Synthesis and Characterization of Ba2Ag2Se2(Se2)

Single crystals and a polycrystalline sample of Ba2Ag2Se2(Se-2) 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 P2(1/c) of a monoclinic system with cell constants of a = 6.1766(2) angstrom, b = 6.1788(2) angstrom, c = 21.5784(8) angstrom, and beta = 90.02(1)degrees (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 Se-2(2-) units. The presence of the Se-2(2-) unit is also supported by an intense band at around 247 cm(-1) in the Raman spectrum of Ba2Ag2Se2(Se-2). 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(Se-2).