Novel quaternary lanthanum bismuth sulfides Pb₂La_xBi_8-xS₁₄, Sr₂La_xBi_8-xS₁₄, and Cs₂La_xBi_10-xS₁₆ with complex structures

The compounds Pb₂La_xBi_8-xS₁₄ (I), Sr₂La_xBi_8-xS₁₄ (II), and Cs₂La_xBi_10-xS₁₆ (III) were synthesized from the corresponding elements or binary sulfides at temperatures above 850 °C. Compounds I and II are isostructural, forming a new structure type, while the structure of III is related to the structure of the mineral kobellite. All compounds crystallize in the orthorhombic space group Pnma (No. 62) with a = 21.2592(4) Å, b = 4.0418(1) Å, c = 28.1718(3) Å, Z = 4 for I, a = 21.190(1) Å, b = 4.0417(2) Å, c = 28.285(2) Å, Z = 4 for II and a = 34.893(4) Å, b = 4.0697(4) Å, c = 21.508(2) Å, Z = 4 for III. All compounds exhibit mixed site occupancy between Bi and La. Furthermore, I and II exhibit disorder between the divalent atom (Sr or Pb) and/or La and/or Bi. The structures of I and II consist of thin walls made of two metal-atom-thick NaCl-type blocks running in two opposite directions in the ac plane, forming rhombus-shaped tunnels. These tunnels are filled with Bi₂Te₃-type fragments. In the points where the walls intersect they form Gd₂S₃-type fragments. The structure of III consists of a complex three-dimensional framework with Cs-filled tunnels. All compounds are semiconductors with band gaps around 1.0 eV, and they melt around 740-860 °C.