Ternary Chalcogenides BaMTe₂ (M = Cu, Ag): Syntheses, Modulated Crystal Structures, Optical Properties, and...

Standard solid-state methods produced black crystals of the compounds BaCu_0.43(3)Te₂ and BaAg_0.77(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 BaCu_0.43(3)Te₂ crystallizes in the monoclinic superspace group P2(αβ1/2)0, having cell dimensions of a = 4.6406(5) Å, b = 4.6596(5) Å, c = 10.362(1) Å, β = 90.000(9)°, and Z = 2 and an incommensurate vector of q = 0.3499(6)b* + 0.5c*. The compound BaAg_0.77(1)Te₂ crystallizes in the orthorhombic P2₁2₁2(α00)000 superspace group with cell dimensions of a = 4.6734(1) Å, b = 4.6468(1) Å, c = 11.1376(3) Å, and Z = 2 and an incommensurate vector of q = 0.364(2)a*. The asymmetric unit of the BaCu_0.43(3)Te₂ structure comprises eight crystallographically independent sites; that for BaAg_0.77(1)Te₂ comprises four. In these two structures, each of the M (M = Cu, Ag) atoms is connected to four Te atoms to make two-dimensional layers of [MTe_4/4] 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 BaAg_0.8Te₂ shows the presence of Ag-Te (83, 116, and 139 cm^-1) and Ba-Te vibrations (667 and 732 cm^-1). A UV-vis-NIR spectroscopic study on a powdered sample of BaAg_0.8Te₂ 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.