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Terbium: structure, properties, uses, obtaining

Terbium was discovered in 1843 by the Swedish chemist Carl Gustav Mosander, in the mineral gadolinite. Mosander treated yttria, an oxide of the metal yttrium, with ammonium hydroxide and found two unknown substances, which he called erbia and terbia, as contaminants: substances that respectively contained the metals erbium and terbium.

Metallic terbium sample. Source: Hi-Res Images of Chemical Elements, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

The name of terbium is due, like that of yttrium, to the Swedish town of Ytterby, where the mineralogical samples came from. It is often the case that ‘terbium’ is easily confused with ‘erbium’ and ‘ytterbium’.

Terbium structure

Terbium forms crystals with compact hexagonal structures (hcp) at room temperature, which is known as the α phase. When these crystals are heated to 1289 ° C, they undergo a transition to the body-centered cubic (bcc) structure, known as the β phase.

Electronic configuration

Terbium has the following electronic configuration:

[Xe] 4f 9 6s 2

By having 9 electrons in its 4f orbitals, and being the ninth member of the lanthanides, this electronic configuration does not present any irregularity compared to the filling order indicated by the Aufbau principle.

Terbium properties

Physical appearance

Solid silver-white metal. It is malleable, ductile, resistant to impacts. Its Tb 3+ cation is fluorescent and emits a bright green light. However, its fluorescence is only visible in the solid state .

Atomic number

65

Molar mass

158.925 g / mol

Melting point

1356 ºC

Boiling point

3123 ºC

Density

8.25 g / cm 3

Heat of fusion

10.15 kJ / mol

Heat of vaporization

391 kJ / mol

Molar caloric capacity

28.91 kJ / mol

Oxidation states

Like the other lanthanides, its main oxidation state is +3 (Tb 3+ ), but it can also have the oxidation state +4 (Tb 4+ ). For example, in the compounds TbO 2 and TbF 4 , terbium has an oxidation state of +4.

Electronegativity

1.2 on the Pauling scale.

Ionization energies

First: 565.8 kJ / mol

Second: 1110 kJ / mol

Third: 2114 kJ / mol

Magnetic order

At room temperature it is a paramagnetic metal that can be picked up with a neodymium magnet. But at a temperature of 230 K (-43 ºC), it becomes antiferromagnetic, becoming ferromagnetic at temperatures below 219 K.

Terbium is stable in air, even at high temperatures, due to the presence of a dark brown oxide that covers it.

This metal is capable of forming three oxides: Tb 2 O 3 , white and dusty, being the common form of the oxides that lanthanides present; TbO 2 , which uses the +4 oxidation state and is generated from atomic oxygen; and Tb 4 O 7 , a dark brown oxide with oxidation states +3 and +4.

Terbium reacts with water to form a hydroxide and liberate hydrogen gas. Likewise, it is attacked by dilute acids, forming salts and releasing hydrogen gas.

Terbium reacts with sulfuric acid, obtaining Tb 2 (SO 4 ) 3 . This salt is capable of emitting a green fluorescence. Terbium combines with all halogens through its +3 oxidation state (TbF 3 , TbCl 3 , etc.).

Applications

Fluorescence

Terbium (III) compounds are characterized by their green fluorescence when absorbing UV radiation. Source: Leiem, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Terbium is used as a green phosphor in trichromatic lighting applications and in color television tubes. Terbium produces the green color of Blackberry cell phones, or other high definition displays.

Tb 3+ ions are used to show the presence of microbes, applying terbium chloride on the sample to be examined, which is then illuminated with ultraviolet light. This causes living endospores to glow green.

Terbium (Tb 3+ ), europium (Eu 3+ ) and thulium (Tm 3+ ) are used to detect counterfeit euro banknotes, since when the banknotes are illuminated with ultraviolet light, they produce a fluorescence of green for terbium, one red for europium, and another blue for thulium.

Alloys

A terbium-iron alloy is used in the construction of metallic films for magneto-optical data recording.

Another neodymium-terbium-dysprosium alloy is used for the manufacture of magnets, capable of conserving their magnetism at high temperatures. This type of magnets are used in electric motors in overhead generators, where high temperatures are produced.

Terphenol is an alloy of terbium, iron, and dysprosium that has the ability to expand or contract based on the existing magnetic field. This alloy is used in “SoundBug” speakers, which allow a table or desk to be used as speakers. Additionally, this alloy is used in magnetically controlled actuators, sonar systems, and pressure sensors.

Other uses

Terbium is used to dope calcium fluoride, calcium tungstate, and strontium molybdate, compounds used in solid-state and fiber-optic devices. Terbium is also used in energy saving light bulbs and mercury lamps.

Terbium has been used to improve the safety of X-rays, since by improving their resolution, it allows the time of exposure to them to be reduced.

In conjunction with gadolinium, terbium has been used for the construction of a two-stage magnetic test refrigerator: gadolinium as a high-temperature stage, and terbium as a low-temperature stage.

Obtaining

Raw material

Terbium has an abundance of 1.2 ppm in the earth’s crust, being an element that is not found in free form. It is present in the minerals monazite, xenotime, bastnäsite and euxenite, the latter being an oxide containing 1% terbium.

Separation

Terbium is commercially extracted from monazite and bastnäsite by an initial crushing of these minerals, followed by a treatment with sulfuric acid and an adjustment of the pH of the solution with sodium hydroxide to a pH between 3 and 4. This produces the separation thorium.

The solution is then treated with ammonium oxalate, for the subsequent formation of rare earth oxides. Subsequently, the oxides dissolve in nitric acid, which causes the separation of cerium. Terbium separates as a double salt of ammonium nitrate by crystallization.

The most efficient method for the separation of terbium salts is by ion exchange chromatography. Rare earth ions are absorbed into a suitable ion exchange resin by interaction with hydrogen, ammonium or cupric ions present in it.

The rare earth ions are separated from the resin by washing them using an agent suitable for each specific metal.

Production

Once the terbium ions are separated from the minerals, their chlorides or fluorides react with the metallic calcium in a tantalum crucible, producing a metallothermic reduction. Calcium and tantalum impurities are removed by applying vacuum distillation.

On the other hand, terbium can also be obtained by electrolysis of terbium oxide in molten calcium chloride.

Isotopes

Terbium has a total of 38 isotopes, between 135 Tb and 172 Tb, of which the only stable isotope is 159 Tb; which corresponds to almost 100% of the terbium obtained from the earth’s crust. The rest of the isotopes of terbium are radioactive.

Most of the radioactive isotopes of terbium are emitters of β  or β + particles . The average life time of most of them is very short, standing out the 138 Tb with a half life of 200 nanoseconds. Meanwhile, its isotopes with the longest half-life are: 158 Tb (58 years) and 157 Tb (71 years).

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