Thulium

Thulium is a chemical element on the periodic table whose symbol is Tm and its atomic number is 69.

In 1879, the Swedish chemist Per Teodor Cleve separated two other previously unknown components from the rare earth oxide erbia, which he called holmia and thulia; they were holmium and thulium oxides, respectively. In 1911, a relatively pure sample of metallic thulium was obtained for the first time. Its name comes from the ancient Greek name for Scandinavia in Latin, Thulium.

Thulium is the second least abundant lanthanide element, after the radioactively unstable promethium, which is found only in trace amounts on Earth. It is an easy to work metal with a brilliant silvery-grey luster. It is quite soft and slowly fogs up in air. Despite its high price and rarity, Thulium is used as a radiation source in portable X-ray devices and some solid-state lasers. It has no important biological function and is not particularly toxic.

History

Per Teodor Cleve, the scientist who discovered the tulio and the holmio.

Thulium was discovered by the Swedish chemist Per Teodor Cleve in 1879 by looking for impurities in the oxides of other rare earth elements (this was the same method Carl Gustaf Mosander had previously used to discover some other rare earth elements). Cleve began by removing all known erbia contaminants (Er
2O
3). Upon further processing, he obtained two new substances; one brown and one green. The brown substance was the oxide of the element holmium and was named holmia by Cleve, and the green substance was the oxide of an unknown element. Cleve named the rust thulia and its element thulium after Thule, an ancient Greek place name associated with Scandinavia or Iceland. The atomic symbol for Thulium was initially Tu, but was later changed to Tm.

Thulium was so rare that none of the early researchers had enough of it to purify it enough to see the color green; they had to be content with observing spectroscopically the strengthening of the two characteristic absorption bands, as the erbium was progressively removed. The first researcher to obtain nearly pure thulium was Charles James, a British expatriate working on a large scale at the College of New Hampshire in Durham, USA. In 1911 he reported his results, having used his discovered method of fractional bromate crystallization to perform the purification. It is well known that he needed 15,000 purification operations to establish that the material was homogeneous.

High purity thulium oxide was first offered commercially in the late 1950s, as a result of the adoption of ion exchange separation technology. Lindsay Chemical Division of American Potash & Chemical Corporation offered it in grades of 99% and 99.9% purity. The price per kilogram ranged from US$4,600 to US$13,300 in the period 1959 to 1998 for 99.9% purity, and was the second highest for lanthanides behind lutetium.

Properties

Physical properties

Pure Thulium metal has a silver luster. It is fairly stable when exposed to air, but must be protected from moisture. The metal is soft, malleable, and ductile. Thulium is ferromagnetic at temperatures below 32 K, antiferromagnetic between 32 and 56 K, and paramagnetic above 56 K.

Chemical Properties

Thulium metal oxidizes slowly when exposed to air and burns at a temperature of 150 °C forming Thulium(III) oxide:

4 Tm + 3 O₂ → 2 Tm₂O₃

Thulium is quite electropositive and reacts slowly with cold water and fairly quickly with hot water to form Thulium hydroxide:

2 Tm (s) + 6 H₂O (l) → 2 Tm(OH)₃ (aq) + 3 H₂ (g)

Thulium reacts with all halogens. The reactions are slow at room temperature, but vigorous above 200 °C:

2 Tm (s) + 3 F₂ (g) → 2 TmF₃ (s) [white]

2 Tm (s) + 3 Cl₂ (g) → 2 TmCl₃ (s) [yellow]

2 Tm (s) + 3 Br₂ (g) → 2 TmBr₃ (s) [white]

2 Tm (s) + 3 I₂ (g) → 2 TmI₃ (s) [yellow]

Thulium dissolves in dilute sulfuric acid to form solutions containing the pale green ions of Tm(III), which exist as complexes of [Tm(OH₂)₉]³⁺:

2 Tm (s) + 3 H₂SO₄ (aq) → 2 Tm³⁺ (aq) + 3 SO₄^2- (aq) + 3 H₂ (g)

Thulium reacts with various metallic and non-metallic elements forming a set of binary compounds, including TmN, TmS, TmC₂, Tm₂C₃ , TmH₂, TmH₃, TmSi₂, TmGe₃, TmB₄ , TmB₆ and TmB₁₂. In these compounds, Thulium presents +2, +3 and +4 valence states, however, the +3 state is more common and this state is the only one that has been observed in Tm solutions.

Occurrence

Tullium is found in the monacita mineral.

The element is never found in nature in its pure form, but is found in trace amounts in minerals with other rare earths. Thulium is often found with yttrium and gadolinium containing minerals. In particular, thulium is found in the mineral gadolinite. However, like many other lanthanides, thulium is also found in the minerals monazite, xenotime, and euxenite. Thulium has not been found in prevalence over the other rare earths in any mineral yet. Its abundance in the Earth's crust is 0.5 mg/kg by weight and 50 parts per billion by moles. Thulium makes up about 0.5 parts per million of the soil, although this value can range from 0.4 to 0.8 parts per million. Thulium makes up 250 parts per quadrillion of seawater. In the Solar System, Thulium exists in concentrations of 200 parts per trillion by weight and 1 part per trillion by moles. Thulium ore is found mostly in China. However, Australia, Brazil, Greenland, India, Tanzania, and the United States also have large reserves of thulium. Total thulium reserves are approximately 100,000 tons. Thulium is the least abundant lanthanide on Earth, except for the radioactive promethium.

Production

Thulium is mainly extracted from the mineral monazite (~0.007% thulium) found in river sands, through ion exchange. Newer ion exchange and solvent extraction techniques have led to easier separation of the rare earths, which has lowered the production costs of thulium. The main sources today are ion adsorption clays from southern China. In these, about two-thirds of the total rare earth content is yttrium, and thulium is about 0.5% (or about even with lutetium for rarity). The metal can be isolated by reduction of its oxide with lanthanum metal or by reduction with calcium in a closed vessel. None of the natural thulium compounds are commercially important. Approximately 50 tons of thulium oxide are produced per year. In 1996, thulium oxide cost US$20 per gram, and in 2005, 99% pure thulium metal powder cost US$70 per gram.

Applications

Laser

Yttrium aluminum garnet triple doped with holmium-chromium-thulium (Ho:Cr:Tm:YAG, or Ho,Cr,Tm:YAG) is an active laser medium material with high efficiency. It launches at 2080 nm in the infrared and is widely used in military, medical, and meteorological applications. Thulium-doped YAG (Tm:YAG) lasers operate at 2010 nm. The wavelength of Thulium-based lasers is very efficient for superficial tissue ablation, with minimal coagulation depth in air or on Water. This makes thulium lasers attractive for laser surgery.

X-ray source

Despite its high cost, portable X-ray devices use thulium that has been bombarded with neutrons in a nuclear reactor to produce the isotope thulium-170, which has a half-life of 128.6 days and five lines of main emission of comparable intensity (to 7. These radioactive sources have a useful life of approximately one year, as medical and dental diagnostic tools, as well as for detecting defects in inaccessible mechanical and electronic components. These sources do not need extensive protection against radiation – just a small cup of lead. They are among the most popular radiation sources for use in industrial radiography. Thulium-170 is gaining popularity as an X-ray source for brachytherapy (sealed source radiation therapy) cancer treatment.

Others

Thulium has been used in high-temperature superconductors in a similar way to yttrium. Thulium potentially has a use in ferrites, ceramic magnetic materials used in microwave equipment. Thulium is also similar to scandium in that it is used in arc lighting because of its unusual spectrum, in this case, its green emission lines, which are not covered by other elements. Because thulium fluoresces blue when exposed to ultraviolet light, thulium is placed on euro banknotes as a measure to prevent counterfeiting. The blue fluorescence of Tm-doped calcium culphate has been used in personal dosimeters for visual monitoring of radiation. Tm-doped halides in which Tm is in its 2+ valence state appear as promising luminescent materials that can constitute and enable the design of efficient electricity-generating windows based on the principle of a luminescent solar concentrator.