Synthesis, structure, band gap, and electronic structure of CsAgSb ₄S₇

The compound CsAgSb₄S₇ has been synthesized by the reaction of the elements in a Cs₂S₃ flux at 773 K. The compound crystallizes in a new structure type with eight formula units in space group C2/c of the monoclinic system in a cell at 153 K of dimensions a = 26.127(2) Å, b = 8.8599(7) Å, c = 12.100(1) Å, β = 97.650(1)°, and V = 2776.0(4) Å3. The structure contains two-dimensional _∞²[AgSb₄S₇] layers separated by Cs atoms. Each _∞²[AgSb ₄S₇] layer is built from edge-sharing one-dimensional _∞¹[AgS₃] and _∞ ¹[Sb₄S₇] chains. Each Ag atom is tetrahedrally coordinated to four S atoms. Each Sb³⁺ center is pyramidally coordinated to three S atoms to form an SbS₃ group. CsAgSb ₄S₇ is insulating with an optical band gap of 2.04eV. Extended Hückel calculations indicate that the band gap in CsAgSb ₄S₇ is dominated by the Sb 5s and S 3p states above and below the Fermi level.