Sm 3 O 2 Sm[SeO 3 ] 4 and Sm 3 O 2 Cl[SeO 3 ] 2 : A Monoclinic Form of Samarium Oxide Oxoselenate(IV) and a Samarium Oxide Chloride Oxoselenate(IV) both with 1 ∞ {[O 2 Sm 3 ] 5+ } Chains

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