Cs(3)Sm(7)Se(12)

The title compound, tricaesium hepta­samarium(III) dodeca­selenide, is setting a new starting point for realization of the channel structure of the Cs(3) M (7)Se(12) series, now with M = Sm, Gd–Er. This Cs(3)Y(7)Se(12)-type arrangement is structurally based on the Z-type sesquiselenides M (2)Se(3) adopting the Sc(2)S(3) structure. Thus, the structural set-up of Cs(3)Sm(7)Se(12) consists of edge- and vertex-connected [SmSe(6)](9−) octa­hedra [d (Ø)(Sm(3+) – Se(2−)) = 2.931 Å], forming a rock-salt-related network [Sm(7)Se(12)](3−) with channels along [001] that are apt to take up monovalent cations (here Cs(+)) with coordination numbers of 7 + 1 for one and of 6 for the second cation. The latter cation has a trigonal–prismatic coordination and shows half-occupancy, resulting in an impossible short distance [2.394 (4) Å] between symmetrically coupled Cs(+) cations of the same kind. While one Sm atom occupies Wyckoff position 2b with site symmetry ..2/m, all other 11 crystallographically different atoms (namely 2 × Cs, 3 × Sm and 6 × Se) are located at Wyckoff positions 4g with site symmetry ..m.