Cadmium Iodide

Cadmium iodide is an ionic compound, the ionic radii of Cd and I being 0.97 Å and 2.16 Å respectively². The structure consists of a close- packing of the I ions with the Cd ions distributed among the octahedral voids. The radius ratio r _Cd/r _I = 0.45 permits the Cd ions to occupy the octahedral voids. Since there are only half as many Cd ions as I ions in the structure, only half of the total octahedral voids are occupied. Thus the Cd and I layers are not stacked alternately; there is one Cd layer after every two I layers as shown in Fig. 12. The structure therefore consists of molecular sheets (called minimal sandwiches) with a layer of Cd ions sandwiched between two close-packed layers of I ions. The binding within the minimal sandwich is ionic in character and is much stronger than the binding between successive sandwiches which is of van der Waals type. It is because of the weak van der Waals bonding between the successive minimal sandwiches that the material possesses the easy cleavage characteristic of a layer structure. Cadmium iodide structures can have a centre of symmetry in octahedral voids, but cannot have a symmetry plane perpendicular to [00.1]. Cadmium iodide can therefore have five possible space groups - P 3m 1, P $\overline{3}$m , R 3m , R $\overline{3}$m and P 6₃mc . Cubic symmetry is not possible in CdI₂ on account of the presence of Cd atoms. The most common modifications of CdI₂ are 4H and 2H with stacking sequences /A$\gamma$B C$\alpha$B/...., and /A$\gamma$B/A$\gamma$B/.... respectively, where the Greek letters denote the positions of Cd ions. In addition, this material also displays^2,19 a number of polytype modifications of large repeat periods. From the structure of CdI₂ it follows that the identity period of all such modifications must consist of an even number of iodine layers. The h/a ratio in all these modifications of CdI₂ is 0.805 which differs considerably from the ideal value of 0.8165.

Figure 12: The layer structure of CdI₂: small circles represent Cd ions and larger ones the I ions (after A. F. Wells, Structural Inorganic Chemistry, Clarendon Press: Oxford, 1945).

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