Crystal structures caesium chloride

Although the caesium chloride arrangement always occurs when the ratio of radii is approximately unity, the crystal structure observed is not always the one that is expected, and the other alkali... 

All the alkali metal halides except the cliloride, bromide and iodide of caesium form cubic crystals with the rock salt lattice and show a co-ordination number of 6. The exceptions are also cubic, but have the caesium chloride structure (Fig. 133) characterised by a co-ordination number of 8. The radius ratio for CsCl, Cs /Cl" = 0.93, allows 8 co-ordination, but is so near the ratio for 6 co-ordination that caesium chloride is dimorphous, changing, at 445°, from the caesium chloride to the rock salt structure. The crystalline halides are generally markedly ionic, though, as expected, lithium iodide is somewhat covalent, for iodide is the largest and most easily polarised simple anion and lithium, the smallest alkali metal cation, possesses the strongest polarising power. 

We have already considered in detail the structures of the alkali halides, all of which crystallize with either the sodium chloride or the caesium chloride arrangement ( 3.04 and 3.05). All of these compounds are essentially ionic, and the degree of ionic character depends on the difference in electronegativity of the atoms concerned it is thus a maximum in caesium fluoride and a minimum in lithium iodide. As we have seen, the radius ratio r+jr is the primary factor in determining whether a given halide possesses the sodium chloride or the caesium... 

To meet the difficulties presented by metal systems, various wider definitions of chemical combination in terms of crystal structure have been proposed. For example, it has been suggested that we should regard an ideal chemical compound as one in which structurally equivalent positions are occupied by chemically identical atoms, and an ideal solid solution as a structure in which all atoms are structurally equivalent. It is clear that such a definition of chemical combination embraces all the generally accepted compounds, but it is not without objection when applied to metal systems. Thus, to take only one example, the ft phase in the silver-cadmium system already discussed has, in its ordered state, the simple caesium chloride structure and must therefore... 

The relative sizes of the cation and anion determine the type of lattice an ionic compound adopts. For example, although caesium and sodium are both in the same group of the periodic table, the chlorides crystallize with different types of lattice. Sodium chloride adopts the simple cubic structure (Chapter 4), whereas caesium chloride adopts the lattice shown in Figure 15.10. In caesium chloride, the caesium ions cannot get as close to the chloride ions as the smaller sodium ions. Eight caesium ions can pack around a chloride ion if they are positioned at the corners of a cube. The structure of ionic lattices is determined by X-ray crystallography (see Chapter 21, and Chapter 22 on the accompanying website). 

Caesium is a large ion (ionic radii are discussed in detail later in Section 1.6.4) and so is able to coordinate eight chloride ions around it. Other compounds with large cations that can also accommodate eight anions and crystallize with this structure include CsBr, Csl, TlCl, TlBr, TII, and NH4CI. 

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