The Unique Crystal Structure of the Triclinic Samarium(III) Oxoselenate(IV) Sm2[SeO3]3

Pale yellow, needle‐shaped single crystals of Sm2[SeO3]3 were obtained by heating stoichiometric mixtures of Sm2O3 and SeO2 (molar ratio: 1:3) along with substantial amounts of CsCl as fluxing agent in evacuated sealed silica tubes at 830 °C for one week. According to X‐ray single‐crystal diffraction data, Sm2[SeO3]3 crystallizes triclinic (space group: $P{\bar 1}$ ) with two formula units per unit cell of the dimensions a = 698.62(7), b = 789.71(8), c = 910.34(9) pm, α = 96.693(5), β = 104.639(5), γ = 115.867(5)°. Its crystal structure contains two crystallographically distinct Sm3+ cations in eight‐ and ninefold coordination with oxygen atoms arranged as distorted uncapped or capped square antiprisms (d(Sm3+−O2−) = 232−271 pm). These [(Sm1)O8] and [(Sm2)O9] polyhedra share opposite edges and faces to form zigzag chains along [100] with discrete pyramidal [SeO3]2− anions bridging units. Further linkage by [SeO3]2− anions in [010] direction leads to a three‐dimensional network, which exhibits almost rectangular channels along [111]. These tunnels offer width enough to incorporate the free non‐bonding electron pairs („lone pairs”︁) at the Se4+ cations, since all nine different Ψ1‐tetrahedral [SeO3]2− groups (d(Se4+−O2−) = 165−173 pm, ∢(O–Se–O) = 94 – 108°) exhibit a pronounced stereochemical „lone‐pair”︁ activity. For not being isotypic with neither triclinic Er2[SeO3]3 (CN(Er3+) = 7 and 8) nor the remainder rare‐earth metal(III) oxoselenates(IV) of the composition M2[SeO3]3 (≡ M2Se3O9; M = Sc, Y, La, Ce – Lu), Sm2[SeO3]3 claims a unique crystal structure among them.