projection of one cation layer (layer 1). The displayed region is 3 × 3 unit cells wide. The atom and polyhedra colors correspond to those used in Fig. 6. The purple polyhedra on the left are Pb and Cu polyhedra belonging to the above layer (layer 2) and are displayed to show the way in which the layers superimpose. The letters Sn and Zn represent straight and zigzag chains (see text), while the subscript indicates the layer.