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Europium: structure, properties, obtaining, uses

It is one of the rarest and most expensive metals, due to all the procedures that must be done to extract it free of impurities. Physically it shows a greyish golden sheen, caused by its rapid oxidation as soon as it is exposed to air. Therefore, it is seldom possible to store it in its true silver color.

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

Europium is so reactive that it must be stored in ampoules or sealed containers under inert argon atmospheres. Even immersed in mineral oil it oxidizes due to the little dissolved oxygen.

Discovery of europium

The discovery of europium was conceived in parts, throughout the 19th century, and in different countries (Sweden, Germany and Switzerland) on the European continent. Europium ions were difficult to identify because their absorption spectra mixed with that of other rare earth metals, such as cerium, gadolinium, and lanthanum, as well as samarium.

Therefore, the identification and isolation of all those metals carried europium as an uncharacterized impurity. For example, the isolation of a pure sample of cerium, in 1839 by Carl Gustav Mosander, resulted in the recognition of other new elements: lanthanum and didymium.

While didymium was studied and it was concluded that it actually consisted of a mixture of other elements (praseodymium and neodymium), samarium appeared, found in 1879 by Paul Èmile Lecoq.

Later, in 1886, Swiss chemist Jean Charles Galissard purified samples of samarium by finding gadolinium. Lecoq by 1892 had already studied the spectra of gadolinium and that of another new element.

Chemical structure of europium

The europium atoms are held together thanks to the metallic bond, with the electrons from their 4f and 6s orbitals participating in it. As a result of its electronic characteristics, atomic radii, and the way it is packed, europium ends up adopting a body-centered cubic crystalline structure (bcc), being one of the least dense.

There are no bibliographic sources that mention another polymorph for europium, apart from said bcc phase, within other temperature ranges; but yes under different pressures.

For example, europium subjected to a pressure close to 18 GPa adopts a compact hexagonal structure, to later finally transform to a monoclinic phase at pressures higher than 31.5 GPa.

Electronic configuration

The abbreviated electron configuration of europium is:

[Xe] 6s 2 4f 7

Being in position or group 7 of the lanthanide series, it has seven electrons occupying its 4f orbitals; and therefore, it does not correspond to any of the deviations that we find in the electron configurations for the elements of the periodic table.

Properties of europium

Physical appearance

Silvery white metal, with a soft hardness similar to that of lead, and that turns golden when exposed to air, being covered with a layer of oxide and carbonate.

Atomic number

63

Molar mass

151.96 g / mol

Melting point

826 ºC

Boiling point

1529 ºC

Density

Solid: 5.264 g / cm 3

At melting point: 5.13 g / cm 3

Oxidation states

The main oxidation states of europium are +2 (Eu 2+ ) and +3 (Eu 3+ ), with +1 (Eu + ) being the least common of the three.

Ionization energies

-First: 547.1 kJ / mol (Eu + gas)

-Second: 1085 kJ / mol (Eu 2+ gaseous)

-Third: 2404 kJ / mol (Eu 3+ gaseous)

Electronegativity

1.2 on the Pauling scale.

Magnetic order

Paramagnetic

Reactivity

The reactivity of europium is comparable to that of lithium and is therefore the most reactive metal in rare earths. For example, it reacts quickly with water to form its corresponding hydroxide, Eu (OH) 3 , yellow in color, which, unlike alkali hydroxides, is insoluble in water:

2 Eu + 6 H 2 O → 2 Eu (OH) 3  + 3 H 2

Likewise, when burned in a lighter it oxidizes to Eu 2 O 3 and gives off a reddish flame, reminiscent of lithium:

4 Eu + 3 O 2  → 2 Eu 2 O 3

Europium is capable of forming many compounds with the oxidation state of +2, due to its half-full f orbitals (4f 7 ), which gives its atom an unusual electronic stability.

The Eu 2+ cation exhibits a chemical behavior similar to that of Ba 2+ , but unlike the latter, it acts as a moderately strong reducing agent, oxidizing to Eu 3+ .

Obtaining

Raw material

Europium is present in rare earth minerals such as bastnasite and monazite. However, because it bears some similarity to alkali and alkaline earth metals with respect to its reactivity, its ions are widely dispersed in the earth’s crust along with minerals of calcium or other metals, so there is no mineral by itself that is rich. in europium. That is why it is very expensive to obtain it.

Processes

Eu 3+ ions are part of many rare earth oxides and phosphates. Therefore, the first step is to separate them from the other metals present. To do this, minerals are processed, especially bastnasite; they are roasted, dissolved in strong acids or bases, and subjected to fractional precipitation using various reagents. Likewise, ion exchange chromatography is used to separate the Eu 3+ .

As the mineral is processed, a concentrate of Eu 3+ ions is obtained , which can be reduced using metallic zinc or an amalgam thereof, so that they are reduced to Eu 2+ . Then, the Eu 2+ is coprecipitated on the carbonate or barium sulfate.

This precipitate is roasted and subjected to a separation to obtain the oxide Eu 2 O 3 , which is reduced with lanthanum in a tantalum crucible, to finally distil and condense the metallic europium.

Another method to obtain europium is by electrolysis of a mixture of melted EuCl 3 and NaCl or CaCl 2 . Thus, chlorine gas is produced at the anode, while metallic europium is formed at the cathode.

Uses / applications

The reddish luminescence seen on this euro note, under a UV lamp, is due to a europium compound. Source: repro by H. Grobe / CC BY (https://creativecommons.org/licenses/by/3.0)

Europium in its metallic form has no routine uses. However, its compounds are another story, especially its oxide Eu 2 O 3 , whose phosphorescence has made it an indispensable component of screens in devices, monitors and televisions. This is because it is a red phosphor, emitting a characteristic red light.

The reddish phosphorescence europium (III) is also used to prevent euro banknotes from being counterfeited, by being illuminated with UV light to confirm its legitimacy.

On the other hand, when it is mixed with europium (II) compounds, which are bluish phosphors, a white light is obtained, very recurrent in the glass of fluorescent lamps.

Europium is added in small quantities to strontium aluminate to prepare phosphors of different colors, which stand out for having a long-lasting phosphorescence.

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