Cs3Sm7Se12

The title compound, tricaesium heptasamarium(III) dodecaselenide, is setting a new starting point for realization of the channel structure of the Cs3 M 7Se12 series, now with M = Sm, Gd–Er. This Cs3Y7Se12-type arrangement is structurally based on the Z-type sesquiselenides M 2Se3 adopting the Sc2S3 structure. Thus, the structural set-up of Cs3Sm7Se12 consists of edge- and vertex-connected [SmSe6]9− octahedra [d Ø(Sm3+ – Se2−) = 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.

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 ] 9À octahedra [d Ø (Sm 3+ -Se 2À ) = 2.931 Å ], forming a rock-salt-related network [Sm 7 Se 12 ] 3À 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 Â Cs, 3 Â Sm and 6 Â Se) are located at Wyckoff positions 4g with site symmetry ..m.

Experimental
Yellow, transparent, needle-shaped single crystals of Cs 3 Sm 7 Se 12 were obtained as the main product of a reaction between 0.10 g Sm, 0.08 g Se and 0.50 g CsCl added as flux and caesium source upon heating at 1073 K for 10 days in a sealed, evacuated fused-silica vessel.

Refinement
In the final difference Fourier map the highest peak is 1.24 Å away from Se2 and the deepest hole is located 0.83 Å away from Sm2.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.