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Samarium: characteristics, structure, obtaining, uses

Lecoq de Boisbaudran is often given credit for the discovery of samarium, although there were other chemists and mineralogists who previously charted the way to find it. It was not until 1901 that the French chemist Eugène Anatole managed to produce the first pure sample of samarium.

Samarium sample stored tightly within an ampoule. Source: Hi-Res Images of Chemical Elements / CC BY (https://creativecommons.org/licenses/by/3.0)

The reason for this delay in its isolation was due to the fact that samarium is a relatively reactive metal, so it is not pure in nature, but rather as part of many mineral masses. It is also closely geologically related to other rare earth elements such as europium and neodymium, making it difficult to separate it from such contaminants.

Characteristics of samarium

Physical

Samarium has a silvery-white luster, but it quickly turns golden (see image above) due to the fact that it is covered with a layer of oxide, Sm 2 O 3 , which is called samaria. It is one of the hardest and most volatile lanthanide metals, having melting and boiling points of 1072 and 1900 ºC, respectively.

It oxidizes relatively slowly when exposed to air or immersed in mineral oil. That is why it should be stored in ampoules, or inside sealed containers with argon or any other inert gas. When heated to 150 ° C, it oxidizes vigorously, giving off fiery sparks if roughly filed.

Chemical

Samarium, like the other lanthanides, exhibits an oxidation state of +3 in almost all of its compounds; that is, it is found as a Sm 3+ cation . However, it is also capable of adopting the oxidation state of +2, Sm 2+ , being found in compounds such as SmO (samarium monoxide), SmS (samarium monosulfide) and SmI 2 (samarium diiodide).

It dissolves in hot water and especially in dilute acids, such as HCl, H 2 SO 4 and CH 3 COOH; With the exception of HF, because it forms a protective layer of SmF 3 that slows its dissolution. Its oxide, Sm 2 O 3 , is moderately basic, so when dissolved in water it will release significant amounts of OH  ions through the action of the hydroxide Sm (OH) 3 .

Most of the compounds of samarium +3 are characterized by having yellowish-greenish colors, and some even stand out for being luminescent.

At room temperature, samarium adopts a rhombohedral crystalline structure, which corresponds to the polymorph or α phase. When it is heated to 731 ºC, a phase transition occurs, densifying its crystals to a compact hexagonal structure (hcp), called the β phase.

Following heating to a temperature of 922 ° C, samarium undergoes another transition to a body-centered cubic structure (bcc), called the γ phase.

Samarium crystals can also undergo other transitions when they are compressed under high pressure, in the order of thousands of kilobars, being the tetragonal and the double compact hexagonal (dhcp) some of the structures obtained in these studies.

Electronic configuration

The abbreviated electron configuration of samarium is:

[Xe] 6s 2 4f 6

It has six electrons in its 4f orbitals, which is consistent with its position in the sixth group of lanthanides. Therefore, its electron configuration is not about any of the many deviations that we see in the periodic table.

Obtaining

Sand from the mineral monazite, the main raw material for obtaining samarium. Source: D. Kemp, AC Cilliersslightly modified by Gretarsson (replaced red, poorly visible scale bar by a white one) / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Despite being part of the rare earths, the abundance of samarium is higher than that of tin and other metals. It is associated with rare earth metal oxides, composing these minerals such as cerite, gadolinite, monazite and bastnasite, monazite being one of its main mineralogical sources, as it contains around 2.8% samarium.

There are several methods to obtain it. One of them consists of processing the monazite sands and separating the Sm 3+ ions , either through dissolutions and subsequent solvent extractions, or using ion exchange chromatography.

Samarium ions when obtained as SmCl 3 , are subjected to electrolysis with a molten mixture of NaCl or CaCl 2 . On the other hand, if these ions are obtained as Sm 2 O 3 , then the oxide is reduced in a tantalum crucible using lanthanum, where the samarium vapors are distilled due to its lower boiling point . The equation for this reduction is as follows:

Sm 2 O 3 + 2La → 2Sm + La 2 O 3

The reduction is carried out hot (close to 1400 ºC) and inside a vacuum induction furnace, which further speeds up the distillation of the resulting samarium vapors.

Uses / applications of samarium

Magnets

SmCo magnets are used as part of the functional components of quartz watches. Source: © 2010 by Tomasz Sienicki [user: tsca, mail: tomasz.sienicki at gmail.com] / CC BY (https://creativecommons.org/licenses/by/3.0)

Samarium is alloyed with cobalt to give rise to SmCo alloys, whose magnetization is permanent and around 10,000 times greater than that of iron.

These samarium-cobalt magnets are used mainly in camera shutters, headphones, motors, electric guitar pickups, as well as in military applications where they withstand temperatures above 400ºC.

Medicine

Molecular structure of the drug Quadramet, 153Sm-EDTMP.

Samarium itself is relatively toxic. However, one of its radioactive isotopes, 153 Sm, chelated by a molecule of EDTMP (ethylenediaminetetramethylenephosphonate, pictured above), is used to combat pain in the treatment of prostate, lung, and breast cancers. This medicine is called samarium (153Sm) lexidronam, commercially known as Quadramet.

Nuclear reactors

The 149 Sm isotope is an excellent neutron absorber, which is why it is used in nuclear reactors to control reactions and prevent an explosion.

electronics

The SmS converts the difference in temperature into electricity, which is why it is used as a thermoelectric in different equipment. It also has the peculiarity of turning metallic under relatively low pressures.

Dating

The alpha decay of isotope 147 Sm ( 1/2 = 1.06 × 10 11 ) to isotope 143 Nd, is used to date rock or meteorite samples inside or outside the Earth. It has the advantage that the atoms 147 Sm and 143 Nd share the same geological characteristics, that is, they do not undergo large separations during metamorphic processes.

Catalysis

Samarium is used in organic syntheses as SmI 2 , acting as a reducing agent in numerous syntheses of synthetic versions of natural products. On the other hand, Sm 2 O 3 is a catalyst for the dehydration and dehydrogenation of ethanol.

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