Ag3.5Bi7.5S13, a new member (N = 8) of the homologous series [Bi2S3]2.AgBiS2]((N-1)/2).

The silver bismuth tridecasulfide Ag3.5Bi7.5S13 crystallizes in the monoclinic space group C2/m. Its structure is built up of two alternating kinds of layered modules parallel to (001). In the module denoted A, octahedra around the metal positions (M = Ag/Bi, M2 and an S atom on 2/m, other atoms on m) alternate with paired monocapped trigonal prisms around Bi. The NaCl-type module B is composed of parallel eight-membered chains of edge-sharing octahedra running diagonally across it. Ag3.5Bi7.5S13 is the member with N = 8 of the pavonite homologous series (N)P of ternary compounds with the general formula [Bi2S3]2.[AgBiS2]((N-1)/2).