Solid State Chemistry and its Applications. Anthony R. West

2.9581 0.305 CoF₂ 4.6951 3.1796 0.306 CrO₂ 4.41 2.91 FeF₂ 4.6966 3.3091 0.300 GeO₂ 4.395 2.859 0.307 MgF₂ 4.623 3.052 0.303 IrO₂ 4.49 3.14 MnF₂ 4.8734 3.3099 0.305 β‐MnO₂ 4.396 2.871 0.302 NiF₂ 4.6506 3.0836 0.302 MoO₂ 4.86 2.79 PdF₂ 4.931 3.367 NbO₂ 4.77 2.96 ZnF₂ 4.7034 3.1335 0.303 OsO₂ 4.51 3.19 SnO₂ 4.7373 3.1864 0.307 PbO₂ 4.946 3.379 TaO₂ 4.709 3.065 RuO₂ 4.51 3.11 WO₂ 4.86 2.77

      R. W. G. Wyckoff, Crystal Structures, Vols 1 to 6, Wiley (1971).

      Two main groups of compounds exhibit the rutile structure, Table 1.15: oxides of tetravalent metals and fluorides of divalent metals. In both cases, the metals are too small to have eight coordination and form the fluorite structure. The rutile structure may be regarded as essentially ionic.

      The CdI₂ structure is nominally similar to that of rutile because it has an hcp anion array with also, half of the octahedral sites occupied by M²⁺ ions. The manner of occupancy of the octahedral sites is quite different, however; entire layers of octahedral sites are occupied and these alternate with layers of empty sites, Fig. 1.39. CdI₂ is therefore a layered material in both its crystal structure and properties, in contrast to rutile, which has a more rigid, 3D character.

      Two I layers in a hcp array are shown in Fig. 1.39(a) with the octahedral sites in between occupied by Cd. To either side of the I layers, the octahedral sites are empty. Compare this with NiAs [Fig. 1.35(d) and (h)] which has the same anion arrangement but with all octahedral sites occupied. The layer stacking sequence along c in CdI₂ is shown schematically in Fig. 1.39(b) and emphasises the layered nature of the CdI₂ structure: I layers form an … ABABA … stacking sequence. Cd occupies octahedral sites which may be regarded as the C positions relative to the AB positions for I. The CdI₂ structure is, effectively, a sandwich structure in which Cd²⁺ ions are sandwiched between layers of I^– ions; adjacent sandwiches are held together by weak van der Waals bonds between the I layers. In this sense, CdI₂ has certain similarities to molecular structures. For example, solid CCl₄ has strong C–Cl bonds within the molecule but only weak Cl–Cl bonds between adjacent molecules. Because the intermolecular forces are weak, CCl₄ is volatile with low melting and boiling points. In the same way, CdI₂ may be regarded as an infinite sandwich ‘molecule’ in which there are strong Cd–I bonds within the molecule but weak van der Waals bonds between adjacent molecules.

      The coordination of I in CdI₂ is shown in Fig. 1.39(c). An I at (shaded) has three close Cd neighbours to one side at c = 0. The next nearest neighbours are 12 I that form the hcp array: six are in the same plane, forming a hexagonal ring, at ; three are at and three at .

The layered nature of CdI₂ is emphasised further in a model of polyhedra: CdI₆ octahedra link at their edges to form infinite sheets, Fig. 1.39(d), but there are no direct polyhedral linkages between adjacent sheets. A self‐supporting, 3D model of octahedra cannot be made for CdI₂, therefore. Some compounds which have the CdI₂ structure are listed in Table 1.16. It occurs mainly in transition metal iodides, bromides, chlorides and hydroxides. TiS₂ has the CdI₂ structure and was considered as a potential intercalation host cathode for use in lithium batteries (see Section 8.4): Li⁺ ions are able to diffuse into the empty layers that separate adjacent TiS₂ sheets at the same time as electrons enter, and migrate through, the 3d band composed of d _xy orbitals on Ti.

       Figure 1.39 The CdI2 structure: (a) the basal plane of the hexagonal unit cell is outlined, with two possible choices of origin; (b) the layer stacking sequence; (c) the coordination environment of I; (d) a layer of close packed octahedra; empty tetrahedral sites are arrowed.

       Table 1.16 Some compounds with the CdI2 structure

Compound	a/Å	c/Å	Compound	a/Å	c/Å
CdI2	4.24	6.84	VBr2	3.768	6.180