Samarium

Samarium, with the symbol Sm and atomic number 62, is a moderately hard, silvery metal from the lanthanide series. It slowly tarnishes in air and most commonly appears in the +3 oxidation state, though compounds where it takes the +2 form—such as the monoxide SmO, the monochalcogenides SmS, SmSe, and SmTe, and samarium(II) iodide—are also known.

French chemist Paul-Émile Lecoq de Boisbaudran discovered the element in 1879. He named it after the mineral samarskite, from which he isolated it. That mineral, in turn, was named for Vassili Samarsky-Bykhovets, a Russian mine official, making him the first person (indirectly) to have a chemical element named after him.

Samarium makes up as much as 2.8% of several minerals, including cerite, gadolinite, samarskite, monazite, and bastnäsite. The last two are the main commercial sources. These minerals are found in China, the United States, Brazil, India, Sri Lanka, and Australia, with China dominating global mining and production.

Commercially, samarium is best known for samarium–cobalt magnets, which have permanent magnetization second only to neodymium magnets. However, samarium compounds can withstand much higher temperatures—above 700 °C—without losing their magnetic properties. The radioisotope samarium-153 is the active ingredient in the drug samarium (153Sm) lexidronam (Quadramet), used to kill cancer cells in lung, prostate, breast, and bone cancers. Samarium-149 strongly absorbs neutrons, so it is added to control rods in nuclear reactors; it also forms as a decay product during reactor operation and is a key factor in reactor design. Other uses include chemical catalysis, radioactive dating, and X-ray lasers. Samarium(II) iodide is a common reducing agent in chemical synthesis. The element has no biological role, and some of its salts are slightly toxic.

**Physical properties** Samarium is a rare earth element with hardness and density similar to zinc. Its boiling point of 1,794 °C makes it the third most volatile lanthanide, after ytterbium and europium, and comparable to lead and barium—a property that aids its separation from ores. Freshly prepared, it has a silvery luster, but it dulls as it oxidizes in air. With an atomic radius of 238 pm, samarium is among the largest elements; only potassium, praseodymium, barium, rubidium, and caesium are larger.

At room temperature, samarium takes a rhombohedral structure (α form). Heating to 731 °C shifts it to a hexagonal close-packed (hcp) structure, though the exact temperature depends on metal purity. Further heating to 922 °C produces a body-centered cubic (bcc) phase. Applying 40 kbar of pressure at 300 °C yields a double-hexagonally close-packed (dhcp) structure. Much higher pressures—hundreds or thousands of kilobars—trigger more phase changes, including a tetragonal phase around 900 kbar. In one study, the dhcp phase appeared without compression by using a rapid temperature change between about 400 °C and 700 °C, confirming its transient nature. Thin films made by vapor deposition can contain hcp or dhcp phases at room temperature.

Samarium and its sesquioxide are paramagnetic at room temperature, with effective magnetic moments below 2 Bohr magnetons—the third-lowest among lanthanides (and their oxides), after lanthanum and lutetium. Cooling to 14.8 K makes the metal antiferromagnetic. Individual samarium atoms can be isolated inside fullerene molecules, or intercalated into bulk C60 to form a solid solution of nominal composition Sm3C60, which becomes superconductive at 8 K. Doping iron-based high-temperature superconductors with samarium raises their transition temperature to 56 K, the highest yet achieved in that series.

**Chemical properties** Samarium oxidizes slowly in air at room temperature and ignites spontaneously at 150 °C. Even under mineral oil, it gradually forms a grayish-yellow powder of oxide-hydroxide mixture on its surface. To preserve its metallic appearance, it must be sealed under an inert gas like argon.

The metal is quite electropositive. It reacts slowly with cold water and rapidly with hot water to produce samarium hydroxide: 2Sm(s) + 6H2O(l) → 2Sm(OH)3(aq) + 3H2(g) It dissolves readily in dilute sulfuric acid, forming yellow to pale green Sm(III) ions that exist as [Sm(OH2)9]3+ complexes: 2Sm(s) + 3H2SO4(aq) → 2Sm3+(aq) + 3SO2−4(aq) + 3H2(g) Samarium is one of the few lanthanides (along with europium and ytterbium) with a relatively accessible +2 oxidation state. Sm2+ ions appear blood-red in aqueous solution.

**Compounds**

**Oxides** The most stable oxide is the sesquioxide Sm2O3. Like many samarium compounds, it has several crystalline phases. The trigonal form appears when the melt is cooled slowly. Sm2O3 melts at a high temperature (2345 °C), so it is usually melted by induction heating with a radio-frequency coil rather than direct heating. Monoclinic Sm2O3 crystals can be grown by the flame fusion method (Verneuil process) from Sm2O3 powder, producing cylindrical boules up to several centimeters long and about one centimeter in diameter. These boules are transparent when pure and defect-free.