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Bi 2 S 3 Bipyramids in Layered Sulfides M 2 Bi 2 S 3 (AlCl 4 ) 2 ( M = Ag, Cu)
Author(s) -
Groh Matthias F.,
Knies Maximilian,
Isaeva Anna,
Ruck Michael
Publication year - 2015
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.201400522
Subject(s) - crystallography , chemistry , stacking , space group , acentric factor , electron diffraction , octahedron , crystal structure , x ray crystallography , diffraction , physics , organic chemistry , optics
Black Cu 2 Bi 2 S 3 (AlCl 4 ) 2 and orange Ag 2 Bi 2 S 3 (AlCl 4 ) 2 were synthesized by solvent‐free reaction (polycrystalline powders) as well as in Lewis‐acidic ionic liquids (crystals) at temperatures of 200 °C or lower. X‐ray diffraction on single‐crystals of Cu 2 Bi 2 S 3 (AlCl 4 ) 2 revealed two centrosymmetric polytypes: a rhombohedral one, space group R $\bar{3}$ c [ a = 658.02(3) pm, c = 6794.3(3) pm], with six formula units in the unit cell (6R polytype), and a hexagonal one, space group P 6 3 / m [ a = 658.71(6) pm, c = 2265.5(3) pm], with two formula units (2H polytype). Ag 2 Bi 2 S 3 (AlCl 4 ) 2 is homeotypic and crystallizes in the acentric hexagonal space group P $\bar{3}$ 2 c with a = 691.65(3) pm, c = 2207.86(9) pm, and two formula units per unit cell (2H′ polytype). All structures consist of 2 ∞ [( M + ) 2 Bi 2 S 3 ] layers ( M = Cu, Ag) separated by double layers of AlCl 4 – tetrahedra and differ mainly in their stacking sequences and the orientation of the AlCl 4 – groups. The sulfidometalate layer is a honeycomb‐like network with M + and S 2– ions in plane, whereas pairs of Bi 3+ cations occupy positions above and below the plane. The analysis of chemical bonding reveals strong covalent two‐center two‐electron bonds in the five‐atomic bipyramidal Bi 2 S 3 unit ( D 3 h symmetry) and covalent bonding with much higher ionic component to the coin metal cations. The DFT‐optimized shape of an isolated hypothetical Bi 2 S 3 molecule differs only marginally from that in the layer. Hence, the compounds might be interpreted as Bi 2 S 3 molecules embedded in M AlCl 4 salts. Optical bandgaps of 1.6 eV ( M = Cu) and 2.2 eV ( M = Ag) were deduced from diffuse reflectance measurements. DFT‐based quantum chemical calculations indicate direct bandgaps of the same magnitude.