R. W. G. Wyckoff, Crystal Structures, Vols 1 to 6, Wiley (1971).
Figure 1.40 The CdCl2 structure.
The CdCl2 structure is closely related to that of CdI2 and differs only in the nature of the anion packing: Cl– ions are ccp in CdCl2 whereas I– is hcp in CdI2.
The CdCl2 structure may be represented by a hexagonal unit cell, although a smaller rhombohedral cell can also be chosen. The base of the hexagonal cell is of similar size and shape to that in CdI2 but its c axis is three times longer than c in CdI2. This is because in CdCl2, the Cd positions, and the CdCl6 octahedra, are staggered along c and give rise to a three‐layer repeat for Cd (CBA) and a six‐layer repeat for Cl (ABCABC), Fig. 1.40. In contrast, in CdI2, the Cd positions and the CdI6 octahedra are stacked on top of each other and the c repeat contains only two I layers (AB) and one Cd layer (C).
The unit cell of CdCl2 in projection down c is shown in Fig. 1.40(b). Cl layers occur at c = 0 (A), 2/12 (B) and 4/12 (C), and this sequence is repeated at c = 6/12, 8/12 and 10/12. Between those Cl layers at 0 and 2/12, Cd occupies octahedral sites at 1/12. However, the octahedral sites between Cl layers at 2/12 and 4/12 are empty (these sites, at c = 3/12, are directly below Cd at 9/12).
The CdCl2 structure is layered, similarly to CdI2, and many of the comments made about structure and bonding in CdI2 apply equally well. It also occurs with a variety of transition metal halides (Table 1.17).
The structure of Cs2O is most unusual as it is anti‐CdCl2. Cs forms ccp layers and O occupies the octahedral sites between alternate pairs of Cs layers. This raises some interesting questions because Cs is the most electropositive element and Cs salts are usually regarded as highly ionic. However, the structure of Cs2O clearly shows that Cs is not surrounded by oxygens, as expected for an ionic structure, but has only three O neighbours, all located at one side. The structure is held together, in 3D, by bonding between Cs in adjacent layers.
Table 1.17 Some compounds with the CdCl2 structure
| Compound | a/Å | c/Å | Compound | a/Å | c/Å |
|---|---|---|---|---|---|
| CdCl2 | 3.854 | 17.457 | NiCl2 | 3.543 | 17.335 |
| CdBr2 | 3.95 | 18.67 | NiBr2 | 3.708 | 18.300 |
| CoCl2 | 3.544 | 17.430 | NiI2 | 3.892 | 19.634 |
| FeCl2 | 3.579 | 17.536 | ZnBr2 | 3.92 | 18.73 |
| MgCl2 | 3.596 | 17.589 |
ZnI2
|
