Discovered in 1879 by French chemistPaul-Émile Lecoq de Boisbaudran, samarium was named after the mineralsamarskite from which it was isolated. The mineral itself was named after a Russian mine official, ColonelVassili 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 includingcerite,gadolinite, samarskite,monazite andbastnäsite, 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.
Samarium is arare earth element with a hardness and density similar tozinc. With a boiling point of 1,794 °C (3,261 °F), samarium is the third mostvolatile lanthanide afterytterbium andeuropium and comparable in this respect tolead andbarium; this helps separation of samarium from its ores.[12][13] When freshly prepared, samarium has a silverylustre, and takes on a duller appearance when oxidized in air. Samarium is calculated to have one of the largestatomic radii of the elements; with a radius of 238 pm, onlypotassium,praseodymium,barium,rubidium andcaesium are larger.[14]
In ambient conditions, samarium has arhombohedral structure (α form). Upon heating to 731 °C (1,348 °F), its crystal symmetry changes tohexagonal close-packed (hcp),; it has actual transition temperature depending on metal purity. Further heating to 922 °C (1,692 °F) transforms the metal into abody-centered cubic (bcc) phase. Heating to 300 °C (572 °F) 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 atetragonal phase appearing at about 900 kbar.[15] In one study, thedhcp phase could be produced without compression, using a nonequilibrium annealing regime with a rapid temperature change between about 400 °C (752 °F) and 700 °C (1,292 °F), confirming the transient character of this samarium phase. Thin films of samarium obtained by vapor deposition may contain thehcp ordhcp phases in ambient conditions.[15]
Samarium and itssesquioxide areparamagnetic at room temperature. Their corresponding effective magnetic moments, below 2bohr magnetons, are the third-lowest among lanthanides (and their oxides) after lanthanum and lutetium. The metal transforms to anantiferromagnetic state upon cooling to 14.8 K.[16][17] Individual samarium atoms can be isolated by encapsulating them intofullerene molecules.[18] They can also be intercalated into the interstices of the bulk C60 to form a solid solution of nominal composition Sm3C60, which issuperconductive at a temperature of 8 K.[19] Samarium doping ofiron-based superconductors – a class ofhigh-temperature superconductor – increases their transition to normal conductivity temperature up to 56 K, the highest value achieved so far in this series.[20]
In air, samarium slowly oxidizes at room temperature and spontaneously ignites at 150 °C (302 °F).[11][13] Even when stored undermineral oil, samarium gradually oxidizes and develops a grayish-yellow powder of theoxide-hydroxide mixture at the surface. The metallic appearance of a sample can be preserved by sealing it under an inert gas such asargon.
Samarium is quite electropositive and reacts slowly with cold water and rapidly with hot water to form samarium hydroxide:[21]
2Sm(s) + 6H2O(l) → 2Sm(OH)3(aq) + 3H2(g)
Samarium dissolves readily in dilutesulfuric acid to form solutions containing the yellow[22] to pale green Sm(III) ions, which exist as[Sm(OH2)9]3+ complexes:[21]
Samarium is one of the few lanthanides with a relatively accessible +2 oxidation state, alongside Eu and Yb.[23]Sm2+ ions are blood-red in aqueous solution.[24]
The most stable oxide of samarium is thesesquioxide 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 withinduction 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 1,900 °C (3,450 °F) converts it to the more stable monoclinic phase.[27] Cubic Sm2O3 has also been described.[28]
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.[26][46]
Samarium forms a trivalentsulfide,selenide andtelluride. 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.[47] 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.[48] It exhibitshysteresis, i.e., when the pressure is released, SmS returns to the semiconducting state at a much lower pressure of about 0.4 kbar.[11][49]
Samarium metal reacts with all thehalogens, forming trihalides:[50]
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.[39] The diiodide can also be prepared by heating SmI3, or by reacting the metal with1,2-diiodoethane in anhydroustetrahydrofuran at room temperature:[51]
Sm (s) + ICH2-CH2I → SmI2 + CH2=CH2.
In addition to dihalides, the reduction also produces manynon-stoichiometric samarium halides with a well-defined crystal structure, such as Sm3F7, Sm14F33, Sm27F64,[38] Sm11Br24, Sm5Br11 and Sm6Br13.[52]
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),[53] and similar treatment results in a new phase of samarium triiodide (density 5.97 g/cm3).[54]
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.[55] The powder can be converted into larger crystals of samarium borides usingarc melting orzone 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.[34] 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.[32]
Samarium hexaboride is a typical intermediate-valence compound where samarium is present both as Sm2+ and Sm3+ ions in a 3:7 ratio.[55] It belongs to a class ofKondo 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 narrowband gap of about 4–14 meV.[56] The cooling-induced metal-insulator transition in SmB6 is accompanied by a sharp increase in thethermal 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 byphonons, but the decrease in electron concentration reduces the rate of electron-phonon scattering.[57]
Samariumcarbides are prepared by melting a graphite-metal mixture in an inert atmosphere. After the synthesis, they are unstable in air and need to be studied under an inert atmosphere.[36] Samarium monophosphide SmP is asemiconductor with a bandgap of 1.10 eV, the same as insilicon, and electrical conductivity ofn-type. It can be prepared by annealing at 1,100 °C (2,010 °F) an evacuated quartz ampoule containing mixed powders of phosphorus and samarium. Phosphorus is highly volatile at high temperatures and may explode, thus the heating rate has to be kept well below 1 °C/min.[44] A similar procedure is adopted for the monarsenide SmAs, but the synthesis temperature is higher at 1,800 °C (3,270 °F).[45]
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.[58][59]
Samarium forms acyclopentadienideSm(C5H5)3 and its chloroderivativesSm(C5H5)2Cl andSm(C5H5)Cl2. They are prepared by reacting samarium trichloride withNaC5H5 intetrahydrofuran. Contrary to cyclopentadienides of most other lanthanides, inSm(C5H5)3 someC5H5 rings bridge each other by forming ring vertexes η1 or edges η2 toward another neighboring samarium, thus creating polymeric chains.[24] The chloroderivativeSm(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 byCN groups.[60]
The (C5H5)− ion in samarium cyclopentadienides can be replaced by the indenide (C9H7)− orcyclooctatetraenide (C8H8)2− ring, resulting inSm(C9H7)3 orKSm(η(8)−C8H8)2. The latter compound has a structure similar touranocene. There is also a cyclopentadienide of divalent samarium,Sm(C5H5)2 a solid that sublimates at about 85 °C (185 °F). Contrary toferrocene, theC5H5 rings inSm(C5H5)2 are not parallel but are tilted by 40°.[60][61]
Naturally occurring samarium is composed of five stableisotopes:144Sm,149Sm,150Sm,152Sm and154Sm, and two extremely long-livedradioisotopes,147Sm (half-lifet1/2 = 1.06×1011 years) and148Sm (7×1015 years), with152Sm being the most abundant (26.75%).[9]149Sm is listed by various sources as being stable,[9][62] but some sources state that it is radioactive,[63] with a lower bound for its half-life given as2×1015 years.[9] Someobservationally stable samarium isotopes are predicted to decay toisotopes of neodymium.[64] The long-lived isotopes146Sm,147Sm, and148Sm undergoalpha decay toneodymium isotopes. Lighter unstable isotopes of samarium mainly decay byelectron capture topromethium, while heavier onesbeta decay toeuropium.[9] The known isotopes range from129Sm to168Sm.[9][65] The half-lives of151Sm and145Sm are 90 years and 340 days, respectively. All remainingradioisotopes have half-lives that are less than 2 days, and most these have half-life less than 48 seconds. Samarium also has twelve knownnuclear isomers, the most stable of which are141mSm (half-life 22.6 minutes),143m1Sm (t1/2 = 66 seconds), and139mSm (t1/2 = 10.7 seconds).[9] Natural samarium has aradioactivity of 127 Bq/g, mostly due to147Sm,[66] whichalpha decays to143Nd with ahalf-life of 1.06×1011 years and is used insamarium–neodymium dating.[67][68]146Sm is anextinct radionuclide, with the half-life of 9.20×107 years.[10] There have been searches of samarium-146 as aprimordial nuclide, because its half-life is long enough such that minute quantities of the element should persist today.[69] It can be used in radiometric dating.[70]
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 thefission product149Nd (yield 1.0888%).149Sm is a decay product andneutron-absorber innuclear reactors, with aneutron poison effect that is second in importance for reactor design and operation only to135Xe.[71][72] Itsneutron cross section is 41000barns forthermal neutrons.[73] Because samarium-149 is not radioactive and is not removed by decay, it presents problems somewhat different from those encountered with xenon-135. The equilibrium concentration (and thus the poisoning effect) builds to an equilibrium value during reactor operations in about 500 hours (about three weeks), and since samarium-149 is stable, its concentration remains essentially constant during reactor operation.[74]
Samarium-153 is a beta emitter with a half-life of 46.3 hours. It is used to kill cancer cells inlung cancer,prostate cancer,breast cancer, andosteosarcoma. For this purpose, samarium-153 ischelated 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.[11] The corresponding drug has several names includingsamarium (153Sm) lexidronam; itstrade name is Quadramet.[75][76][77]
Detection of samarium and related elements was announced by several scientists in the second half of the 19th century; however, most sources give priority toFrench chemistPaul-Émile Lecoq de Boisbaudran.[78][79] Boisbaudran isolated samarium oxide and/or hydroxide inParis in 1879 from the mineralsamarskite((Y,Ce,U,Fe)3(Nb,Ta,Ti)5O16) and identified a new element in it via sharp optical absorption lines.[13] Swiss chemistMarc Delafontaine announced a new elementdecipium (fromLatin:decipiens meaning "deceptive, misleading") in 1878,[80][81] but later in 1880–1881 demonstrated that it was a mix of several elements, one being identical to Boisbaudran's samarium.[82][83] Though samarskite was first found in theUral Mountains inRussia, by the late 1870s it had been found in other places, making it available to many researchers. In particular, it was found that the samarium isolated by Boisbaudran was also impure and had a comparable amount ofeuropium. The pure samarium(III) oxide was produced only in 1901 byEugène-Anatole Demarçay,[84][85][86] and in 1903 Wilhelm Muthmann isolated the element.
Boisbaudran named his elementsamarium after the mineral samarskite, which in turn honoredVassili Samarsky-Bykhovets (1803–1870). Samarsky-Bykhovets, as the Chief of Staff of theRussian Corps of Mining Engineers, had granted access for two German mineralogists, the brothersGustav andHeinrich Rose, to study the mineral samples from the Urals.[87][88][89] Samarium was thus the first chemical element to be named after a person.[84][90] The wordsamaria is sometimes used to mean samarium(III) oxide, by analogy withyttria,zirconia,alumina,ceria,holmia, etc. The symbolSm was suggested for samarium, but an alternativeSa was often used instead until the 1920s.[84][91]
Before the advent ofion-exchange separation technology in the 1950s, pure samarium had no commercial uses. However, a by-product of fractional crystallization purification of neodymium was a mix of samarium and gadolinium that got the name "Lindsay Mix" after the company that made it, and was used for nuclearcontrol rods in some early nuclear reactors.[92] Nowadays, a similar commodity product has the name "samarium-europium-gadolinium" (SEG) concentrate.[90] It is prepared by solvent extraction from the mixedlanthanides isolated from bastnäsite (or monazite). Since heavier lanthanides have more 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 a large enough scale to continue by separating the components of SEG, which typically makes up only 1–2% of the original ore. Such producers therefore make SEG with a view to marketing it to the specialized processors. In this manner, the valuable europium in the ore is rescued for use in makingphosphor. Samarium purification follows the removal of the europium. As of 2012[update], being in oversupply, samarium oxide is cheaper on a commercial scale than its relative abundance in the ore might suggest.[93]
Samarium concentration in soils varies between 2 and 23 ppm, and oceans contain about 0.5–0.8 parts per trillion.[11] The median value for itsabundance in the Earth's crust used by the CRC Handbook is 7 parts per million (ppm)[94] and is the 40th most abundant element.[95] 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.[96]
Samarium is not found free in nature, but, like other rare earth elements, is contained in many minerals, includingmonazite,bastnäsite,cerite,gadolinite andsamarskite; monazite (in which samarium occurs at concentrations of up to 2.8%)[13] and bastnäsite are mostly used as commercial sources. World resources of samarium are estimated at two milliontonnes; they are mostly located in China, US, Brazil, India, Sri Lanka and Australia, and the annual production is about 700 tonnes.[11] Country production reports are usually given for all rare-earth metals combined. By far, China has the largest production with 120,000 tonnes mined per year; it is followed by the US (about 5,000 tonnes)[96] and India (2,700 tonnes).[97] Samarium is usually sold as oxide, which at the price of about US$30/kg is one of the cheapest lanthanide oxides.[93] Whereasmischmetal – a mixture of rare earth metals containing about 1% of samarium – has long been used, relatively pure samarium has been isolated only recently, throughion exchange processes,solvent extraction techniques, andelectrochemical deposition. The metal is often prepared by electrolysis of a molten mixture ofsamarium(III) chloride withsodium chloride orcalcium chloride. Samarium can also be obtained by reducing its oxide withlanthanum. The product is then distilled to separate samarium (boiling point 1794 °C) and lanthanum (b.p. 3464 °C).[79]
Very few minerals have samarium being the most dominant element. Minerals with essential (dominant) samarium includemonazite-(Sm) andflorencite-(Sm). These minerals are very rare and are usually found containing other elements, usuallycerium orneodymium.[98][99][100][101] It is also made byneutron capture by samarium-149, which is added to thecontrol rods of nuclear reactors. Therefore,151Sm is present in spentnuclear fuel and radioactive waste.[96]
An important use of samarium issamarium–cobalt magnets, which are nominallySmCo5 orSm2Co17.[102] They have high permanent magnetization, about 10,000 times that of iron and second only toneodymium magnets. However, samarium magnets resist demagnetization better; they are stable to temperatures above 700 °C (1,292 °F) (cf. 300–400 °C for neodymium magnets). These magnets are found in small motors, headphones, and high-end magneticpickups for guitars and related musical instruments.[11] For example, they are used in the motors of asolar-poweredelectric aircraft, theSolar Challenger, and in theSamarium Cobalt Noiseless electric guitar and bass pickups.
In its usual oxidized form, samarium is added to ceramics and glasses where it increases absorption of infrared light. As a (minor) part ofmischmetal, samarium is found in the "flint" ignition devices of manylighters and torches.[11][13]
Samarium-149 has a highcross section for neutron capture (41,000 barns) and so is used in control rods ofnuclear reactors. Its advantage compared to competing materials, such as boron and cadmium, is stability of absorption – most of the fusion products of149Sm are other isotopes of samarium that are also goodneutron absorbers. For example, the cross section of samarium-151 is 15,000 barns, it is on the order of hundreds of barns for150Sm,152Sm, and153Sm, and 6,800 barns for natural (mixed-isotope) samarium.[13][96][105]
Samarium-dopedcalcium fluoride crystals were used as an active medium in one of the firstsolid-state lasers designed and built byPeter Sorokin (co-inventor of thedye laser) and Mirek Stevenson atIBM 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 saturatedX-ray laser operating at wavelengths shorter than 10 nanometers. It gave 50-picosecond pulses at 7.3 and 6.8 nm suitable for uses inholography, high-resolutionmicroscopy of biological specimens,deflectometry,interferometry, andradiography of dense plasmas related to confinement fusion andastrophysics. 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 infraredNd-glass laser (wavelength ~1.05 μm).[108]
In 2007 it was shown that nanocrystalline BaFCl:Sm3+ as prepared by co-precipitation can serve as a very efficient X-raystorage 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. The5DJ–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]
The change in electrical resistivity insamarium 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 inthermoelectric power converters.[115]
Analysis of relative concentrations of samarium and neodymium isotopes147Sm,144Nd, and143Nd allows determination of the age and origin of rocks and meteorites insamarium–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 theionic radii of said elements.[116]
The Sm3+ ion is a potentialactivator for use in warm-white light emitting diodes. It offers highluminous efficacy due to narrow emission bands; but the generally lowquantum efficiency and too little absorption in theUV-A to blue spectral region hinders commercial application.[117]
Samarium is used forionosphere 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 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.[11][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.[96]
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