Cadmium iodide structure type

It is clear that the bonding in the cadmium chloride and cadmium iodide structures cannot be purely ionic, for adjacent atoms in neighbouring layers are of the same kind and the forces between them can be only of the van der Waals type. Indeed, this is also evident from... 

Titanium Dlbromide, Titanium dibromide [13873-04-5], a black crystalline solid, density 4310 kg/m3, mp 1025°C, has a cadmium iodide-type structure and is readily oxidized to trivalent titanium by water. Spontaneously flammable in air (142), it can be prepared by direct synthesis from the elements, by reaction of the tetrabromide with titanium, or by thermal decomposition of titanium tribromide. This last reaction must be carried out either at or below 400°C, because at higher temperatures the dibromide itself disproportionates. 

Fig. 7.8 Stereoview of the unit cell of the cadmium iodide, CdU, structure type hexagonal, space group P3m. Large circles, I small circles, Cd. (From Ladd, M. F. C. Structure and Bonding in Solid Stale Chemistry Ellis Horwood Chicester, 1979.

Many inorganic solids crystallize in layer-lattice structures cadmium iodide is frequently cited as the type example. The cadmium ions are arranged hexagonally in sheets, each with a sheet of hexagonally arranged iodide ions above and below. The separation of adjacent sheets of... 

The ionic lattices include also a large number of layer lattices in which the same heteropolar type of linkage is present in the layer plane as perpendicular to it but such that, through the polarizability of the ions, the linkages arc considerably greater in a given, network plane than they are perpendicular to it. An example of such a heteropolar layer lattice is the lattice of cadmium iodide, represented in Fig. 47, in which the strongly anisotropic conductivity, solubility, etc. are readily understandable from the structure. 

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