Received 14 November 2011
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 [dØ(Sm3+ - Se2-) = 2.931 Å], forming a rock-salt-related network [Sm7Se12]3- with channels along  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.
For prototypic Cs3Y7Se12 or Rb3Yb7Se12, see: Folchnandt & Schleid (1996); Kim et al. (1996). For other representatives of the A3M7Ch12 series, see: Folchnandt & Schleid (1997, 1998, 2000); Tougaît et al. (2001); Lissner et al. (2002). A detailed description of the relation between the crystal structures of the Cs3M7Se12 series and Z-type Sc2Ch3 (Dismukes & White, 1964) is provided by Folchnandt & Schleid (1998).
Data collection: DIF4 (Stoe & Cie, 1992); cell refinement: DIF4; data reduction: REDU4 (Stoe & Cie, 1992); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXL97.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2561 ).
This work was supported by the State of Baden-Württemberg (Stuttgart) and the German Research Foundation (DFG; Bonn) within the funding programme Open Access Publishing. We thank Dr Falk Lissner for the data collection.
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