Ba3Zr2Cu4S9: the first quaternary phase of the Ba–Zr–Cu–S system

Abstract

We report the synthesis of red-colored crystals of Ba₃Zr₂Cu₄S₉via a high-temperature reaction of elements at 1223 K. The title compound is the first quaternary phase of the Ba–Zr–Cu–S system. A single-crystal X-ray diffraction study has elucidated its crystal structure, which shows that it crystallizes in a new structure type in the centrosymmetric triclinic P space group with cell dimensions of a = 6.9150(3) Å, b = 10.3863(5) Å, c = 12.2417(6) Å, α = 109.524(1)°, β = 105.132(2)°, and γ = 96.809(2)° with Z = 2. The asymmetric unit of the structure contains a total of eighteen crystallographic distinct atomic positions: 3 × Ba, 2 × Zr, 4 × Cu, and 9 × S. All these atoms are present at the general positions. The structure is best described as pseudo-two-dimensional with infinite layers of ^∝₂[Zr₂Cu₄S₉]⁶⁻ and Ba²⁺ cations occupy the space between two such layers. The Zr atoms are bonded with six S atoms to form ZrS₆ octahedra. Each Cu(2) and Cu(4) atom sits at the center of a distorted tetrahedron of sulfur atoms. Interestingly, the Cu(1) and Cu(3) atoms are only three coordinated and form distorted CuS₃ trigonal polyhedra. The ab initio DFT calculations are performed to explore the electronic structure and the optical properties. An experimental optical bandgap study established the semiconducting nature (E_g = 1.9(1) eV) of a polycrystalline Ba₃Zr₂Cu₄S₉ sample in agreement with the theoretical DFT study. The computed cell constants and atomic positions are found to be in good agreement with the experiments. The computed bandgap is indirect with a ∼1.1 to 1.5 eV magnitude. The COHP values are obtained to assess the relative strength of chemical bonding between atoms. The relative charge transfer between bonding atoms is estimated from the analysis of the Bader charge.