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Lutetium: structure, properties, uses, obtaining

Although its physical properties may be outstanding, the truth is that chemically it is very similar to the rest of its lanthanide counterparts. One consequence of this is that lutetium was the last of the lanthanides to be discovered, isolated, and produced.

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

The year of its discovery dates back to 1907, the product of the independent work of three scientists: the French Georges Urbain, the Austrian Carl Welsbach, and the American Charles James. However, the greatest credit goes to Georges Urbain, who christened this metal ‘lutetia’ from ‘lutetia’, the Latin name for Paris. It was not until 1953 that the first pure lutetium metal sample was obtained.

Structure

Lutetium atoms are held together thanks to their metallic bond. As a product of its interactions, its atomic radii and the order of its packing, lutetium ends up adopting a compact hexagonal crystalline structure (hcp).

The hcp structure is the only known lutetium at ambient pressure. It is therefore said to be a monoform metal, that is, it lacks polymorphs and phase transitions under other temperatures.

Electronic configuration

The electron configuration of lutetium is as follows:

[Xe] 4f 14 5d 1 6s 2

Note that its 4f orbitals are completely filled with electrons. Lutetium participates in chemical reactions using its valence electrons, hovering around the 5d and 6s orbitals.

This configuration is somewhat reminiscent of lanthanum ([Xe] 5d 1 6s 2 ), metal d , and because of this there are those who consider that lutetium shares a chemistry more akin to transition metals than to lanthanides. Electronically, lutetium is a smaller version of lanthanum, which also has all of its 4f orbitals filled.

When lutetium reacts, it loses the three valence electrons from its 5d 1 and 6s 2 orbitals , transforming into the Lu 3+ cation .

Physical appearance

Silvery white metal, which darkens when slowly oxidized. It is characterized by being very dense and hard.

Atomic number

71

Molar mass

174.97 g / mol

Melting point

1652 ºC

Boiling point

3402 ºC

Density

At room temperature: 9.841 g / cm 3

Right at the melting point: 9.3 g / cm 3

Heat of fusion

22 kJ / mol

Heat of vaporization

414 kJ / mol

Molar caloric capacity

26.86 kJ / mol K

Oxidation states

Lutetium is capable of forming compounds with the following oxidation states: 0, +1 (Lu + ), +2 (Lu 2+ ) and +3 (Lu 3+ ), the latter being by far the most common and stable of everybody. Therefore, almost all lutetium compounds contain the Lu 3+ cation , either forming complexes, or interacting electrostatically with other anions.

Electronegativity

1.27 on the Pauling scale.

Ionization energies

First: 523.5 kJ / mol

Second: 1340 kJ / mol

Third: 2022.3 kJ / mol

Magnetic order

Paramagnetic. However, it becomes superconducting at a temperature of 0.022 K, and under a pressure of 45 kilobars.

Reactivity

Chemically, lutetium bears a close resemblance to scandium and yttrium, forming Lu 3+ cations whose solid compounds and solutions are, for the most part, colorless. This peculiarity contradicts the rest of the lanthanides, which generally produce very colorful and fluorescent solutions.

The reactivity of lutetium can also be compared to that of calcium and magnesium, which is why it dissolves easily in dilute acids; such as hydrochloric acid, to produce lutetium chloride, LuCl 3 .

Applications

Gas remover

Lutetium oxide, Lu 2 O 3 , is a good absorber of moisture and carbon dioxide, so its powder is used to remove these gases from some compartments.

Oil catalysis

Lu 2 O 3 is used to prepare catalysts that accelerate the cracking of petroleum hydrocarbons .

Organic catalysis

Lutetium triflate is used in organic syntheses as a catalyst in aqueous media, having the advantage of dispensing with organic solvents, and making reactions more ecological.

Dopant

Lu 2 O 3 and Lu 3+ ions are used as dopants for glasses, ceramics, garnets and alloys. For example, Lutetium Aluminum Garnet (LuAG) is used as a blue phosphor in LED bulbs, and Lutetium Aluminum Gadolinium Garnet is used in bubble memory devices .

On the ceramic side, lutetium oxyortosilicate (LSO) is used in positron emission tomography detectors. Thanks to this material, it is possible to obtain 3D images of the cellular activity of the patients subjected to these analyzes.

Dating

The radioactive decay of the 176 Lu isotope is used to date meteorites present on Earth.

Medicine

The radioactive isotope 177 Lu, prepared by neutron bombardment from 176 Lu, is coordinated to an organic molecule ( 177 Lu-DOTATATE) to target its radioactive action on neuroendocrine tumors, or in the treatment of prostate cancer. This is perhaps the most promising app for lutetium.

Obtaining

Lutetium is the least abundant of the lanthanides. There is no mineral that contains a concentration above 0.1% for this metal. That is why it is extracted from many rare earth minerals, such as euxenite, xenotime, lateritic clays and monazite, being a by-product of the processing of the other lanthanides.

These minerals are dissolved in sulfuric acid, the solution of which is then treated with ammonium oxalate to precipitate various oxalates, which are heated to transform into their metal oxides. The oxides are then dissolved with nitric acid, leaving out the cerium oxide, which is insoluble in this acid.

The new solution is mixed with ammonium nitrate to form a set of double salts, to be finally refined and separated by ion exchange chromatography techniques or fractional crystallizations using various solvents. Thus, the Lu 3+ ions are separated as anhydrous halides.

Lutetium is obtained by reducing its halides with calcium:

2 LuCl 3  + 3 Ca → 2 Lu + 3 CaCl 2

Isotopes

Lutetium occurs in nature as two isotopes: 175 Lu and 176 Lu, whose respective abundances are 97.4% and 2.6%. The 176 Lu is radioactive, but its 1/2 is 3.76·10 10 years, so that their beta emissions are harmless for those working with samples or lutetium salts.

Lutetium, apart from 176 Lu, has 33 other artificial radioisotopes, of which 177 Lu is the most famous and useful, and 150 Lu the most unstable, with a 1/2 of just about 45 milliseconds. The atomic masses of these radioisotopes are between 150 and 184 u.

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