Cs₃Sm₇Se₁₂

The title compound, tricaesium hepta­samarium(III) dodeca­selenide, is setting a new starting point for realization of the channel structure of the Cs₃M₇Se₁₂ series, now with M = Sm, Gd-Er. This Cs₃Y₇Se₁₂-type arrangement is structurally based on the Z-type sesquiselenides M₂Se₃ adopting the Sc₂S₃ structure. Thus, the structural set-up of Cs₃Sm₇Se₁₂ consists of edge- and vertex-connected [SmSe₆]^9- octa­hedra [d_Ø(Sm³⁺ - Se^2-) = 2.931 Å], forming a rock-salt-related network [Sm₇Se₁₂]^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.