elements: the lanthanoids. Chemists at the time didn't know if it was a transition metal or a rare earth metal.

The French chemist Georges Urbain, discoverer of lutetium, a neighboring metal of hafnium, claimed in 1911 to have discovered element 72, which he called celtium and proclaimed that it was a rare earth metal. But three years later it was concluded that his results were wrong, and that he had only isolated a mixture of lanthanoids.

It was not until the elements were ordered by their atomic numbers, thanks to the work of Henry Moseley in 1914, that the neighborhood between lutetium and element 72 was put in evidence, agreeing with Mendeleev's predictions when this last element was located in the same group as the metals titanium and zirconium.

Detection in Copenhagen

In 1921, after Niels Bohr's studies of the atomic structure and his prediction of the X-ray emission spectrum for element 72, the search for this metal in rare earth minerals was stopped; Instead, he focused his search on zirconium minerals, since both elements must share various chemical properties.

The Danish chemist Dirk Coster and the Hungarian chemist Georg von Hevesy in 1923 finally managed to recognize the spectrum predicted by Niels Bohr in zircon samples from Norway and Greenland. Having made the discovery in Copenhagen, they called element 72 by the Latin name of this city: hafnia, from which it was later derived 'hafnium'.

Isolation and production

However, it was not an easy task to separate the hafnium atoms from those of the zirconium, since their sizes are similar and they react in the same way. Although a fractional recrystallization method had been devised in 1924 to obtain hafnium tetrachloride, HfCl ₄, it was the Dutch chemists Anton Eduard van Arkel and Jan Hendrik de Boer who reduced it to hafnium metal.

For this, the HfCl ₄ was subjected to a reduction using metallic magnesium (Kroll process):

HfCl ₄ + 2 Mg (1100 ° C) → 2 MgCl ₂ + Hf

On the other hand, starting from hafnium tetraiodide, HfI ₄, this was vaporized to undergo thermal decomposition on an incandescent tungsten filament, on which metallic hafnium was deposited to produce a bar with a polycrystalline appearance (process of the crystalline bar or Arkel- De Boer process):

HfI ₄ (1700 ° C) → Hf + 2 I ₂

Hafnium structure

The hafnium atoms, Hf, clump together at ambient pressure in a crystal with a compact hexagonal structure, hcp, as do the metals titanium and zirconium. This hcp hafnium crystal becomes its α phase, which remains constant up to a temperature of 2030 K, when it undergoes a transition to the β phase, with a cubic structure centered in the body, bcc.

This is understood if it is considered that the heat "relaxes" the crystal and, therefore, the Hf atoms seek to position themselves in such a way as to decrease their compaction. These two phases are sufficient to consider the polymorphism of hafnium.

Likewise, it presents a polymorphism that depends on high pressures. The α and β phases exist at a pressure of 1 atm; while the ω phase, hexagonal but even more compacted than ordinary hcp, appears when pressures exceed 40 GPa. Interestingly, when pressures continue to increase, the less dense β phase reappears.

Properties

Physical appearance

Silvery-white solid, which shows dark tones if it has an oxide and nitride coating.

Molar mass

178.49 g / mol

Melting point

2233 ºC

Boiling point

4603 ºC

Density

At room temperature: 13.31 g / cm ³, being twice as dense as zirconium

Right at the melting point: 12 g / cm ³

Heat of fusion

27.2 kJ / mol

Heat of vaporization

648 kJ / mol

Electronegativity

1.3 on the Pauling scale

Ionization energies

First: 658.5 kJ / mol (Hf ⁺ gaseous)

Second: 1440 kJ / mol (Hf ²⁺ gaseous)

Third: 2250 kJ / mol (Hf ³⁺ gaseous)

Thermal conductivity

23.0 W / (mK)

Electrical resistivity

331 nΩ m

Mohs hardness

5.5

Reactivity

Unless the metal is polished and burns, giving off sparks at a temperature of 2000 ° C, it has no susceptibility to rust or corrode, as a thin layer of its oxide protects it. In this sense, it is one of the most stable metals. In fact, neither strong acids nor strong bases can dissolve it; With the exception of hydrofluoric acid, and halogens capable of oxidizing it.

Electronic configuration

The hafnium atom has the following electronic configuration:

4f ¹⁴ 5d ² 6s ²

This coincides with the fact of belonging to group 4 of the periodic table, together with titanium and zirconium, because it has four valence electrons in the 5d and 6s orbitals. Also note that hafnium could not be a lanthanoid, since it has its 4f orbitals completely filled.

Oxidation numbers

The same electron configuration reveals how many electrons a hafnium atom is theoretically capable of losing as part of a compound. Assuming that it loses its four valence electrons, it would remain as a tetravalent cation Hf ⁴⁺ (in analogy to Ti ⁴⁺ and Zr ⁴⁺), and therefore would have an oxidation number of +4.

This is in fact the most stable and common of its oxidation numbers. Other less relevant are: -2 (Hf ^2-), +1 (Hf ⁺), +2 (Hf ²⁺) and +3 (Hf ³⁺).

Isotopes

Hafnium occurs on Earth as five stable isotopes and one radioactive with a very long lifetime:

- ¹⁷⁴ Hf (0.16%, with a mean life time of 2 · 10 ¹⁵ years, so it is considered practically stable)

- ¹⁷⁶ Hf (5.26%)

- ¹⁷⁷ Hf (18.60%)

- ¹⁷⁸ Hf (27.28%)

- ¹⁷⁹ Hf (13.62%)

- ¹⁸⁰ Hf (35.08%)

Note that there is as such no isotope that stands out in abundance, and this is reflected in the average atomic mass of hafnium, 178.49 amu.

Of all the radioactive isotopes of hafnium, which together with the natural ones add up to a total of 34, ^178m2 Hf is the most controversial because in its radioactive decay it releases gamma radiation, which is why these atoms could be used as a weapon of war.

Applications

Nuclear reactions

Hafnium is a metal resistant to humidity and high temperatures, as well as being an excellent absorber of neutrons. For this reason, it is used in pressurized water reactors, as well as in the manufacture of control rods for nuclear reactors, whose coatings are made of ultra-pure zirconium, since it must be capable of transmitting neutrons through it..

Alloys

Hafnium atoms can integrate other metallic crystals to give rise to different alloys. These are characterized by being tough and thermally resistant, which is why they are intended for space applications, such as in the construction of engine nozzles for rockets.

On the other hand, some alloys and solid hafnium compounds have special properties; such as its carbides and nitrides, HfC and HfN, respectively, which are highly refractory materials. Tantalum hafnium carbide, Ta ₄ HfC ₅, with a melting point of 4215 ° C, is one of the most refractory materials ever known.

Catalysis

Hafnium metallocenes are used as organic catalysts for the synthesis of polymers such as polyethylene and polystyrene.

Risks

It is unknown to date what impact Hf ⁴⁺ ions could have on our body. On the other hand, because they are found in nature in zirconium minerals, they are not believed to alter the ecosystem by releasing their salts into the environment.

However, it is recommended to handle hafnium compounds with care, as if they were toxic, even if there are no medical studies that prove that they are harmful to health.

The real danger of hafnium lies in the finely ground particles of its solid, which can barely burn when they come into contact with oxygen in the air.

This explains why when it is polished, an action that scrapes its surface and releases particles of pure metal, burning sparks are released with a temperature of 2000 ºC; that is, hafnium has pyrophoricity, the only property that carries fire or serious burns risks.

References

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