Samarium

Samarium is a chemical element with the symbol Sm and atomic number 62.

Characteristics

Physical

Samarium is a rare earth metal, with a bright silver luster. Three crystal modifications of the metal also exist, with transformations at 734 and 922 °C, making it polymorphic. Individual samarium atoms can be isolated by encaspulating them into fullerene molecules.[2]

Chemical

Samarium oxidizes in air and ignites at 150 °C. Even with long-term storage under mineral oil, samarium is gradually oxidized, with a grayish-yellow powder of the oxide-hydroxide being formed. The metallic appearance of a sample can be preserved by sealing it under an inert gas such as argon.

Samarium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form samarium hydroxide:

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

Samarium metal reacts with all the halogens:

2 Sm (s) + 3 F₂ (g) → 2 SmF₃ (s) [white]

2 Sm (s) + 3 Cl₂ (g) → 2 SmCl₃ (s) [yellow]

2 Sm (s) + 3 Br₂ (l) → 2 SmBr₃ (s) [yellow]

2 Sm (s) + 3 I₂ (s) → 2 SmI₃ (s) [orange]

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

2 Sm (s) + 3 H₂SO₄ (aq) → 2 Sm³⁺ (aq) + 3 SO2−4 (aq) + 3 H₂ (g)

Compounds
See also: Category:Samarium compounds

Compounds of samarium include

• Fluorides: SmF₂, SmF₃
• Chlorides: SmCl₂, SmCl₃
• Bromides: SmBr₂, SmBr₃
• Iodides: SmI₂, SmI₃
• Oxides: Sm₂O₃
• Sulfides: Sm₂S₃
• Selenides: Sm₂Se₃
• Tellurides: Sm₂Te₃

The most common oxidation state of samarium is +3, but +2 compounds are known too, such as SmI2.

Isotopes
Main article: Isotopes of samarium

Naturally occurring samarium is composed of four stable isotopes, 144Sm, 150Sm, 152Sm and 154Sm, and three extremely long-lived radioisotopes, 147Sm (1.06 × 1011y), 148Sm (7 × 1015y) and 149Sm (>2 × 1015y), with 152Sm being the most abundant (26.75% natural abundance).

151Sm has a halflife of 90 years, and 145Sm has a halflife of 340 days. All of the remaining radioisotopes have half-lives that are less than 2 days, and the majority of these have half-lives that are less than 48 seconds. This element also has 5 meta states with the most stable being 141mSm (t½ 22.6 minutes), 143m1Sm (t½ 66 seconds) and 139mSm (t½ 10.7 seconds).

The long lived isotopes,146Sm, 147Sm, and 148Sm primarily decay by alpha decay to isotopes of neodymium. Lighter unstable isotopes of samarium primarily decay by electron capture to isotopes of promethium, while heavier ones decay by beta minus decay to isotopes of europium

Natural Samarium has an activity of 128 Bq/g.

History

Samarium was discovered in 1853 by Swiss chemist Jean Charles Galissard de Marignac by its sharp absorption lines in didymium, and isolated in Paris in 1879 by French chemist Paul Émile Lecoq de Boisbaudran from the mineral samarskite ((Y,Ce,U,Fe)3(Nb,Ta,Ti)5O16). Although samarskite was first found in the Urals, by the late 1870s a new deposit had been located in North Carolina, and it was from that source that the samarium-bearing didymium had originated.

The samarskite mineral was named after Vasili Samarsky-Bykhovets, the Chief of Staff (Colonel) of the Russian Corps of Mining Engineers in 1845–1861. The name of the element is derived from the name of the mineral, and thus traces back to the name Samarsky-Bykhovets. In this sense samarium was the first chemical element to be named after a living person.

Prior to the advent of ion-exchange separation technology in the 1950s, samarium had no commercial uses in pure form. However, a by-product of the fractional crystallization purification of neodymium was a mixture of samarium and gadolinium that acquired the name of "Lindsay Mix" after the company that made it. This material is thought to have been used for nuclear control rods in some of the early nuclear reactors. Nowadays, a similar commodity product has the name "samarium-europium-gadolinium" (SEG) concentrate. It is prepared by solvent extraction from the mixed lanthanides extracted from bastnäsite (or monazite). Since the heavier lanthanides have the greater affinity for the solvent used, they are easily extracted from the bulk using relatively small proportions of solvent. Not all rare earth producers who process bastnäsite do so on large enough scale to continue onward with the separation of the components of SEG, which typically makes up only one or two percent of the original ore. Such producers will therefore be making SEG with a view to marketing it to the specialized processors. In this manner, the valuable europium content of the ore is rescued for use in phosphor manufacture. Samarium purification follows the removal of the europium. Currently, being in oversupply, samarium oxide is less expensive on a commercial scale than its relative abundance in the ore might suggest.

Occurrence
Samarskite

Samarium is not found free in nature, but, like other rare earth elements, is contained in many minerals, including monazite, bastnäsite and samarskite; monazite (in which it occurs up to an extent of 2.8%) and bastnäsite are also used as commercial sources. Mischmetal containing about 1% of samarium has long been used, but it was not until recent years that relatively pure samarium has been isolated through ion exchange processes, solvent extraction techniques, and electrochemical deposition. The metal is often prepared by electrolysis of a molten mixture of samarium(III) chloride with sodium chloride or calcium chloride. Samarium can also be obtained by reducing its oxide with lanthanum.[4]

Applications

Uses of samarium include:

* Carbon-arc lighting for the motion picture industry (together with other rare earth metals).
* CaF2 crystals for use in lasers.
* As a neutron absorber in nuclear reactors.
* For headphone magnets.
* Samarium-cobalt magnets; SmCo5 and Sm2Co17 are used in making permanent magnet materials that have high resistance to demagnetization when compared to other permanent magnet materials. These materials have high coercivities and intrinsic coercivities. Samarium-cobalt combinations have recently found use in high-end magnetic pickups for guitars and related musical instruments.
* Samarium(II) iodide is used as a reducing agent and coupling agent chemical reagent in organic synthesis, for example in the Barbier reaction.[5]
* Samarium oxide is used in optical glass to absorb infrared light.
* Samarium compounds act as sensitizers for phosphors excited in the infrared.
* Samarium oxide is a catalyst for the dehydration and dehydrogenation of ethanol.
* Samarium-neodymium dating is useful for determining the age relationships of rocks and meteorites.
* Radioactive samarium-153 is used in medicine to treat the severe pain associated with cancers that have spread to bone. The drug is called Quadramet.[6]

Precautions

As with the other lanthanides, samarium compounds are of low to moderate toxicity, although their toxicity has not been investigated in detail.

References

1. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
2. ^ Okazaki , T (2002). "Electronic and geometric structures of metallofullerene peapods". Physica B 323: 97. doi:10.1016/S0921-4526(02)00991-2.
3. ^ "Chemical reactions of Samarium". Webelements. https://www.webelements.com/samarium/chemistry.html. Retrieved 2009-06-06.
4. ^ N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, UK, 1984.
5. ^ Cotton (2007). Advanced inorganic chemistry, 6th ed. Wiley-India. p. 1128. ISBN 8126513381. http://books.google.com/books?id=U3MWRONWAmMC&pg=PA1128.
6. ^ "Centerwatch About drug Quadramet". http://www.centerwatch.com/patient/drugs/dru267.html. Retrieved 2009-06-06.

External links

* WebElements.com – Samarium
* It's Elemental – Samarium
* Reducing Agents > Samarium low valent