Samarium

Samarium — Material Page

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Samarium (Sm, atomic number 62) is a moderately hard, silvery-white lanthanide with a rhombohedral (9R) crystal structure unique among the lanthanides, a melting point of 1,072 °C, and a 4f⁶6s² configuration that supports both +2 and +3 oxidation states. It has seven natural isotopes spanning ¹⁴⁴Sm–¹⁵⁴Sm; ¹⁴⁷Sm is radioactive (alpha, t½ = 1.06 × 10¹¹ yr) and is the basis of the Sm-Nd geochronological system. ¹⁴⁹Sm has the largest thermal neutron absorption cross-section of any stable nuclide in the relevant mass range (σ ≈ 41,000 barn) and is the dominant reactor poison in uranium-fuelled fission reactors after xenon. Sm is extracted from bastnäsite, monazite, and ion-adsorption clays, with China the dominant producer.

SmCo₅ and Sm₂Co₁₇ permanent magnets are the preferred choice where NdFeB is unsuitable: their Curie temperatures (720 °C for SmCo₅, 820 °C for Sm₂Co₁₇) and superior coercivity retention at elevated temperature make them the standard magnet in aerospace sensors, military guidance systems, turbomachinery, and satellite reaction wheels, where NdFeB's performance degradation above 80–150 °C is unacceptable. SmCo magnets also have better corrosion resistance than NdFeB and do not require protective coatings in most environments. Sm₂Co₁₇-based magnets achieve energy products up to ~34 MGOe — lower than the best NdFeB but competitive where thermal stability is the design constraint.

SmI₂ (samarium diiodide, Kagan's reagent) is one of the most widely used single-electron reductants in organic synthesis — its mild reduction potential (E° ≈ −1.33 V vs. NHE in THF/HMPA) enables chemoselective ketone and aldehyde reductions, ketyl radical cyclisations, pinacol couplings, and Barbier-type reactions that are inaccessible with conventional reagents. Sm₂O₃ is used in optical glass to absorb IR radiation (the Sm³⁺ ⁶H → ⁶F manifold transitions) and as a neutron-absorbing additive in nuclear fuel rod matrix materials and speciality ceramics.

General Properties

Property	Value	Notes
Atomic Number	62	4f⁶6s²; +3 dominant; +2 accessible in solution (SmI₂ as reductant, E° ≈ −1.33 V in THF) and in some solid compounds (SmO, SmS). Unlike most lanthanides, Sm²⁺ is a practical synthetic handle. ¹⁴⁹Sm (I = 7/2) is NMR-active.
Atomic Mass	150.36 u	Seven natural isotopes: ¹⁴⁴Sm (3.07%, stable), ¹⁴⁷Sm (14.99%, radioactive), ¹⁴⁸Sm (11.24%, Stable), ¹⁴⁹Sm (13.82%, Stable), ¹⁵⁰Sm (7.38%, stable), ¹⁵²Sm (26.75%, stable), ¹⁵⁴Sm (22.75%, stable). ¹⁴⁷Sm→¹⁴³Nd decay underpins Sm-Nd geochronology.
Density (20 °C)	7.52 g/cm³	Higher than the light lanthanides (La: 6.15, Nd: 7.01 g/cm³), reflecting progressive lanthanide contraction. Relevant to SmCo magnet rotor mass calculations.
Melting Point	1,072 °C (1,345 K)	Moderate for a mid-lanthanide. Sm has an anomalously low boiling point (1,794 °C) relative to its melting point — the smallest liquid range of any lanthanide — which complicates arc-melting of SmCo alloys and requires controlled-atmosphere induction melting.
Boiling Point	1,794 °C	Unusually low boiling point; Sm evaporates readily during SmCo alloy melting and sintering, requiring Sm-rich compositions and Ar-atmosphere processing to compensate for volatility losses.
Thermal Conductivity	13 W/m·K	Low conductivity typical of lanthanides. Relevant to thermal modelling of SmCo magnets in high-temperature aerospace and turbomachinery environments.
Electrical Resistivity	100 nΩ·m (20 °C)	High resistivity. SmS undergoes a pressure-induced semiconductor-to-metal transition (black-to-golden SmS) — a rare example of valence transition in a binary compound, studied in condensed matter physics.
Crystal Structure	α-Sm: rhombohedral (9R), a = 3.629 Å, c = 26.207 Å	The 9R structure (ABABCBCAC stacking, 9 layers per repeat) is unique to Sm among the elements. Transforms to HCP above ~734 °C and BCC above ~922 °C.

Mechanical Properties

Property	Value	Notes
Tensile Strength	~220 MPa	Moderate strength for a mid-lanthanide. Sm is not used structurally; mechanical properties are relevant to SmCo magnet sintering and to Sm foil and rod fabrication for alloy precursors.
Young's Modulus	54 GPa	Low-moderate modulus. Used in thermal stress modelling of SmCo magnet assemblies under temperature cycling in aerospace applications.
Hardness	~55–65 HB (annealed)	Harder than the light lanthanides La, Ce, Pr, and Nd; can be machined under inert atmosphere. SmCo magnets are much harder (~600 HV) and brittle due to their intermetallic microstructure.
Elongation at Break	~18%	Moderate ductility. Sm foil (99.9%+) is used as a sputtering target for SmCo thin-film deposition in micro-magnet research.
Poisson's Ratio	0.27	Typical for a mid-lanthanide.

Chemical Properties

Property	Value / Behavior	Notes
Oxidation States	+3 dominant (SmCl₃, Sm₂O₃); +2 accessible (SmI₂, SmO, SmS)	Sm²⁺ in THF solution (SmI₂) is the most widely used lanthanide +2 reagent in organic synthesis (Kagan's reagent), enabling single-electron reductions, ketyl radical cyclisations, and pinacol couplings. SmO and SmS are stable solid +2 compounds; SmS undergoes a dramatic black→gold colour change and resistivity drop under pressure as Sm²⁺→Sm³⁺ (mixed-valence intermediate state).
Corrosion Resistance	Oxidises slowly in dry air; reacts with moist air and water; dissolves in dilute acids	More air-stable than La or Ce but should be stored under inert atmosphere for prolonged periods. Sm powder is flammable. SmCo magnets are intrinsically corrosion-resistant — a key advantage over NdFeB in marine and chemical-process environments.
Surface Oxide	Sm₂O₃ (monoclinic B-type at RT) forms in air	Sm₂O₃ is used in IR-absorbing optical glass, as a neutron-absorbing ceramic additive, and as a precursor for SmCo magnet alloy preparation. The monoclinic B-type structure is the stable RT polymorph; transforms to cubic C-type above ~750 °C.

Identifier	Value
Symbol	Sm
Atomic Number	62
CAS Number	7440-19-9
UN Number	UN3089 (powder)
EINECS Number	231-128-7

Isotope	Type	Notes
¹⁴⁴Sm	Stable	3.07%; I = 0. Lightest stable Sm isotope; used as reference isotope in Sm-Nd isotope ratio normalisation. Low abundance makes it useful as an enriched IDMS spike for high-precision Sm concentration measurements.
¹⁴⁷Sm	Radioactive	14.99%; I = 7/2; alpha decay to ¹⁴³Nd, t½ = 1.06 × 10¹¹ yr. The basis of the Sm-Nd geochronological system: ¹⁴⁷Sm/¹⁴⁴Sm → ¹⁴³Nd/¹⁴⁴Nd isochrons date igneous and metamorphic rocks and are used to calculate ε_Nd, the principal tracer of mantle vs. crustal sources in igneous petrology. Also used in Sm-Nd model age calculations for crustal formation timing.
¹⁴⁸Sm	Stable*	11.24%; I = 0. Alpha decay to ¹⁴⁴Nd, t½ = 7 × 10¹⁵ yr. Negligible alpha decay in all practical contexts. Used as reference isotope in Sm-Nd measurements and as an IDMS spike.
¹⁴⁹Sm	Stable*	13.82%; I = 7/2, NMR-active; alpha decay to ¹⁴⁵Nd, t½ > 2 × 10¹⁵ yr. σ(thermal neutron) ≈ 41,000 barn — one of the largest of any nuclide. ¹⁴⁹Sm is the dominant fission product reactor poison after ¹³⁵Xe, building up in the fuel over hours to days after startup and causing reactivity suppression that must be compensated by control rod withdrawal. Its accumulation and burnout are explicitly modelled in reactor physics codes for PWR/BWR operations. Also the primary neutron absorber in Sm-based neutron shielding ceramics and Sm₂O₃-UO₂ burnable absorber fuel.
¹⁵⁰Sm	Stable	7.38%; I = 0. Used in isotope dilution mass spectrometry (IDMS) as a Sm spike and in reactor physics codes tracking Sm isotope evolution during fuel burnup (¹⁴⁹Sm→¹⁵⁰Sm by neutron capture).
¹⁵²Sm	Stable	26.75%; I = 0. Most abundant Sm isotope. Primary ICP-MS monitoring isotope for Sm in REE analysis. σ(thermal) = 206 barn — moderate neutron absorption.
¹⁵⁴Sm	Stable	22.75%; I = 0. Second most abundant; used as an alternative ICP-MS monitoring isotope. Low neutron cross-section (σ ≈ 8.4 barn); essentially transparent to thermal neutrons.

Scientific & Research Applications

Use Case	Form Typically Used	Description
SmI₂ Organic Synthesis (Kagan's Reagent)	SmI₂ (0.1 M in THF, prepared from Sm metal + ICH₂CH₂I); Sm metal (99.9%+) as precursor	SmI₂ is the most broadly applicable single-electron reductant in synthetic chemistry. Key reactions include Barbier-type allylations, ketyl radical cyclisations, pinacol couplings, and reductive cleavage of C–X bonds. The reduction potential is tunable with HMPA or alcohol additives. Used in synthesis of complex natural products and pharmaceuticals where conventional reductants lack chemoselectivity.
Sm-Nd Geochronology	¹⁴⁷Sm/¹⁴³Nd ratio by TIMS or MC-ICP-MS	The ¹⁴⁷Sm→¹⁴³Nd system (t½ = 1.06 × 10¹¹ yr) is less fractionated than Rb-Sr or U-Pb in most geological processes, making it particularly useful for dating mafic and ultramafic igneous rocks and for tracing mantle-crust mixing via ε_Nd. Model ages based on depleted-mantle evolution lines estimate when crustal material was extracted from the mantle.
SmCo Thin-Film Magnet Research	SmCo₅ and Sm₂Co₁₇ sputtering targets; Sm metal foil (99.9%+)	SmCo thin films are studied for MEMS microactuators, magneto-optical recording media, and exchange-spring magnet nanocomposites. The high coercivity of SmCo₅ (>2 T) in thin-film form makes it attractive for micro-scale permanent magnet applications where NdFeB is too corrosion-susceptible.
Neutron Detector & Reactor Physics Research	Sm₂O₃ powder; ¹⁴⁹Sm-enriched targets	¹⁴⁹Sm's large σ(thermal) makes it a useful neutron detector material in scintillator composites and reactor safety instrumentation. Sm₂O₃ additions to UO₂ fuel pellets (1–8 wt%) act as burnable absorbers — ¹⁴⁹Sm burns to ¹⁵⁰Sm (low σ) over the first reactor cycle, compensating initial excess reactivity without producing long-lived fission products.

Industrial & Commercial Applications

Sector	Form / Grade Used	Description
SmCo Permanent Magnets	Sm metal (95–99%+) in SmCo₅ (1:5) and Sm₂Co₁₇ (2:17) sintered and bonded magnet alloys	SmCo magnets are the standard for high-temperature, high-reliability permanent magnet applications: aerospace gyroscopes, satellite reaction wheels, military guidance systems, turbomachinery bearings, downhole oil and gas sensors, and high-temperature servo motors. Sm₂Co₁₇ grades achieve up to ~34 MGOe with operating temperatures to 350 °C — unmatched by NdFeB.
Optical Glass (IR Absorption)	Sm₂O₃ (1–5 wt%) in borosilicate or phosphate glass batch	Sm³⁺ 4f-4f transitions produce absorption bands in the near-IR (900–1,100 nm range), used to filter IR radiation in protective eyewear for laser operators, in spectral-shaping filters for optical instruments, and in glass for solar control glazing.
Neutron-Absorbing Ceramics & Nuclear Fuel	Sm₂O₃ powder (95–99%) mixed with UO₂ fuel or Al₂O₃ ceramic matrix	Sm₂O₃-UO₂ burnable absorber fuel rods are used in PWR and BWR reactors to flatten the initial power distribution and reduce shutdown margin requirements. Sm₂O₃-Al₂O₃ composites are used in neutron shielding panels in nuclear facility construction.

Purity	Applications	Notes
95% (1N5)	Sm–Co magnet alloys, nuclear shielding materials, and specialty glass.	Mid-grade for industrial magnet production and nuclear applications.

Synonym / Alternative Name	Context
Sm	Chemical symbol; from samarskite (the mineral in which Sm was discovered), itself named after Russian mining engineer Vasily Samarsky-Bykhovets. Used in SmCo magnet alloy specs, SmI₂ synthetic chemistry protocols, and ICP-MS REE databases (¹⁵²Sm as primary analytical isotope).
Sm metal	Commercial form designation for ingot, rod, foil, or powder. Used in SmCo alloy procurement specs, sputtering target datasheets, and SmI₂ precursor documentation.
Sm element	Scientific designation distinguishing elemental Sm from Sm compounds; used in condensed matter physics literature on SmS valence transitions and Sm crystal structure allotropy.
Samarium metal	Full commercial designation in REACH/RoHS documentation and ASTM REE metal standards.
Samarium element	Used in academic databases, Sm-Nd geochronology literature, and reactor physics texts discussing ¹⁴⁹Sm fission product poisoning and burnup modelling.
Samarium rare earth metal	Trade designation; Sm is classified as critical on EU and US materials lists for its role in SmCo magnets used in aerospace and defence applications where NdFeB cannot meet thermal performance requirements.
Samarium rare earth element	Geochemical designation in REE deposit assessments and chondrite-normalised REE pattern databases. The Sm/Nd ratio is the primary geochemical parameter tracked in Sm-Nd crustal evolution studies.
Element 62	Periodic table designation used in XRF/ICP-MS software, nuclear data libraries (ENDF/B-VIII for ¹⁴⁹Sm neutron cross-section and ¹⁴⁷Sm alpha decay), and reactor physics codes modelling ¹⁴⁹Sm accumulation and burnout during power plant operation.

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