Synthesis, Crystal Chemistry, and Semiconducting Properties of A3B14Se24 (A = Sn/Pb; B = In/Sb/Bi) with Step-Layered NaCl(111)-Type Slabs Interleaved by NaCl(100)-Type Ribbons

Abstract

New multinary selenides A3B14Se24 (A = Sn/Pb; B = In/Sb/Bi) were discovered by exploratory synthesis in the (Sn/Pb)–In–(Sb/Bi)–Se systems. Single-crystal X-ray diffraction established this structure type for Pb3In5Sb9Se24, Pb3In6Sb8Se24, Sn3In5Sb9Se24, and Sn3.7Pb1.3In3.9Bi1.6Sb6.6Se24, 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 Pb3InxSb14-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 Se²⁻. 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 electronic structure.