Multinary selenides with unusual coordination environment of bismuth.

New multinary selenides Ae3SnPn2Se8 (Ae = Sr, Ba; Pn = Sb, Bi), Sr8.01Ge2.04Bi7.95Se24, and Sr8YGe2Bi7Se24 were synthesized by solid-state reaction, and their structures were determined by single-crystal X-ray diffraction. These compounds crystallize in orthorhombic space group Pnma (no. 62) for Ae3SnPn2Se8 (Ae = Sr, Ba; Pn = Sb, Bi) and in Pna21 (no. 33) for Sr8.01Ge2.04Bi7.95Se24 and Sr8YGe2Bi7Se24. The structures feature one-dimensional corner sharing tetrahedral (∞)(1)[MSe3] units, and one-dimensional edge sharing octahedral ∞1[M4Se10], packed with the alkaline earth or rare earth cations. Sr8.01Ge2.04Bi7.95Se24 and Sr8YGe2Bi7Se24 contain a triple cell superlattice structure derived from a special arrangement of Bi and Ge in the tetrahedrally coordinated ∞1[MSe3] chain. Diffuse reflectance spectra and electronic resistivity measurements indicate semiconducting behaviors; the Sr8YGe2Bi7Se24 Seebeck coefficient is −180 μV/K at 303 K. Electronic structure calculations confirm that the electron count for Sr8YGe2Bi7Se24 is optimal for interatomic bonding in the ionic network.