Solid State Chemistry and its Applications. Anthony R. West

4.48 6.96 TiBr₂ 3.629 6.492 CoI₂ 3.96 6.65 MnBr₂ 3.82 6.19 FeI₂ 4.04 6.75 FeBr₂ 3.74 6.17 MgI₂ 4.14 6.88 CoBr₂ 3.68 6.12 MnI₂ 4.16 6.82 TiCl₂ 3.561 5.875 PbI₂ 4.555 6.977 VCl₂ 3.601 5.835 ThI₂ 4.13 7.02 Mg(OH)₂ 3.147 4.769 TiI₂ 4.110 6.820 Ca(OH)₂ 3.584 4.896 TmI₂ 4.520 6.967 Fe(OH)₂ 3.258 4.605 VI₂ 4.000 6.670 Co(OH)₂ 3.173 4.640 YbI₂ 4.503 6.972 Ni(OH)₂ 3.117 4.595 ZnI₂(l) 4.25 6.54 Cd(OH)² 3.48 4.67

      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	4.25	21.5
MnCl2	3.686	17.470	Cs2Oa	4.256	18.99

^a Cs₂O has an anti‐CdCl₂ structure.

      R. W. G. Wyckoff, Crystal