Structural Chemistry of Lead-Antimony and Lead-Bismuth Sulphides

Abstract

<p>The structures in the PbS-rich part of the Ag₂S-PbS-Bi₂S₃ system are composed of PbS-like slabs obtained by twinning the parent PbS (NaCl structure) along the (311)PbS planes. A number of phases, some new and some already encountered in mineral samples, have been found by HREM. In the observed structures the slabs 4 and 7 octahedra wide were the most common. Slabs with width of 8 and 5 octahedra were also encountered, but not those with 6 octahedra. The material was sensitive to the electron beam and the structures often decomposed to galena during the observation. Disappearance of the twin boundary was accompanied by formation of an edge dislocation.</p> <p>The structures in the PbS-rich part of the PbS-Sb₂S₃ system are composed of ribbons made of back to back square pyramids and arranged in three kinds of topology thus forming three homologous series. The structures of 5 phases were determined and the cation occupancies in the ribbons show that lead atoms occupy the 8- or 7-coordinated sites where the ribbons join, edge to face, while antimony atoms prefer the 7-coordinated sites where the parallel parts of the ribbons overlap.</p> <p>The observations of the topology and strength of bonding in the PbS-Sb₂S₃ structures served as basis for developing models in which the structures. built of ribbons, are bonded together in variety of ways. The model correctly predicts: 1) stability of three kinds of ribbon arrangements 2) stability limits of the structures in each homologous series 3) lead and antimony distribution in the structures.</p>