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What is thulium?

The thulium (Tm) is a chemical element belonging to the lanthanide series and it is the most scarce natural and nonradioactive metal rare earth. Therefore, its cost has always been high, even being more expensive than platinum itself. Its name comes from the word ‘Thule’, designated to the northernmost part of the old European maps, where the Scandinavian region is currently located.

It was discovered and named in 1879 by the Swedish chemist Per Teodor Cleve, who studied rare earth oxides, specifically erbium, from which he extracted negligible amounts of thulium oxide, identified thanks to its absorption spectrum and associated characteristic lines. to the color green.

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

The first relatively pure sample of thulium was obtained in 1911, after 15,000 fractional crystallizations with bromate salts carried out by the chemist Charles James, then resident in the United States. As separation techniques and ion exchange chromatography evolved, increasingly pure and inexpensive samples of metallic thulium were produced.

Thulium is an element that is often ignored because it is considered strange. It is useful in medicine, being an important source of X-rays, as well as a doping element for the production of alloys and special ceramics.

Physical

Thulium has a silvery-gray surface, which gradually darkens as it oxidizes. When hard-filed, it gives off fiery sparks and greenish flashes, the color of which is reminiscent of the excited state of copper. It is soft, malleable and ductile, having a Mohs hardness between 2 and 3, so it can be cut using a knife.

It is a strongly paramagnetic metal, and its molten liquid exhibits high vapor pressures, somewhat unusual for many metals.

Chemical

Thulium, like the other lanthanides, participates in most of its compounds with an oxidation state or number of +3 (Tm 3+ ). For example, its only oxide, Tm 2 O 3 , contains the cations Tm 3+ and forms rapidly when a sample of metallic thulium is heated to 150 ° C:

4 Tm (s) + 3 O 2  (g) → 2 Tm 2 O 3 (s)

On the other hand, thulium reacts with cold or hot water to produce its respective hydroxide:

2 Tm (s) + 6 H 2 O (l) → 2 Tm (OH) 3  (aq) + 3 H 2  (g)

The aqueous solutions of the Tm 3+ ions are greenish due to the formation of the complex aqueous [Tm (OH 2 ) 9 ] 3+ . These also exhibit bluish luminescence when irradiated with ultraviolet light.

The hydrates of the thulium (III) compounds, likewise, are characterized by having greenish colors, since the water molecules manage to coordinate with part of the Tm 3+ present in the crystals.

Chemical structure

In some sources, thulium is cited as having a single allotropic form, corresponding to a compact hexagonal structure, hcp. However, reference is made to another second allotropic form, called α-Tm, whose structure is tetragonal; while thulium hcp is called β-Tm, being by far the most stable and reported.

Under high pressures (in the order of GPa), thulium undergoes transitions to denser crystalline phases, passing from hcp or β-Tm to a hexagonal structure isomorphic to that of samarium, to later become double compact hexagonal (dhcp), and finally compacting to distorted forms of fcc crystals.

Electronic configuration

The electron configuration of thulium is as follows:

[Xe] 6s 2 4f 13

Note that it only lacks a single electron to complete the filling of its 4f orbitals. By having 13 electrons in this subshell, and by being located in position or group 13 of the lanthanide series, it is said that its electronic configuration does not present any deviation.

The electrons in its 4f orbitals are responsible for the metallic bond that joins the thulium atoms. As there are 13 of them, the attractions between the Tm atoms are large, explaining why their melting and boiling points are higher compared to those of europium, for example, this metal also being a member of the lanthanides.

Obtaining thulium

Raw material

Thulium is found in many of the minerals where other rare earth metals (gadolinium, erbium, samarium, cerium, etc.) predominate. In none of them is it found in a considerable proportion to serve as the sole mineralogical source.

The mineral monazite contains around 0.007% thulium, making it one of the raw materials from which this metal is obtained. But the clays of the southeast of China have a concentration of up to 0.5% of thulium, being therefore the most used raw material for its extraction and production.

Extraction and production method

Thulium was one of the last metals to be produced with a high degree of purity (> 99%). It is first necessary to separate the Tm 3+ ions from the rest of the mineralogical matrix, enriched with unimaginable amounts of ions from other rare earth metals. Without ion exchange chromatography, accompanied by solvent extraction techniques, such separation is not possible.

When the clays or monazite are chemically processed to obtain the separated Tm 3+ ions as Tm 2 O 3 , a reduction is used using lanthanum in order to reduce thulium oxide to metallic thulium.

Applications

Dopant for ceramics and alloys

Thulium in its pure state has no uses. However, its neutral atoms are used as dopants in many ceramic materials and metal alloys composed of other rare earth elements.

In ceramics, it is used for the production of superconducting materials at high temperatures and for the production of microwave components; while in alloys, such as yttrium aluminum garnet (YAG), it is used for the manufacture of powerful lasers to carry out surgeries.

Bluish luminescence

The bluish, shiny bits of euros under UV light are due to the fluorescence of thulium. Source: repro by H. Grobe / CC BY (https://creativecommons.org/licenses/by/3.0)

Like europium, thulium oxide is impregnated on euro notes to emit bluish luminescence when exposed under a UV lamp. In this way, the euros are prevented from being counterfeited.

On the other hand, its luminescence or fluorescence is also used in personal dosimeters, in which thulium is added to calcium sulfate so that the salt shines against a source of ultraviolet radiation.

X-ray emitter

Thulium has a single natural isotope: 169 Tm. But when bombarded with neutrons, it becomes the 170 Tm isotope , which emits moderate gamma radiation and has a 1/2 of 128 days.

This 170 Tm is used in portable devices such as X-ray emitters, used to visualize cancers using brachytherapy, and also to detect cracks in structures or electronic equipment.

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