samarium

samarium (Sm), chemical element, a rare-earth metal of the lanthanide series of the periodic table.

Samarium is a moderately soft metal, silvery white in colour. It is relatively stable in air, slowly oxidizing to Sm₂O₃. It rapidly dissolves in diluted acids—except hydrofluoric acid (HF), in which it is stable because of formation of a protective trifluoride (SmF₃) layer. Samarium is a moderately strong paramagnet above 109 K (−164 °C, or −263 °F). Below 109 K, antiferromagnetic order develops for the cubic sites in the samarium lattice, and the hexagonal site atoms finally order antiferromagnetically below 14 K (−259 °C, or −434 °F).

Samarium was isolated as an impure oxide and spectroscopically identified as a new element in 1879 by French chemist Paul-Émile Lecoq de Boisbaudran. Samarium occurs in many other rare-earth minerals but is almost exclusively obtained from bastnasite; it is also found in products of nuclear fission. In Earth’s crust, samarium is as abundant as tin.

The seven naturally occurring isotopes of samarium are samarium-144 (3.1 percent), samarium-147 (15.0 percent), samarium-148 (11.2 percent), samarium-149 (13.8 percent), samarium-150 (7.4 percent), samarium-152 (26.8 percent), and samarium-154 (22.0 percent). Samarium-144, samarium-150, samarium-152, and samarium-154 are stable, but the other three naturally occurring isotopes are alpha emitters. A total of 34 (excluding nuclear isomers) radioactive isotopes of samarium have been characterized. Their mass ranges from 128 to 165, and their half-life can be as short as 0.55 second for samarium-129 or as long as 7 × 10¹⁵ years for samarium-148.

Liquid-liquid and ion-exchange techniques are used for the commercial separation and purification of samarium. The metal is conveniently prepared by metallothermic reduction of its oxide, Sm₂O₃, with lanthanum metal, followed by distillation of the samarium metal, which is one of the most volatile rare-earth elements. Samarium exists in three allotropic (structural) forms. The α-phase (or Sm-type structure) is a rhombohedral arrangement that is unique among the elements, with a = 3.6290 Å and c = 26.207 Å at room temperature. (The unit cell dimensions are given for the non-primitive hexagonal unit cell of the primitive rhombohedral lattice.) The β-phase is hexagonal close-packed with a = 3.6630 Å and c = 5.8448 Å at 450 °C (842 °F). The γ-phase is body-centred cubic with a = 4.10 Å (estimated) at 922 °C (1,692 °F).

The most common use of samarium is with cobalt (Co) in high-strength SmCo₅- and Sm₂Co₁₇-based permanent magnets suitable for high-temperature applications. The energy product of samarium-based permanent magnets is second to those based on neodymium, iron, and boron (Nd₂Fe₁₄B), but the latter have much lower Curie points than the samarium magnets and therefore are unsuitable for applications above approximately 300 °C (570 °F). Because of its high absorption cross section for thermal neutrons (samarium-149), samarium is used as an addition in nuclear reactor control rods and for neutron shielding. Other uses are in phosphors for displays and TV screens that use cathode-ray tubes, in special luminescent and infrared-absorbing glasses, in inorganic and organic catalysis, and in the electronics and ceramics industries.

In addition to its more stable +3 oxidation state, samarium, unlike most of the rare earths, has a +2 oxidation state. The Sm²⁺ ion is a powerful reducing agent that rapidly reacts with oxygen, water, or hydrogen ions. It can be stabilized by precipitation as the extremely insoluble sulfate SmSO₄. Other salts of samarium in the +2 state are SmCO₃, SmCl₂, SmBr₂, and Sm(OH)₂; they are reddish brown in colour. In its +3 oxidation state, samarium behaves as a typical rare-earth element; it forms a series of yellow salts in solutions.