Samarium is a chemical element; it has symbol Sm and atomic number 62. It is a moderately hard silvery metal that slowly oxidizes in air. Being a typical member of the lanthanide series, samarium usually has the oxidation state +3. Compounds of samarium(II) are also known, most notably the monoxide SmO, monochalcogenides SmS, SmSe and SmTe, as well as samarium(II) iodide.
Discovered in 1879 by French chemist Paul-Émile Lecoq de Boisbaudran, samarium was named after the mineral samarskite from which it was isolated. The mineral was named after a Russian mine official, Vassili Samarsky-Bykhovets, who thus became the first person to have a chemical element named after him, though the name was indirect.
Samarium occurs in concentration up to 2.8% in several minerals including cerite, gadolinite, samarskite, monazite and bastnäsite, the last two being the most common commercial sources of the element. These minerals are mostly found in China, the United States, Brazil, India, Sri Lanka and Australia; China is by far the world leader in samarium mining and production.
The main commercial use of samarium is in samarium–cobalt magnets, which have permanent magnetization second only to neodymium magnets; however, samarium compounds can withstand significantly higher temperatures, above , without losing their permanent magnetic properties. The radioisotope samarium-153 is the active component of the drug samarium (153Sm) lexidronam (Quadramet), which kills cancer cells in lung cancer, prostate cancer, breast cancer and osteosarcoma. Another isotope, samarium-149, is a strong neutron absorber and so is added to control rods of nuclear reactors. It also forms as a decay product during reactor operation and is one of the important factors considered in reactor design and operation. Other uses of samarium include catalysis of chemical reactions, radioactive dating and X-ray lasers. Samarium(II) iodide, in particular, is a common reducing agent in chemical synthesis.
Samarium has no biological role; some samarium salts are slightly toxic.
Physical properties Samarium is a rare earth element with a hardness and density similar to zinc. With a boiling point of , samarium is the third most volatile lanthanide after ytterbium and europium and comparable in this respect to lead and barium; this helps separation of samarium from its ores. When freshly prepared, samarium has a silvery lustre, and takes on a duller appearance when oxidized in air. Samarium is calculated to have one of the largest atomic radii of the elements; with a radius of 238 pm, only potassium, praseodymium, barium, rubidium and caesium are larger.
In ambient conditions, samarium has a rhombohedral structure (α form). Upon heating to , its crystal symmetry changes to hexagonal close-packed (hcp), though its actual transition temperature depends on the metal purity. Further heating to transforms the metal into a body-centered cubic (bcc) phase. Heating to plus compression to 40 kbar results in a double-hexagonally close-packed structure (dhcp). Higher pressure of the order of hundreds or thousands of kilobars induces a series of phase transformations, in particular with a tetragonal phase appearing at about 900 kbar. In one study, the dhcp phase could be produced without compression, using a nonequilibrium annealing regime with a rapid temperature change between about and , confirming the transient character of this samarium phase. Thin films of samarium obtained by vapor deposition may contain the hcp or dhcp phases in ambient conditions.
Samarium and its sesquioxide are paramagnetic at room temperature. Their corresponding effective magnetic moments, below 2 bohr magnetons, are the third-lowest among lanthanides (and their oxides) after lanthanum and lutetium. The metal transforms to an antiferromagnetic state upon cooling to 14.8 K. Individual samarium atoms can be isolated by encapsulating them into fullerene molecules. They can also be intercalated into the interstices of the bulk C60 to form a solid solution of nominal composition Sm3C60, which is superconductive at a temperature of 8 K. Samarium doping of iron-based superconductors – a class of high-temperature superconductor – increases their transition to normal conductivity temperature up to 56 K, the highest value achieved so far in this series.
Chemical properties
In air, samarium slowly oxidizes at room temperature and spontaneously ignites at . Even when stored under mineral oil, samarium gradually oxidizes and develops a grayish-yellow powder of the oxide-hydroxide mixture at the surface. 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 rapidly with hot water to form samarium hydroxide:
Samarium dissolves readily in dilute sulfuric acid to form solutions containing the yellowGreenwood, p. 1243 to pale green Sm(III) ions, which exist as complexes:
Samarium is one of the few lanthanides with a relatively accessible +2 oxidation state, alongside Eu and Yb. ions are blood-red in aqueous solution.Greenwood, p. 1248
Compounds
Formula Color Symmetry Space group No. Pearson symbol a (pm) b (pm) c (pm) Z Density, g/cm3 Sm silvery trigonal Rm 166 hR9 362.9 362.9 2621.3 9 7.52 Sm silvery hexagonal P63/mmc 194 hP4 362 362 1168 4 7.54 Sm silvery tetragonal I4/mmm 139 tI2 240.2 240.2 423.1 2 20.46 SmO golden cubic Fmm 225 cF8 494.3 494.3 494.3 4 9.15 Sm2O3 trigonal Pm1 164 hP5 377.8 377.8 594 1 7.89 Sm2O3 monoclinic C2/m 12 mS30 1418 362.4 885.5 6 7.76 Sm2O3 cubic Ia 206 cI80 1093 1093 1093 16 7.1 SmH2 cubic Fmm 225 cF12 537.73 537.73 537.73 4 6.51 SmH3 hexagonal Pc1 165 hP24 377.1 377.1 667.2 6 Sm2B5 gray monoclinic P21/c 14 mP28 717.9 718 720.5 4 6.49 SmB2 hexagonal P6/mmm 191 hP3 331 331 401.9 1 7.49 SmB4 tetragonal P4/mbm 127 tP20 717.9 717.9 406.7 4 6.14 SmB6 cubic Pmm 221 cP7 413.4 413.4 413.4 1 5.06 SmB66 cubic Fmc 226 cF1936 2348.7 2348.7 2348.7 24 2.66 Sm2C3 cubic I3d 220 cI40 839.89 839.89 839.89 8 7.55 SmC2 tetragonal I4/mmm 139 tI6 377 377 633.1 2 6.44 SmF2 purple cubic Fmm 225 cF12 587.1 587.1 587.1 4 6.18 SmF3 white orthorhombic Pnma 62 oP16 667.22 705.85 440.43 4 6.64 SmCl2 brown orthorhombic Pnma 62 oP12 756.28 450.77 901.09 4 4.79 SmCl3 yellow hexagonal P63/m 176 hP8 737.33 737.33 416.84 2 4.35 SmBr2 brown orthorhombic Pnma 62 oP12 797.7 475.4 950.6 4 5.72 SmBr3 yellow orthorhombic Cmcm 63 oS16 404 1265 908 2 5.58 SmI2 greenGreenwood, p. 1241 monoclinic P21/c 14 mP12 SmI3 orange trigonal R 63 hR24 749 749 2080 6 5.24 SmN cubic Fmm 225 cF8 357 357 357 4 8.48 SmP cubic Fmm 225 cF8 576 576 576 4 6.3 SmAs cubic Fmm 225 cF8 591.5 591.5 591.5 4 7.23
Oxides The most stable oxide of samarium is the sesquioxide Sm2O3. Like many samarium compounds, it exists in several crystalline phases. The trigonal form is obtained by slow cooling from the melt. The melting point of Sm2O3 is high (2345 °C), so it is usually melted not by direct heating, but with induction heating, through a radio-frequency coil. Sm2O3 crystals of monoclinic symmetry can be grown by the flame fusion method (Verneuil process) from Sm2O3 powder, that yields cylindrical boules up to several centimeters long and about one centimeter in diameter. The boules are transparent when pure and defect-free and are orange otherwise. Heating the metastable trigonal Sm2O3 to converts it to the more stable monoclinic phase. Cubic Sm2O3 has also been described.
Samarium is one of the few lanthanides that form a monoxide, SmO. This lustrous golden-yellow compound was obtained by reducing Sm2O3 with samarium metal at high temperature (1000 °C) and a pressure above 50 kbar; lowering the pressure resulted in incomplete reaction. SmO has cubic rock-salt lattice structure.Greenwood, p. 1239
Chalcogenides
Samarium forms a trivalent sulfide, selenide and telluride. Divalent chalcogenides SmS, SmSe and SmTe with a cubic rock-salt crystal structure are known. These chalcogenides convert from a semiconducting to metallic state at room temperature upon application of pressure. Whereas the transition is continuous and occurs at about 20–30 kbar in SmSe and SmTe, it is abrupt in SmS and requires only 6.5 kbar. This effect results in a spectacular color change in SmS from black to golden yellow when its crystals of films are scratched or polished. The transition does not change the lattice symmetry, but there is a sharp decrease (~15%) in the crystal volume. It exhibits hysteresis, i.e., when the pressure is released, SmS returns to the semiconducting state at a much lower pressure of about 0.4 kbar.
Halides
Samarium metal reacts with all the halogens, forming trihalides:Greenwood, pp. 1236, 1241 2 Sm (s) + 3 X2 (g) → 2 SmX3 (s) (X = F, Cl, Br or I)
Their further reduction with samarium, lithium or sodium metals at elevated temperatures (about 700–900 °C) yields the dihalides. The diiodide can also be prepared by heating SmI3, or by reacting the metal with 1,2-diiodoethane in anhydrous tetrahydrofuran at room temperature:Greenwood, p. 1240 Sm (s) + ICH2-CH2I → SmI2 + CH2=CH2.
In addition to dihalides, the reduction also produces many non-stoichiometric samarium halides with a well-defined crystal structure, such as Sm3F7, Sm14F33, Sm27F64, Sm11Br24, Sm5Br11 and Sm6Br13.
Samarium halides change their crystal structures when one type of halide anion is substituted for another, which is an uncommon behavior for most elements (e.g. actinides). Many halides have two major crystal phases for one composition, one being significantly more stable and another being metastable. The latter is formed upon compression or heating, followed by quenching to ambient conditions. For example, compressing the usual monoclinic samarium diiodide and releasing the pressure results in a PbCl2-type orthorhombic structure (density 5.90 g/cm3), and similar treatment results in a new phase of samarium triiodide (density 5.97 g/cm3).
Borides Sintering powders of samarium oxide and boron, in a vacuum, yields a powder containing several samarium boride phases; the ratio between these phases can be controlled through the mixing proportion. The powder can be converted into larger crystals of samarium borides using arc melting or zone melting techniques, relying on the different melting/crystallization temperature of SmB6 (2580 °C), SmB4 (about 2300 °C) and SmB66 (2150 °C). All these materials are hard, brittle, dark-gray solids with the hardness increasing with the boron content. Samarium diboride is too volatile to be produced with these methods and requires high pressure (about 65 kbar) and low temperatures between 1140 and 1240 °C to stabilize its growth. Increasing the temperature results in the preferential formation of SmB6.
Samarium hexaboride
Samarium hexaboride is a typical intermediate-valence compound where samarium is present both as Sm2+ and Sm3+ ions in a 3:7 ratio. It belongs to a class of Kondo insulators; at temperatures above 50 K, its properties are typical of a Kondo metal, with metallic electrical conductivity characterized by strong electron scattering, whereas at lower temperatures, it behaves as a non-magnetic insulator with a narrow band gap of about 4–14 meV. The cooling-induced metal-insulator transition in SmB6 is accompanied by a sharp increase in the thermal conductivity, peaking at about 15 K. The reason for this increase is that electrons themselves do not contribute to the thermal conductivity at low temperatures, which is dominated by phonons, but the decrease in electron concentration reduces the rate of electron-phonon scattering.
Other inorganic compounds