Synthesis and characterization of new homologue multinary selenides, M2Sb5Bi5Se17 (M = Sn, Pb)

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New quaternary selenides M2Sb5Bi5Se17 (M = Sn, Pb) were synthesized using solid‐state sintering reactions that crystallize in the monoclinic system with C2/m (No. 12) space group with lattice parameters a = 27.914(7) Å, b = 4.0804(11) Å, c = 15.512(4) Å, and β = 114.881(9)° for M = Sn, and a = 27.987(3) Å, b = 4.1062(5) Å, c = 15.6372(19) Å, and β = 115.318(3)° for M = Pb, respectively. The crystal structure is related to a homologous series [A+22x−4B+34 Se−22x−2][B+32y−2Se−23y−3] with (x, y) = (3, 4) that contains building units of two‐dimensional slabs of NaCl111‐type [Sb2Bi4Se11] separated by 1D ribbons NaCl100‐type [Pb2Sb3BiSe6]. The NaCl111 unit contains edge‐shared octahedra filled with Sb3+ and Bi3+ cations, which are parallel and overlapped to form a step‐layer 2D network stacking alone [001]. The NaCl100 type ribbons containing Pb2+ and Sb3+ in square or trigonal pyramidal environments with the general formula [M6Se6] filled in the space between 2D layers of NaCl111 units. The conductivity measurement revealed semiconducting property with band gaps of ~0.1 eV. Pb2Sb5Bi5Se17 exhibits low thermal conductivity 3,000 μW cm−1 K−1 in a temperature range of 300–480 K.