Solid State Chemistry and its Applications. Anthony R. West. Читать онлайн. Newlib. NEWLIB.NET

Автор: Anthony R. West
Издательство: John Wiley & Sons Limited
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Жанр произведения: Химия
Год издания: 0
isbn: 9781118695579
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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 c equals one quarter (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 c equals one quarter; three are at c equals negative one quarter and three at c equals three quarters.

Schematic illustration of the CdI2 structure.

       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/
CdI2 4.24 6.84 VBr2 3.768 6.180

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