Layer structures with Cdl2 lattice

Zn and Cd halides (apart from fluorides) are based on close-packed lattices of halide ions, with Zn occupying tetrahedral holes and Cd octahedral ones. The Zn compounds are best regarded as polymeric, whereas those of Cd are prototypes of the important CdCl2 and Cdl2 layer structures. 

The CdCl2 lattice is related to the Cdl2 layer-structure but with the Cl ions in a cubic close-packed arrangement. Examples of compounds adopting this structure are FeCl2 and C0CI2. Other layer structures include talc and mica (see Section 13.9). 

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... 

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