Samarium
Samarium | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Pronunciation | /səˈmɛəriəm/ | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Appearance | silvery white | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Standard atomic weight Ar°(Sm) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Samarium in the periodic table | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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kJ/mol | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Heat of vaporization | 192 kJ/mol | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Molar heat capacity | 29.54 J/(mol·K) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Vapor pressure
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Atomic properties | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Discovery and first isolation | Lecoq de Boisbaudran (1879) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Isotopes of samarium | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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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
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 has no biological role; some samarium salts are slightly toxic.[10]
Physical properties
Samarium is a
In ambient conditions, samarium has a
Samarium and its
Chemical properties
In air, samarium slowly oxidizes at room temperature and spontaneously ignites at 150 °C (302 °F).[10][12] 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:[20]
- 2Sm(s) + 6H2O(l) → 2Sm(OH)3(aq) + 3H2(g)
Samarium dissolves readily in dilute sulfuric acid to form solutions containing the yellow[21] to pale green Sm(III) ions, which exist as [Sm(OH2)9]3+ complexes:[20]
- 2Sm(s) + 3H2SO4(aq) → 2Sm3+(aq) + 3SO2−4(aq) + 3H2(g)
Samarium is one of the few lanthanides with a relatively accessible +2 oxidation state, alongside Eu and Yb.[22] Sm2+ ions are blood-red in aqueous solution.[23]
Compounds
Formula | color | symmetry | space group | No | Pearson symbol | a (pm) | b (pm) | c (pm) | Z | density, g/cm3 |
---|---|---|---|---|---|---|---|---|---|---|
Sm | silvery | trigonal[14] | R3m | 166 | hR9 | 362.9 | 362.9 | 2621.3 | 9 | 7.52 |
Sm | silvery | hexagonal[14] | P63/mmc | 194 | hP4 | 362 | 362 | 1168 | 4 | 7.54 |
Sm | silvery | tetragonal[24] | I4/mmm | 139 | tI2 | 240.2 | 240.2 | 423.1 | 2 | 20.46 |
SmO | golden | cubic[25] | Fm3m | 225 | cF8 | 494.3 | 494.3 | 494.3 | 4 | 9.15 |
Sm2O3 | trigonal[26] | P3m1 | 164 | hP5 | 377.8 | 377.8 | 594 | 1 | 7.89 | |
Sm2O3 | monoclinic[26] | C2/m | 12 | mS30 | 1418 | 362.4 | 885.5 | 6 | 7.76 | |
Sm2O3 | cubic[27] | Ia3 | 206 | cI80 | 1093 | 1093 | 1093 | 16 | 7.1 | |
SmH2 | cubic[28] | Fm3m | 225 | cF12 | 537.73 | 537.73 | 537.73 | 4 | 6.51 | |
SmH3 | hexagonal[29] | P3c1 | 165 | hP24 | 377.1 | 377.1 | 667.2 | 6 | ||
Sm2B5 | gray | monoclinic[30] | P21/c | 14 | mP28 | 717.9 | 718 | 720.5 | 4 | 6.49 |
SmB2 | hexagonal[31] | P6/mmm | 191 | hP3 | 331 | 331 | 401.9 | 1 | 7.49 | |
SmB4 | tetragonal[32] | P4/mbm | 127 | tP20 | 717.9 | 717.9 | 406.7 | 4 | 6.14 | |
SmB6 | cubic[33] | Pm3m | 221 | cP7 | 413.4 | 413.4 | 413.4 | 1 | 5.06 | |
SmB66 | cubic[34] | Fm3c | 226 | cF1936 | 2348.7 | 2348.7 | 2348.7 | 24 | 2.66 | |
Sm2C3 | cubic[35] | I43d | 220 | cI40 | 839.89 | 839.89 | 839.89 | 8 | 7.55 | |
SmC2 | tetragonal[35] | I4/mmm | 139 | tI6 | 377 | 377 | 633.1 | 2 | 6.44 | |
SmF2 | purple[36] | cubic[37] | Fm3m | 225 | cF12 | 587.1 | 587.1 | 587.1 | 4 | 6.18 |
SmF3 | white[36] | orthorhombic[37] | Pnma | 62 | oP16 | 667.22 | 705.85 | 440.43 | 4 | 6.64 |
SmCl2 | brown[36] | orthorhombic[38] | Pnma | 62 | oP12 | 756.28 | 450.77 | 901.09 | 4 | 4.79 |
SmCl3 | yellow[36] | hexagonal[37] | P63/m | 176 | hP8 | 737.33 | 737.33 | 416.84 | 2 | 4.35 |
SmBr2 | brown[36] | orthorhombic[39] | Pnma | 62 | oP12 | 797.7 | 475.4 | 950.6 | 4 | 5.72 |
SmBr3 | yellow[36] | orthorhombic[40] | Cmcm | 63 | oS16 | 404 | 1265 | 908 | 2 | 5.58 |
SmI2 | green[36] | monoclinic | P21/c | 14 | mP12 | |||||
SmI3 | orange[36] | trigonal[41] | R3 | 63 | hR24 | 749 | 749 | 2080 | 6 | 5.24 |
SmN | cubic[42] | Fm3m | 225 | cF8 | 357 | 357 | 357 | 4 | 8.48 | |
SmP | cubic[43] | Fm3m | 225 | cF8 | 576 | 576 | 576 | 4 | 6.3 | |
SmAs | cubic[44] | Fm3m | 225 | cF8 | 591.5 | 591.5 | 591.5 | 4 | 7.23 |
Oxides
The most stable oxide of samarium is the
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.[25][45]
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.[46] 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.[47] 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.[10][48]
Halides
Samarium metal reacts with all the halogens, forming trihalides:[49]
- 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.[38] The diiodide can also be prepared by heating SmI3, or by reacting the metal with 1,2-diiodoethane in anhydrous tetrahydrofuran at room temperature:[50]
- 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,[37] Sm11Br24, Sm5Br11 and Sm6Br13.[51]
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),[52] and similar treatment results in a new phase of samarium triiodide (density 5.97 g/cm3).[53]
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.[54] 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.[33] 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.[31]
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.
Other inorganic compounds
Samarium
Numerous crystalline binary compounds are known for samarium and one of the group 14, 15, or 16 elements X, where X is Si, Ge, Sn, Pb, Sb or Te, and metallic alloys of samarium form another large group. They are all prepared by annealing mixed powders of the corresponding elements. Many of the resulting compounds are non-stoichiometric and have nominal compositions SmaXb, where the b/a ratio varies between 0.5 and 3.[57][58]
Organometallic compounds
Samarium forms a cyclopentadienide Sm(C5H5)3 and its chloroderivatives Sm(C5H5)2Cl and Sm(C5H5)Cl2. They are prepared by reacting samarium trichloride with NaC5H5 in tetrahydrofuran. Contrary to cyclopentadienides of most other lanthanides, in Sm(C5H5)3 some C5H5 rings bridge each other by forming ring vertexes η1 or edges η2 toward another neighboring samarium, thus creating polymeric chains.[23] The chloroderivative Sm(C5H5)2Cl has a dimer structure, which is more accurately expressed as (η(5)−C5H5)2Sm(−Cl)2(η(5)−C5H5)2. There, the chlorine bridges can be replaced, for instance, by iodine, hydrogen or nitrogen atoms or by CN groups.[59]
The (C5H5)− ion in samarium cyclopentadienides can be replaced by the indenide (C9H7)− or cyclooctatetraenide (C8H8)2− ring, resulting in Sm(C9H7)3 or KSm(η(8)−C8H8)2. The latter compound has a structure similar to uranocene. There is also a cyclopentadienide of divalent samarium, Sm(C5H5)2 a solid that sublimates at about 85 °C (185 °F). Contrary to ferrocene, the C5H5 rings in Sm(C5H5)2 are not parallel but are tilted by 40°.[59][60]
A
- SmCl3 + 3LiR → SmR3 + 3LiCl
- Sm(OR)3 + 3LiCH(SiMe3)2 → Sm{CH(SiMe3)2}3 + 3LiOR
Here R is a hydrocarbon group and Me =
Isotopes
Naturally occurring samarium is composed of five stable
Samarium-149 is an observationally stable isotope of samarium (predicted to decay, but no decays have ever been observed, giving it a half-life at least several orders of magnitude longer than the age of the universe), and a product of the decay chain from the
Samarium-153 is a beta emitter with a half-life of 46.3 hours. It is used to kill cancer cells in lung cancer, prostate cancer, breast cancer, and osteosarcoma. For this purpose, samarium-153 is chelated with ethylene diamine tetramethylene phosphonate (EDTMP) and injected intravenously. The chelation prevents accumulation of radioactive samarium in the body that would result in excessive irradiation and generation of new cancer cells.[10] The corresponding drug has several names including samarium (153Sm) lexidronam; its trade name is Quadramet.[75][76][77]
History
Detection of samarium and related elements was announced by several scientists in the second half of the 19th century; however, most sources give priority to
Boisbaudran named his element samarium after the mineral samarskite, which in turn honored
Before the advent of
Occurrence and production
Samarium concentration in soils varies between 2 and 23 ppm, and oceans contain about 0.5–0.8 parts per trillion.[10] The median value for its abundance in the Earth's crust used by the CRC Handbook is 7 parts per million (ppm).[96] Distribution of samarium in soils strongly depends on its chemical state and is very inhomogeneous: in sandy soils, samarium concentration is about 200 times higher at the surface of soil particles than in the water trapped between them, and this ratio can exceed 1,000 in clays.[97]
Samarium is not found free in nature, but, like other rare earth elements, is contained in many minerals, including
Very few minerals have samarium being the most dominant element. Minerals with essential (dominant) samarium include monazite-(Sm) and florencite-(Sm). These minerals are very rare and are usually found containing other elements, usually cerium or neodymium.[99][100][101][102] Samarium-151 is produced in nuclear fission of uranium with a yield of about 0.4% of all fissions. It is also made by neutron capture by samarium-149, which is added to the control rods of nuclear reactors. Therefore, 151Sm is present in spent nuclear fuel and radioactive waste.[97]
Applications
Magnets
An important use of samarium is
Chemical reagent
Samarium and its compounds are important as catalysts and
In its usual oxidized form, samarium is added to ceramics and glasses where it increases absorption of infrared light. As a (minor) part of mischmetal, samarium is found in the "flint" ignition devices of many lighters and torches.[10][12]
Neutron absorber
Samarium-149 has a high
Lasers
Samarium-doped calcium fluoride crystals were used as an active medium in one of the first solid-state lasers designed and built by Peter Sorokin (co-inventor of the dye laser) and Mirek Stevenson at IBM research labs in early 1961. This samarium laser gave pulses of red light at 708.5 nm. It had to be cooled by liquid helium and so did not find practical applications.[106][107] Another samarium-based laser became the first saturated X-ray laser operating at wavelengths shorter than 10 nanometers. It gave 50-picosecond pulses at 7.3 and 6.8 nm suitable for uses in holography, high-resolution microscopy of biological specimens, deflectometry, interferometry, and radiography of dense plasmas related to confinement fusion and astrophysics. Saturated operation meant that the maximum possible power was extracted from the lasing medium, resulting in the high peak energy of 0.3 mJ. The active medium was samarium plasma produced by irradiating samarium-coated glass with a pulsed infrared Nd-glass laser (wavelength ~1.05 μm).[108]
Storage phosphor
In 2007 it was shown that nanocrystalline BaFCl:Sm3+ as prepared by co-precipitation can serve as a very efficient X-ray storage phosphor.[109] The co-precipitation leads to nanocrystallites of the order of 100–200 nm in size and their sensitivity as X-ray storage phosphors is increased a remarkable ~500,000 times because of the specific arrangements and density of defect centers in comparison with microcrystalline samples prepared by sintering at high temperature.[110] The mechanism is based on reduction of Sm3+ to Sm2+ by trapping electrons that are created upon exposure to ionizing radiation in the BaFCl host. The 5DJ–7FJ f–f luminescence lines can be very efficiently excited via the parity allowed 4f6→4f55d transition at ~417 nm. The latter wavelength is ideal for efficient excitation by blue-violet laser diodes as the transition is electric dipole allowed and thus relatively intense (400 L/(mol⋅cm)).[111] The phosphor has potential applications in personal dosimetry, dosimetry and imaging in radiotherapy, and medical imaging.[112]
Non-commercial and potential uses
- The change in electrical resistivity in samarium monochalcogenides can be used in a pressure sensor or in a memory device triggered between a low-resistance and high-resistance state by external pressure,[113] and such devices are being developed commercially.[114] Samarium monosulfide also generates electric voltage upon moderate heating to about 150 °C (302 °F) that can be applied in thermoelectric power converters.[115]
- Analysis of relative concentrations of samarium and neodymium isotopes 147Sm, 144Nd, and 143Nd allows determination of the age and origin of rocks and meteorites in samarium–neodymium dating. Both elements are lanthanides and are very similar physically and chemically. Thus, Sm–Nd dating is either insensitive to partitioning of the marker elements during various geologic processes, or such partitioning can well be understood and modeled from the ionic radii of said elements.[116]
- The Sm3+ ion is a potential activator for use in warm-white light emitting diodes. It offers high luminous efficacy due to narrow emission bands; but the generally low quantum efficiency and too little absorption in the UV-A to blue spectral region hinders commercial application.[117]
- Samarium is used for ionosphere testing. A rocket spreads samarium monoxide as a red vapor at high altitude, and researchers test how the atmosphere disperses it and how it impacts radio transmissions.[118][119]
- Samarium hexaboride, SmB6, has recently been shown to be a topological insulator with potential uses in quantum computing.[120][121][122][123]
Biological role and precautions
Hazards[124] | |
---|---|
GHS labelling: | |
Warning | |
H261 | |
P231+P232, P280, P370+P378, P501 | |
NFPA 704 (fire diamond) |
Samarium salts stimulate metabolism, but it is unclear whether this is from samarium or other lanthanides present with it. The total amount of samarium in adults is about 50 μg, mostly in liver and kidneys and with ~8 μg/L being dissolved in blood. Samarium is not absorbed by plants to a measurable concentration and so is normally not part of human diet. However, a few plants and vegetables may contain up to 1 part per million of samarium. Insoluble salts of samarium are non-toxic and the soluble ones are only slightly toxic.[10][126] When ingested, only 0.05% of samarium salts are absorbed into the bloodstream and the remainder are excreted. From the blood, 45% goes to the liver and 45% is deposited on the surface of the bones where it remains for 10 years; the remaining 10% is excreted.[97]
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External links