Syntheses, crystal structures, optical, and theoretical study of two ternary chalcogenides CsSc5Te8 and Cs0.6(1)Ti6Se8 with tunnel structures

Single crystals of two new chalcogenides CsSc5Te8 and Cs0.6(1)Ti6Se8 were synthesized by the high-temperature solid-state sealed tube method using a reactive flux (CsCl) at 1223 K. The crystal structures of CsSc5Te8 and Cs0.6(1)Ti6Se8 were determined by single-crystal X-ray diffraction method. The compound CsSc5Te8 crystallizes in the monoclinic space group C2h3−C2/m with the lattice parameters a = 21.3376(15) Å, b = 4.1434(3) Å, c = 10.2853(7) Å, and β = 103.925(2)° having the two formula units (Z = 2). The asymmetric unit of CsSc5Te8 contains eight crystallographically independent atomic sites: Cs1 (site symmetry: 2/m), Sc1 (m), Sc2 (m), Sc3 (2/m), Te1 (m), Te2 (m), Te3 (m), and Te4 (m). The structure of CsSc5Te8 is built up of the three-dimensional anionic framework of 3∞[Sc5Te8]1− where the Sc atoms are octahedrally coordinated with six Te atoms forming the one-dimensional tunnels approximately along the b-axis where the Cs+ cations are present. The compound Cs0.6(1)Ti6Se8 crystallizes in the Nb3Te4 structure type with Z = 1, in the hexagonal space group C6h2−P63/m having cell dimensions of a = b = 9.9520(1) Å and c = 3.5710(1) Å. The asymmetric unit of Cs0.6(1)Ti6Se8 structure is composed of four crystallographically independent sites with atoms Cs1 (site symmetry: 6‾.), Ti1 (m.), Se1 (m.), and Se2 (6‾.). The Ti atoms are making distorted octahedral units by coordinating with six Se atoms. These TiSe6 distorted octahedra share edges and corners with the adjacent TiSe6 units to form three-dimensional anionic networks that generate one-dimensional tunnels approximately along the c-direction. The optical absorption measurements show that CsSc5Te8 is a semiconductor having a direct bandgap of 1.2(1) eV at room temperature consistent with the DFT studies