Abstract
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.