Sm₃O₂Sm[SeO₃]₄ and Sm₃O₂Cl[SeO₃]₂: A Monoclinic Form of Samarium Oxide Oxoselenate(IV) and a Samarium Oxide Chloride Oxoselenate(IV) both with ¹_∞{[O₂Sm₃]⁵⁺} Chains

Abstract

Appropriate mixtures of Sm2O3 and SeO2 with various chloride fluxes yielded single crystals of Sm3O2Sm[SeO3]4 (monoclinic, P21/n; a = 1497.89(8), b = 556.61(3), c = 1639.63(9) pm, β = 91.054(3)° with Z = 8 for Sm2Se2O7) or Sm3O2Cl[SeO3]2 (orthorhombic, Pnma; a = 565.82(4), b = 1457.14(9), c = 1145.23(7) pm and Z = 4). In contrast to the known tetragonal form of Sm2Se2O7, the monoclinic one exhibits four crystallographically different Sm3+ and Se4+ cations. Nonetheless [OSm4]10+ tetrahedra are connected by trans‐oriented edges to single chains 1∞{[OSm4/2]4+} like in tetragonal Sm2O[SeO3]2. The monoclinic form, however, displays double chains 1∞{[O2Sm3]5+} of edge‐linked [OSm4]10+ tetrahedra, which are also present in the samarium(III) oxide chloride oxoselenate(IV) Sm3O2Cl[SeO3]2. Both tetragonal Sm2O[SeO3]2 and monoclinic Sm3O2Sm[SeO3]4 with the empirical formula Sm2Se2O7 as well as Sm3O2Cl[SeO3]2 contain empty channels in their crystal structures, apt to accommodate the lone‐pair electrons at the Se4+ cations. These are residing in isolated ψ1‐tetrahedral [SeO3]2― anions. Non‐selenium bonded oxide anions (and if necessary chloride anions in the case of Sm3O2Cl[SeO3]2) take care of the charge balance. The much higher density (Dx = 6.207 g·cm–3) of the tetragonal form (P42/ncm; a = 1077.03(8), c = 526.38(4) pm) indicates the new monoclinic modification of Sm2Se2O7 (Dx = 5.546 g·cm–3) as high‐temperature phase, which is confirmed by its synthesis under fast quenching conditions.