Cadmium iodide lattice

The hep structure consists of a stacking of atomic layers in the sequence ABABAB- . The octahedral holes are located between adjacent layers, as shown in Fig. 10.2.3(a). In the crystal structure of nickel asenide, the As atoms constitute a hep lattice, and the Ni atoms occupy all the octahedral holes. In contrast, cadmium iodide, Cdl2, may be described as a hep of 1 anions, in which only half the octahedral holes are occupied by Cd2+ ions. The manner of occupancy of the octahedral interstices is such that entire layers of octahedral interstices are filled, and these alternate with layers of empty... 

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. 

Ta2C has two crystal structures. The low temperature form shows an ordering of the carbon atoms such that the hexagonal metal layers are separated by alternately completely filled and completely empty carbon layers. This is the cadmium iodide antitype structure found by A. L. Bowman et al. (1965). However, the difference between the x-ray pattern of this phase and an L3 structure is very slight. Near 2000° (Rudy and Harmon, 1965) the carbon lattice presumably disorders to give the L 3 structure. 

Solid cadmium(II) iodide Cdlj has a layer lattice —a structure intermediate between one containing Cd " and P ions and one containing Cdl2 molecules—and this on vaporisation gives linear, covalent I—Cd—I molecules. In solution, iodo-complexes exist, for example... 

The halides of cadmium demonstrate the effect on structure of the easier polarisation of an anion by a smaller cation. Cdp2 has the cubic fluorite lattice but the chloride, bromide and iodide form hexagonal crystals based on layer lattices (p. 150). The distances between the layers increase from the chloride to the iodide, with a corresponding reduction in lattice energies. 

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