Samarium compounds are compounds formed by the lanthanide metal samarium (Sm). Samarium most commonly occurs in the +3 oxidation state, as in Sm2O3, SmCl3 and Sm(NO3)3. The +2 oxidation state is unusually accessible for a lanthanide and gives rise to an extensive chemistry of divalent halides, chalcogenides, coordination compounds and organometallic compounds. Among these, samarium(II) iodide (SmI2) is widely used as a single-electron reducing agent in organic synthesis.
Samarium forms a broad range of binary compounds, including oxides, hydrides, halides, chalcogenides, borides, carbides, silicides and pnictides, as well as numerous coordination and organosamarium compounds. Several display unusual electronic behaviour associated with changes in samarium valence. Samarium(II) sulfide undergoes a pressure-induced transition from a black semiconducting state to a golden metallic state, while samarium hexaboride is an intermediate-valence Kondo insulator studied for its low-temperature electronic properties. Samarium compounds also have practical uses in organic synthesis, catalysis and optical materials, while samarium–cobalt intermetallic compounds are important high-temperature permanent-magnet materials.
Properties of samarium compounds
Formula Colour Crystal system 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 green monoclinic P21/c 14 mP12 SmI3 orange trigonal R 148 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
Chalcogenides
Oxides
The most stable oxide of samarium is the sesquioxide Sm2O3, which exists in several crystalline modifications. The trigonal form can be obtained by slow cooling from the melt. Because Sm2O3 has a high melting point of about 2345 °C, it is generally melted by induction heating using a radio-frequency coil rather than by direct heating. Crystals of the monoclinic modification can be grown from Sm2O3 powder by the flame-fusion (Verneuil) method, producing cylindrical boules several centimetres long and about one centimetre in diameter. These are transparent when pure and defect-free, but orange when defects are present. Heating the metastable trigonal modification to about 1900 °C converts it to the more stable monoclinic form. A cubic modification of Sm2O3 has also been described.
Samarium is one of the few lanthanides known to form a monoxide, SmO. This lustrous golden-yellow compound can be obtained by reducing Sm2O3 with samarium metal at about 1000 °C under pressures above 50 kbar; at lower pressures the reaction is incomplete. SmO crystallizes in the cubic rock salt structure.
Other chalcogenides
Samarium forms several sulfides in addition to SmS. The sesquisulfide Sm2S3, containing Sm, is polymorphic. The A- or α-modification is orthorhombic with space group Pnma, whereas the C- or γ-modification has a cubic, cation-deficient Th3P4-type structure with space group I3d. The former can be obtained by oxidation of SmCl2 with sulfur in a sodium chloride flux, while the latter has been prepared by reaction of the elements under related conditions. A mixed-valence sulfide, Sm3S4, is also known. It adopts the Th3P4 structure and contains samarium in both the +2 and +3 oxidation states.
The corresponding sesquiselenide Sm2Se3 is likewise polymorphic. Chemical-transport reactions have yielded red-brown crystals of a cation-deficient Th3P4-type modification with space group I3d, as well as black crystals of an orthorhombic U2S3-type modification with space group Pnma. The trivalent telluride Sm2Te3 is also known.
The divalent monochalcogenides SmS, SmSe and SmTe crystallize in the cubic rock-salt structure and are narrow-gap semiconductors at ambient pressure. Their unusual electronic properties arise from the accessibility of the Sm and Sm configurations, and all three undergo pressure-induced changes toward metallic, intermediate-valence states.
The transition is particularly abrupt in SmS. At room temperature it occurs at about 6.5 kbar and is accompanied by a change from the black semiconducting state to a golden, much more conductive state. The rock-salt symmetry is retained, but the unit-cell volume decreases by about 13.5%, reflecting the change from predominantly divalent samarium toward an intermediate-valence state. Mechanical pressure produced by scratching or polishing can produce the same characteristic golden coloration at the surface. SmSe and SmTe undergo analogous but continuous electronic transitions over considerably broader and higher pressure ranges. SmS additionally exhibits hysteresis: after compression, the semiconducting phase is recovered at a substantially lower pressure than that required to induce the metallic state.
Halides
Samarium forms both trivalent and divalent binary halides. Reaction of the metal with the halogens gives the trihalides (X = F, Cl, Br or I):
Their crystal structures vary across the series. is orthorhombic, while adopts the hexagonal structure type and the orthorhombic type. Samarium(III) iodide belongs instead to the layered structure type characteristic of the heavier lanthanide triiodides.
The +2 oxidation state is considerably more accessible for samarium than for most lanthanides, and all four samarium dihalides , , and are known. Reduction of the trihalides with samarium, lithium or sodium at about 700–900 °C gives the corresponding lower halides. crystallizes in the cubic fluorite structure, whereas and have orthorhombic structures. The diiodide can also be prepared by heating , or by reaction of samarium metal with 1,2-diiodoethane in anhydrous tetrahydrofuran at room temperature:
Sm + ICH2CH2I → SmI2 + CH2=CH2
The samarium–halogen systems also contain intermediate phases between the formal +2 and +3 compositions. In the fluoride system these include , and . Three closely spaced phases occur between and , with compositions , and . Their structures are related to the fluorite structure and accommodate additional bromide ions in long-period superstructures.
Several samarium halides are polymorphic under pressure or at elevated temperature. Compression of the usual monoclinic form of , followed by decompression, produces an orthorhombic PbCl2-type modification with a density of 5.90 g/cm3. High-pressure treatment likewise produces a denser modification of , with a reported density of 5.97 g/cm3.
Hydrides
Samarium reacts directly with hydrogen to form non-stoichiometric hydrides of approximate composition . Hydrogen uptake is strongly dependent on the physical state of the metal: powdered samarium begins to absorb hydrogen rapidly at about 300–330 °C, whereas bulk samples may require temperatures up to about 525 °C. Compositions as hydrogen-rich as approximately have been obtained by direct hydrogenation. The hydrides are sensitive to air and can partially decompose to samarium oxide under ambient conditions.
The samarium–hydrogen system contains several distinct phases between the nominal dihydride and trihydride compositions. The fluorite-related cubic phase extends approximately over , while a hydrogen-rich hexagonal, tysonite-related phase occurs around . An intermediate phase, , has a tetragonally distorted fluorite-related structure analogous to that of . Similar phase relationships occur in the corresponding deuterides. The structural chemistry follows the general hydride–fluoride analogy found among lanthanides; unlike europium, however, samarium is readily oxidized from Sm toward Sm as the hydrogen content increases.
At elevated pressure, samarium trihydride undergoes a reversible transformation from its ambient-pressure hexagonal structure to a cubic high-pressure phase. Much more hydrogen-rich phases have also been obtained at megabar pressures. In 2026, laser heating samarium with ammonia borane in diamond-anvil cells produced phases assigned as , , , , and hydrogen-deficient . The higher hydrides were metallic under the conditions studied, but no superconducting transition was observed.
Borides
Samarium forms several binary borides spanning a wide range of boron content. Early phase-equilibrium studies of the Sm–B system identified a samarium-rich phase of approximate composition , together with , and the boron-rich phase . The tetraboride and hexaboride have measurable homogeneity ranges, and the hardness of the phases generally increases with increasing boron content. A further boride, , has also been structurally characterized.
is difficult to obtain under ordinary conditions because more boron-rich phases are preferentially formed. It has been synthesized at high pressure and high temperature and adopts the hexagonal structure type, with lattice parameters a = 0.3310 nm and c = 0.4019 nm. crystallizes in the tetragonal structure type. It undergoes antiferromagnetic ordering near 25 K, with a second low-temperature anomaly near 7 K.
Powder mixtures of samarium oxide and boron can be sintered under vacuum to give mixtures of boride phases, whose proportions depend on the starting composition. Larger crystals of , and can be produced by arc or zone melting, taking advantage of their different melting and crystallization behaviour. These borides are dark, hard and brittle, with hardness generally increasing with boron content.
Samarium hexaboride
Samarium hexaboride, , crystallizes in the cubic structure type, in which octahedra form a three-dimensional boron framework surrounding the samarium atoms. It is an intermediate valence compound: the samarium 4f configuration fluctuates between states conventionally described as Sm and Sm, giving an average valence of about +2.6 near room temperature rather than distinct crystallographic Sm and Sm sites.
At high temperature SmB6 has comparatively conductive behaviour, but on cooling the hybridization of localized 4f states with conduction states opens a narrow gap and the bulk becomes insulating. It is therefore a prototypical Kondo insulator. Spectroscopic measurements find energy scales of several to several tens of millielectronvolts associated with the hybridization gap, depending on the experimental method. The decrease in the number of mobile bulk charge carriers is accompanied by a marked rise in thermal conductivity at low temperature, with a maximum near 15 K.