Caesium chloride lattice

Attraction,by Coulomb forces does not lead to valency saturation. If a positive ion is attracted by a negative ion, that does not prevent it from being attracted to other negative ions. Thus with electrostatic attraction it is not the normal valency which is characteristic, but the number of surrounding ions. In the crystal lattice of sodium chloride, each chloride ion is surrounded by six sodium ions and in the caesium chloride lattice eight caesium ions surround one ion of chlorine. The numbers 6 and 8 bear no relationship to the number of unpaired electrons. 

Figure 15.10 A model of the caesium chloride lattice red balls represent caesium Ions green balls represent chloride ions... 

The reaction between dry phosphine and hydrogen iodide, first described in 1817 by J. J.Houtonde la Billardiere produces phosphonium iodide. The simplest laboratory preparation of this compound is by the hydrolysis of an intimate mixture of diphosphorus tetraiodide and white phosphorus According to X-ray diffraction investigations, phosphonium iodide crystallises in a caesium chloride type lattice 3m,32s). 326) hydrogen atoms... 

The second common structure is the caesium chloride structure. In this structure, ions of one type are located at the corners of a cubic lattice... 

Table CLH gives the values of d d U for different ammonium salts together with the values of P calculated by means of equation hi. It is interesting to notice that the values increase in the order I, Br, Cl and F, and that the value for the fluoride is significantly greater than the other values. NH4CI, NH4Br, NH4I crystallize in lattices of the sodium chloride and caesium chloride type. The lattice of NH4F, however, is of the tetrahedral, wurtzite type. The NH4 ion is tetrahedral and in NH4F,...

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. 

By examining Figures 3.7 and 3.32, we note that the caesium cations sit on a primitive cubic unit cell (lattice type P) with chloride anion occupying the cubic hole in the body centre. Alternatively, one can view the structure as P-type lattice of chloride anions with caesium cation in cubic hole. Keep in mind that caesium chloride does not have a body centred cubic lattice although it might appear so at a first glance. The body centred lattice has all points identical, whereas in CsCl lattice the ion at fte body centre is different from those at the comers. 

The structure of caesium chloride is included here because, although it is not close packed, it is often confused with, and written as, body centred when it is not. The structure of caesium chloride is shown in Figure 1.17. The chloride ions are on the cube comers and the ion at the centre is a caesium. In Section 1.4 we saw that a body-centred cubic lattice refers to an identical set of points with identical atoms at the comers and at the centre of the cube. This means that the stmcture of caesium chloride is not body-centred cubic. Many alloys, such as brass (copper and zinc) possess the caesium chloride structure. 

Q Use atom counting to determine the number of formula units of caesium chloride per unit cell, and hence determine the lattice type. 

Fig. 3.08. The electrostatic component of the lattice energy of the caesium chloride, sodium chloride and zincblende ionic structures as a function of the radius ratio r+/r (r assumed constant). The energy values are negative on an arbitrary scale, the zero of energy being above the top of the figure.

The radius ratio r+jr for each of the alkali halides is shown in table 3.03. Consideration of these values reveals that CsCl, CsBr and Csl would, indeed, be expected to have the caesium chloride structure, and that the majority of the remaining halides would be expected to show the sodium chloride arrangement. There are, to be sure, a number of halides with r+jr > 0 7 which, nevertheless, have the sodium chloride rather than the caesium chloride structure. Fig. 3.08, however, emphasizes that energetically there is little difference between these two structures when the radius ratio is large, and there are in any case other factors contributing to the lattice energy which we have so far ignored in our discussion. 

The second structure common to a number of T1-B1 systems is that of sodium thallide, sometimes called the Zintlphase. This structure (fig. 13.12) is closely related to that of caesium chloride in that the pattern of sites occupied forms a cubic body-centred lattice. The distribution of the atoms, however, is such that each atom has four neighbours of each kind, and the true cell is therefore the larger unit shown, containing sixteen instead of only two atoms. Some phases in which the sodium thallide structure occurs are LiZn, LiCd, LiAl, LiGa, Liln, Naln and NaTl. It is a characteristic feature of all of these phases that in them the alkali metal atom appears to have a radius considerably smaller than in the structure of the element (even when allowance is made for the change in co-ordination number), suggesting that this atom is present in a partially ionized condition and that forces other than purely metallic bonds are operative in the structure. 

The caesium-chloride structure can be considered to be derived from the ccp structure by having Cl-ions occupy all the primitive lattice points and octahedral sites, with all tetrahedral sites occupied by Cs+ ions. This is exceedingly difficult to visualize and describe without carefully constructed figures or models. Refer to S.-M. Ho and B. E. Douglas, J. Client. Educ. 46, 208, 1969, for the appropriate diagrams. 

Because the Madelung constant has been computed by a summation over all lattice sites, it adopts characteristic values for all structure types [5,8]. To give a few examples, M arrives at (dimensionless) values of 1.6381 (zinc-blende-type), 1.7476 (sodium chloride-type), 1.7627 (caesium chloride-type), 5.0388 (fluorite-type), and 25.0312 (corundum-type) and does not scale with (= is independent of) the interionic distances. For the case of NaCl, the Madelung constant shows that the three-dimensional lattice surpasses the ionic pair in energy by almost 75%. This is what has made the formation of solid NaCl possible, a collective stabilization. 

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). 

---