Modulated Linear Tellurium Chains in Ba₃ScTe₅: Synthesis, Crystal Structure, Optical and Resistivity Studies, and Electronic Structure

A new ternary telluride, Ba₃ScTe₅, with a pseudo-one-dimensional structure, was synthesized at 1173 K by standard solid-state methods. A single-crystal X-ray diffraction study at 100(2) K shows the structure to be modulated. The structure of the subcell of Ba₃ScTe₅ crystallizes with two formula units in the hexagonal space group D6h3-P6₃/mcm with unit cell dimensions of a = b = 10.1190(5) Å and c = 6.8336(3) Å. The asymmetric unit of the subcell structure consists of four crystallographically independent sites: Ba1 (site symmetry: m2m), Sc1 (-3.m), Te1 (m2m), and Te2 (3.2). Its structure is made up of chains of ∞1[ScTe₃ ^3-] that are separated by Ba²⁺ cations. The Sc atoms are bonded to six Te1 atoms that form a slightly distorted octahedral geometry. The structure of the subcell also contains linear infinite chains of Te2 with intermediate Te···Te interactions. The superstructure of Ba₃ScTe₅ is incommensurate and was solved in the hexagonal superspace group P-6(00γ)0 with a = 10.1188(3) Å and c = 6.8332(3) Å and a modulation vector of q = 0.3718(2)c*. The arrangement and coordination geometries of the atoms in the superstructure are very similar to those in the substructure. However, the main difference is that the infinite chains of Te atoms in the superstructure are distorted owing to the formation of long- and short-bonded pairs of Te atoms. The presence of these chains with intermediate Te···Te interactions makes assignment of the formal oxidation states arbitrary. The optical absorption study of a polycrystalline sample of Ba₃ScTe₅ that was synthesized by the stoichiometric reaction of elements at 1173 K reveals a direct band gap of 1.1(2) eV. The temperature-dependent resistivity study of polycrystalline Ba₃ScTe₅ shows semiconducting behavior corroborating the optical studies, while density functional theory calculations report a pseudo band gap of 1.3 eV.