Synthesis, crystal chemistry, and semiconducting properties of A₃B₁₄Se₂₄ (A = Sn/Pb; B = In/Sb/Bi) with step-layered...

New multinary selenides A₃B₁₄Se₂₄ (A = Sn/Pb; B = In/Sb/Bi) were discovered by exploratory synthesis in (Sn/Pb)-In-(Sb/Bi)-Se systems. Single-crystal X-ray diffraction established this structure type for Pb₃In₅Sb₉Se₂₄, Pb₃In₆Sb₈Se₂₄, Sn₃In₅Sb₉Se₂₄, and Sn_3.7Pb_1.3In_3.9Bi_1.6Sb_6.6Se₂₄, which reveals a monoclinic structure (C2/m) composed of two-dimensional step-layered NaCl(111)-type slabs interleaved with one-dimensional NaCl(100)-type ribbons. The NaCl(111)-derived unit contains nine edge-sharing octahedra per step; three octahedra on each side overlap with adjacent steps, leaving three isolated overlapped octahedra at the step center. The NaCl(100)-derived ribbons host A and Sb(Bi) in trigonal- and square-pyramidal Se coordination, consistent with stereoactive ns² lone pairs. A single-phase field was established for Pb₃In_xSb_14-xSe₂₄ (x = 5-6). Synchrotron Rietveld refinement confirms phase purity and site-occupancy trends, while SEM/EDS and ICP-OES verify bulk stoichiometry; XPS is consistent with Pb²⁺, In³⁺, Sb³⁺, and Se^2-. UV-Vis-NIR diffuse reflectance gives optical band gaps of 0.76(2) and 0.80(2) eV for x = 5 and 6, respectively, and four-probe measurements yield activation energies of 0.15-0.16 eV, indicating narrow-gap semiconducting behavior. These compounds expand NaCl-fragment homologous chemistry and provide a modular step-layer/ribbon platform for tuning the electronic structure.