The crystal structure is related to Proudite-Felbertalite homologous series and is made up of layered units ∞ 2 M 16 Se 23 ½ � derived from the octahedrally coordinated MSe 6 with stepped layers of 16 octahedra overlapped by 8 octahedra units, and 1D ribbons ∞ 1 M 12 Se 14 ½ � with metal centers coordinated by 3 or 5 Se 2À anions. Structural anisotropy leads to their layered shape and strong preferred orientation. The quinary selenide Sn 3.84 Pb 6.16 Sb 5.68 Bi 12.32 Se 37 exhibits n-type semiconducting behavior and thermoelectric properties with an optimized thermoelectric figure of merit (zT) reaching ~0.09 at 440 K. Density functional theory (DFT) calculation on a chargebalanced model with a triple supercell indicates its indirect band gap.
were prepared by solid-state reactions. These compounds adopt the Felbertalite structure type that crystallizes in the monoclinic space group C2/m (No. 12). Single-crystal and powder X-ray diffraction measurements studied the crystal structure and site preference of metal sites. They exhibit stacking of stepped layers ∞ 2 M 8 Se 13 ½ � (NaCl 111 -type) and ribbon-shape ∞ 1 M 6 Se 6 ½ � (NaCl 100 -type) units that are connected via shared Se 2À ions. Electronic structure calculations indicated semiconducting properties and were verified by temperature-dependent conductivity and Seebeck coefficient measurements. The thermoelectric properties show a maximum power factor of 1.06 μW • m À 1 • K À 2 at 550 K for Pb 4 Sb 6 Bi 4 Se 19 .
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