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Gadolinium: structure, properties, obtaining, uses

Gadolinium forms fluorescent complexes and has other particular physical properties: it is magnetocaloric, that is, its temperature is dependent on the existing magnetic field. It is also a paramagnetic element that becomes ferromagnetic at low temperatures.

Gadolinium metal sample. Source: Hi-Res Images of Chemical Elements / CC BY (https://creativecommons.org/licenses/by/3.0)

Gadolinium has a Curie point of 17ºC. It has an abundance of 5.2 ppm in the earth’s crust, higher than that of elements such as cesium, beryllium and tin. Its presence has been shown in some vegetables such as dill, red beets and romaine lettuce.

Discovery

Gadolinium was discovered in 1880 by the Swiss chemist Jean Charles Gelissard de Marignac. This scientist was able to identify in an oxide, obtained from the mineral samarskite, a new spectroscopic record, which was later shown to correspond to the one presented by the metal gadolinium.

There is a claim that Marignac prepared gadolinium oxide from the mineral cerite, rather than samarskite, calling the oxide “gadolinia.” In 1886, the French chemist Paul Émile Lacog de Boisbaudran succeeded in isolating the metal gadolinium from its oxide.

This served to confirm Marignac’s findings and to attribute the discovery of gadolinium to him. De Boisbaudran, in consultation with Marignac, named the new metal gadolinium in honor of the 18th century mineralogist John Gadolin.

John Gadolin (1760-1752) was a Finnish chemist who in 1792 examined a black mineral collected near Stockholm, and found that it contained 38% of a rare earth oxide which he called yttria.

In 1800 the mineral that Gadolin examined was named gadolinite. However, it was later established that it was not particularly rich in gadolinium, but only had traces of this metal.

Gadolinium can adopt two crystalline structures:

-Compact Hexagonal (hcp) at room temperature, called α-Gd

-Body-centered cubic (bcc) above 1235 ºC, which is represented as β-Gd

Electronic configuration

The abbreviated electron configuration of gadolinium is:

[Xe] 4f 7 5d 1 6s 2

It should have eight electrons in the 4f orbitals, as it is the eighth member of the lanthanides; but instead it has seven, with one electron in the 5d orbital. This is one of the many irregularities in the order of filling of the orbitals.

Gadolinium properties

Physical appearance

Solid silver-white metal. Gadolinium is a ductile and malleable metal.

Atomic number

64

Molar mass

157 g / mol

Melting point

1312 ºC

Boiling point

3000 ºC

Density

7.90 g / cm 3

Heat of fusion

10.05 kJ / mol

Heat of vaporization

301.3 kJ / mol

Oxidation states

0, +1, +2 and +3, the latter being (Gd 3+ ) the most important oxidation state.

Electronegativity

1.2 on the Pauling scale

Ionization energies

First: 593.4 kJ / mol

Second: 1170 kJ / mol

Third: 1190 kJ / mol

Magnetism

At temperatures below 20 ºC (Curie point 17 ºC), it behaves like a ferromagnetic metal, that is, it is attracted by magnets. And at temperatures above 20 ºC, it behaves like a paramagnetic metal.

Gadolinium has the property of being thermo-magnetic, since it increases its temperature when entering a magnetic field; and decreases it when leaving it. Furthermore, gadolinium has a high electrical resistivity value (131 µΩ-cm).

Reactivity

Most of the compounds formed by gadolinium are with the valence +3. The metal is stable in dry air, but is clouded by humid air, forming a flaky white oxide, Gd 2 O 3 , which then darkens and does not protect it from further oxidation.

Gadolinium is not soluble in cold water, but is capable of reacting with hot water to form gadolinium hydroxide, Gd (OH) 3 . Gadolinium is a strong reducing agent that works by reducing metal oxides.

It also reacts with all halogens to form white halides; except for gadolinium iodide, which is yellow. Reacts with acids with the exception of hydrofluoric acid, with which it forms a protective layer.

Obtaining

Like many rare earths, gadolinium is obtained economically from the minerals monazite and bastnäsite. Once these minerals are obtained, they are crushed to reduce them to fragments and thus start the isolation process.

The first step is to treat the mineral fragments with hydrochloric acid to transform the insoluble oxides into soluble chlorides. The filtrate is then neutralized with the addition of sodium hydroxide to adjust the pH between 3 and 4, causing precipitation of thorium hydroxide.

Gadolinium shavings. Source: W. Oelen, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

The supernatant is then treated with ammonium oxalate so that the formation of the insoluble rare earth oxalates occurs. These oxalates are heated to convert them into oxides, which are in turn treated with nitric acid, which produces the precipitation of cerium.

The supernatant is treated with magnesium nitrate to produce double crystallized salts of gadolinium, samarium, and europium, which can be separated using ion exchange chromatography.

The metallic gadolinium can finally be obtained from its oxides or salts by bringing them to 1450 ºC, and reducing them with calcium in an inert argon atmosphere .

Uses / applications

Magnetic cooling

Gadolinium, silicon and germanium alloys, fused by arc, demonstrate a magnetocaloric effect. That is, their temperature is affected by the intensity of the magnetic field to which they are exposed. This property has served as the basis for the establishment of magnetic refrigeration.

Industrial

Gadolinium is used in alloys with iron and chromium to improve resistance to high temperatures and corrosion.

Its compounds are used as green phosphor in picture tubes of color television. Likewise, gadolinium is used as a source of phosphors in fluorescent lamps, X-ray intensifying screens, and scintillators for X-ray tomography.

Gadolinium is used with yttrium in the manufacture of garnets for microwave applications. It is also used in the manufacture of magnets, electronic components such as video recorder heads, and compact discs (CDs) and computer memories.

Nuclear reactors

Due to its cross section, gadolinium has a great capacity to capture neutrons, thus allowing its use as a shield and control rod in nuclear reactors.

Doctors

The magnetic characteristics of gadolinium have allowed it to be used to form contrast complexes, useful in magnetic resonance imaging (MRI). The contrast material is injected intravenously, allowing some of the following medical studies:

-State of evolution of cancerous tumors

-Cardiac perfusion imaging, with the characterization of the cardiac tissue and the quantification of myocardial fibrosis

-Diagnosis in patients with abnormalities of the central nervous system , etc.

Gadolinium contrast solution is injected directly into the knee, elbow, and shoulder joints to achieve magnetic resonance imaging of their integrity and function.

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