Copper/silver-containing chalcogenides have recently attracted significant interest for their promising thermoelectric applications. In this article, we report the syntheses of five new layered quaternary Cu/Ag-containing chalcogenides: SrScCuSe3, SrScCuTe3, BaScCuSe3, BaScCuTe3,...

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 D 3 ,6h−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 Ba 2+ cations. The Sc atoms are bonded to six Te1 atoms that form a slightly distorted octahedral geometry. The structure of 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 atoms in the superstructure are very similar to those in the substructure. However, the main difference is that the infinite chains of Te 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 the assignment of 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 bandgap of 1.1(2) eV. The temperature-dependent resistivity study of polycrystalline Ba3ScTe5 shows semiconducting behavior corroborating the optical studies while DFT calculations report a pseudo bandgap of 1.3 eV.

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Single crystals and a polycrystalline sample of Ba2Ag2Se2(Se2) were synthesized by standard solid-state chemistry methods at 1173 and 973 K, respectively. The crystal structure of this ternary compound was established by single-crystal X-ray diffraction studies at 100(2) K. The superstructure of this compound is commensurate and crystallizes in the space group P21/c of a monoclinic system with cell constants of a = 6.1766(2) Å, b = 6.1788(2) Å, c = 21.5784(8) Å, and β = 90.02(1)° (Z = 4). The asymmetric unit of the superstructure comprises eight atoms occupying general positions: two Ba atoms, two Ag atoms, and four Se atoms. In this structure, each Ag atom is tetrahedrally coordinated with four adjacent Se atoms to form distorted AgSe4 units that share edges with the neighboring tetrahedra to form a two-dimensional [AgSe4/4]− layer. These layers are separated by Ba2+ and Se2 2– units. The presence of the Se2 2– unit is also supported by an intense band at around 247 cm–1 in the Raman spectrum of Ba2Ag2Se2(Se2). A density functional theory study shows that the compound is a semiconductor with a calculated band gap of 1.1 eV. As determined by UV–visible spectroscopy, the direct and indirect band gaps are 1.23(2) and 1.10(2) eV, respectively, in good agreement with the theory and consistent with the black color of the compound. A temperature-dependent resistivity study also confirms the semiconducting nature of Ba2Ag2Se2(Se2).

Standard solid-state methods produced black crystals of the compounds BaCu0.43(3)Te2 and BaAg0.77(1)Te2 at 1173 K; the crystal structures of each were established using single-crystal X-ray diffraction data. Both crystal structures are modulated. The compound BaCu0.43(3)Te2 crystallizes in the monoclinic superspace group P2(αβ1/2)0 having cell dimensions a = 4.6406(5) Å, b = 4.6596(5) Å, c = 10.362(1) Å, Z = 2, and an incommensurate vector q = 0.3498(6)b* + 1/2c*. The compound BaAg0.77(1)Te2 crystallizes in the orthorhombic P21212(α00)000 superspace group with cell dimensions a = 4.6734(1) Å, b = 4.6468(1) Å, c = 11.1376(3) Å, Z = 2, and an incommensurate vector q = 0.364(2)a*. The asymmetric unit of the BaCu0.43(3)Te2 structure comprises eight crystallographically independent sites; that for BaAg0.77(1)Te2 comprises four. In these two structures each of the M (= Cu, Ag) atoms is connected to four Te atoms to make two-dimensional layers of [MxTe4/4] n− that are separated by layers of Ba atoms and square nets of Te. A Raman spectroscopic study at 298(2) K on a pelletized polycrystalline sample of BaAg0.8Te2 shows the presence of Ag−Te (83, 116, and 139 cm −1) and Ba−Te vibrations (667 cm −1 and 732cm −1). A UV−vis-NIR spectroscopic study on a powdered sample of BaAg0.8Te2 shows the semiconducting nature of the compound with a direct band gap of 1.0(2) eV consistent with its black color. DFT calculations give a pseudo bandgap with a weak value of the DOS at the Fermi level.