Multinary Selenides with Unusual Coordination Environment of Bismuth

New multinary selenides Ae₃SnPn₂Se₈ (Ae = Sr, Ba; Pn = Sb, Bi), Sr_8.01Ge_2.04Bi_7.95Se₂₄, 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 <i>Pnma</i> (no. 62) for Ae₃SnPn₂Se₈ (Ae = Sr, Ba; Pn = Sb, Bi) and in <i>Pna</i>2₁ (no. 33) for Sr_8.01Ge_2.04Bi_7.95Se₂₄ 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. Sr_8.01Ge_2.04Bi_7.95Se₂₄ 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