Samarium(III) selenate is an inorganic compound of samarium and the selenate ion, with the chemical formula Sm2(SeO4)3. Several hydrates are known, including the hexahydrate, heptahydrate and octahydrate.
Preparation Samarium(III) selenate can be prepared by reaction of samarium(III) carbonate with selenic acid:
Sm2(CO3)3 + 3 H2SeO4 → Sm2(SeO4)3 + 3 CO2↑ + 3 H2O
Crystallization from aqueous solution gives hydrated samarium selenates. The octahydrate, Sm2(SeO4)3·8H2O, is the stable solid phase in equilibrium with saturated aqueous solution at 25 °C.
Hydrates and structure Several hydrates of samarium(III) selenate have been reported, including the hexa-, hepta- and octahydrates.
The octahydrate crystallizes in the monoclinic crystal system, in space group C2/c (No. 15). Reported lattice parameters are a = 7.014(1) Å, b = 13.878(1) Å, c = 18.121(2) Å and β = 99.40(1)°.
The octahydrate is structurally analogous to several other rare-earth selenate octahydrates. The crystal structure contains Sm–O coordination polyhedra linked to SeO4 tetrahedra, together with coordinated and lattice water molecules.
At 25 °C, the solubility of Sm2(SeO4)3·8H2O in water is about 0.582 mol of Sm2(SeO4)3 per kilogram of water. Its solubility decreases strongly with increasing temperature, as is typical for a number of rare-earth selenates.
Thermal decomposition Rare-earth selenate hydrates dehydrate in stages on heating before the anhydrous selenates themselves decompose at higher temperature.
For samarium selenate, heating first removes the water of crystallization. At higher temperatures, Se(VI) is reduced and selenium oxides are evolved, ultimately leaving samarium oxide as the stable high-temperature residue.
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