1 Natural History Museum of Denmark, Faculty of Science, Københavns Universitet2 Faculty of Agriculture, University of Belgrade (UB)3 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet4 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet
The intermediate member of the (Bi,Sb)2S3 solid-solution series was prepared by dry synthesis at low temperature (200 °C) with a long annealing period in sealed silica tube. The EDS analysis yielded an empirical formula (Bi0.96Sb1.04)S3, which is very close to the formula (Bi0.94Sb1.06)S3 obtained by crystal structure analysis. The crystal structure was refined to the R-factors: R1 = 0.0288 for I > 2 s(I) and wR2 = 0.0542 for all data. (Bi0.94Sb1.06)S3 has orthorhombic symmetry with space group Pnma (No. 62). The asymmetric unit contains two independent mixed cation sites, both coordinated to seven S atoms in a form of monocapped trigonal prism. They combine into infinite (M4S6)n rods parallel to the  direction with only weak interactions between rods where the lone electron pairs are directed. Our data and the revaluation of previous data resolve a controversy about the distribution of Sb and Bi over the two structural sites which exists in previously published works. It is shown that the structural constraints favour a slight preference of the larger cation for the marginal M1 site over the whole span of the solid-solution series. The unit cell parameters show a deviation form the Vegard’s law connected with different rates of the parallel increase of lengths of the strong bonds and the decreasing influence of the lone electron pair with the replacement of Sb by Bi. The changes in the character of the two independent cation sites introduced by this replacement are similar to those produced during compression of Sb2S3.