Synthesis and structure of new Cd-Bi-S homologous series: A study in intergrowth and the control of twinning patterns

Four compounds, CdBi2S4, CdBi4S7, Cd2.8Bi8.1S15, and Cd2Bi6S11, have been synthesized using a RbCl/LiCl flux. Single-crystal X-ray structure refinements were made for CdBi2S4 and Cd2.8Bi8.1S15. The final R values for these two crystals were respectively 3.05% and 5.53%. The compounds CdBi2S4 and CdBi4S7 are isotypic with respectively HgBi2S4 and Y5S7. Cd2.8Bi8.1S15 can be considered as an ordered intergrowth between the two phases CdBi2S4 and CdBi4S7 and belongs to a new structure type as does Cd2Bi6S11. All four structures can be viewed as chemically twinned NaCl superstructures where the twinning plane is the (113) plane. We find there is segregation of the larger Bi atoms toward the twinning planes, whereas smaller Cd atoms occupy the interior sites of the NaCl domain. We also find that the average oxidation state of the cations controls the twinning patterns even in the presence of metal disorder. We propose the possible twinning patterns for Cd2Bi6S11 based on the observed average oxidation state of metal cations. The space groups of CdBi2S4, CdBi4S7, and Cd2.8Bi8.1S15 are C2/m for all three crystals. Cell axes for the three phases are respectively a = 13.095(2) Angstrom, b = 3.9792(7) Angstrom, c = 14.611(3) Angstrom, beta = 116.30(1)degrees; a = 13.112(2) Angstrom; b = 4.0032(7) Angstrom, c = 11.765(2) Angstrom, beta = 105.21(1)degrees; and a = 13.111(2) Angstrom, b = 3.9885(8) Angstrom, c = 24.710(5) Angstrom, beta = 97.84(2)degrees. Cell axes for Cd2Bi6S11 are a = 13.112(3) Angstrom, b = 3.997(1) Angstrom, c = 35.84(1) Angstrom, and beta = 90.39(2)degrees.