Synthesis, Structure, and Thermoelectric Properties of Quaternary Selenides M ₂ In ₅ Sb ₉ Se ₂₃ (M = Sn, Pb) with NaCl-Type Ribbon Motifs

Two quaternary selenides, M2In5Sb9Se23 (M = Sn, Pb), were synthesized by solid-state routes and characterized by single-crystal and synchrotron powder X-ray diffraction. Both compounds crystallize in orthorhombic Pbam (No. 55) and comprise alternating NaCl-(111) ribbons of edge-sharing (In/Sb)Se6 octahedra and NaCl-(100) ribbons of distorted (Pb/Sn/Sb)–Se polyhedra that pack in a herringbone-like structure feature. Diffuse-reflectance spectroscopy indicates a narrow optical gap of 0.91 eV for Pb2In5Sb9Se23. Transport measurements on cold-pressed, unoriented Pb2In5Sb9Se23 indicate p-type semiconducting behavior with an Arrhenius activation energy of Ea ∼ 0.25 eV, Seebeck coefficients up to +460 μV K–1 (≈440 K), and intrinsically low total thermal conductivity of ∼0.5 W m–1 K–1, yielding zT ∼ 0.005 at 440 K. Density functional calculations for both analogues predict an indirect band gaps and ribbon-localized band-edge states, suggesting anisotropic electronic transport. These results identify mixed NaCl-(111)/(100) ribbon assembly as a motif in multinary selenides and establish M2In5Sb9Se23 as a useful platform for structure–property analysis and future thermoelectric optimization.