Synthesis and Characterization of Ba₂Ag₂Se₂(Se₂)

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 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 β = 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 (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). (authors)