Cesium chloride coordination number

Cesium, chloride (CsCl) structure (Fig. 4-H)- The CsCl structure can be described as interpenetrating simple cubic arrays of Cs+ and CP. Again, the Cs+ and CP positions are fully interchangeable. The structure is sometimes wrongly called body-centered cubic (bcc). The terminology is appropriate only when the shaded and unshaded atoms of Fig. 4.11 are identical, as in Fig. 4.8. In any case, the coordination number is eight for any atom. The unit cell of CsCl contains one net CsCl unit. 

With larger cations, such as cesium, the radius ratio (rCl /rcl-= 181 pm/ 167 pm = 1.08) increases beyond the acceptable limit for a coordination number of 6 the coordination number of the cations (and anions) increases to 8, and the cesium chloride lattice (Fig. 4.1b) results. As we have seen, although this is an efficient structure for cations and anions of about the same size, it cannot be directly related to a closest packed structure of anions... 

Alternatively, we might examine the radius ratio of Oj BF and get a crude estimate of = 0.8. The accuracy of our values does not permit us to choose between coordination number 6 and 8, but since the value of the Madelung constant does not differ appreciably between the sodium chloride and cesium chloride structures, a value of 1.75 may be taken which will suffice for our present rough calculations. We may then use the Bom-Lande equation (Eq. 4.13), which provides an estimate of —616 kJ mor1 for the attractive energy, which will be decreased by about 10% (if... 

Fig. 5.44). The coordination number of each type of ion is 8, and overall the structure has (8,8)-coordination. The cesium-chloride structure is much less common than the rock-salt structure, but it is found for Csl as well as CsCl. 

Cesium chloride crystallizes with a structure derived from the simple cubic primitive cell. Ch ions occupy the 8 comer sites with Cs+ in the center of the cell note that this is not a body-centered cubic unit cell since the ion at the center is not the same as those at the comers. Thus there is one CsCl unit per unit cell and the coordination numbers of Cs+ and Ch are both 8. Crystals of CsBr and Csl adopt the CsCl structure, but all other alkali halides crystallize in the NaCl structure. 

Solution X-ray diffraction measurements for saturated aqueous solutions of the KCl-MgCl2-6H20 and CsCl-MgCl2-6H20 double salts at 25°C reveal that magnesium(II) ions in the solutions are fully hydrated as [Mg(H20)6]2+ with a Mg-0 bond length of 208-209 pm. This is essentially the same bond length as in the double salt crystals, and the K+ and Cs+ ions have both water molecules and chloride ions in their first coordination sphere. The coordination numbers for water molecules and chloride ions around a K+ ion are 4.7 and 2.4, respectively, and those around a Cs+ ion are 4.7 and 2.0, respectively. The K+-OH2 and K+-C1 interatomic distances are found to be 227 and 320 pm, respectively, and the Cs+-OH2 and Cs + -Cl distances are 315 and 339 pm, respectively (58). The interatomic distances determined are essentially the same as those that have been reported in the literature for aqueous solutions of potassium and cesium salts. 

The only alkali metal halides that do not adopt the NaCl structure are CsCl, CsBr, and Csl, formed from the largest alkali metal cation and the three largest halide ions. These crystallize in the cesium chloride structure (shown here for CsCl). This structure has been used as an example of how dispersion forces can dominate in the presence of ionic forces. Use the ideas of coordination number and polarizability to explain why the CsCl structure exists. 

Three-dimensional lattices of the same symmetry class characterize the two polymorphic forms, but the unit cells exist with different coordination numbers and different coordination polyhedra. The classic example of this type of behavior is given by cesium chloride, which undergoes a reversible transformation from a cubic body-centered lattice to a cubic face-centered lattice at 445°C [14], On the body-centered modification, two simple primitive cubic lattices (one of Cs cations and one of Cl anions) are placed inside one another so that the comers of one kind of cube are situated at the centers of cubes of the other kind. The face-centered modification is built up from face-centered ionic lattices situated inside one another. 

Figure 9.7 shows the lattices, packing models, and unit cells of both sodium chloride and cesium chloride crystals. Sodium chloride crystallizes as a face-centered cubic compound lattice with alternating sodium and chloride ions. (However, see Problems 12 and 13, page 164.) The coordination number of each ion is 6 the nearest neighbors of any one ion are 6 ions of the other kind. Cesium chloride is a body-centered cubic lattice in which the central ion s nearest neighbors are the eight ions of the other kind. 

What is the coordination number in the cesium chloride cubic structure ... 

Determine the coordination number(s) of the ions in the fluorite and rutile unit cells. Why are there two unequal coordination numbers, whereas for cesium chloride, sodium chloride, and zincblende unit cells there is only one coordination number ... 

Although some compounds would appear to have a coordination number 3, only a few, on close inspection, actually do. For example, cesium trichlorocuprate(II), Cs[CuCl3], is actually made up of chloride-bridged chains of tetrahedral [CuCb] units. Genuine examples of coordination number 3 often involve large, bulky ligands. Two relatively simple examples are given in Table 3.1. 

Cesium chloride has a radius ratio of 1.08 because, using Shannon-Prewitt radii, the cesium cation is larger than the chloride anion. In this case, we should actually calculate r lr (= 0.93) and assume that the cations form the A-type lattice and the chlorides fill the appropriate holes. Note that 0.93 falls in the cubic hole/C.N. = 8 range of Table 7.4. As shown in Figure 7.21e, the cesium cations form a simple cubic lattice, and the chloride anions occupy the cubic holes. Alternatively, the chloride anions can be pictured as forming the A-type lattice with the cesium cations in the cubic holes. Using the solid lines as the unit cell, note that the coordination number of both the cation and anion is 8. Note also that there is a total of one [8( )] chloride per unit cell and, of course, one cesium cation in the body consistent with a 1 1 stoichiometry. Table 7.9 shows that the greatest correlation (100%) between the known structure and calculated radius ratios occurs for the CsCl structure. 

If the cation to anion ratio is between 1 and 0.732, the coordination number can be 8 and the compound will form an interpenetrating simple cubic structure with the anions (or cations) at the comers of the cube and the counterion in the center of the cube as in Figure 5.5. This is known as the cesium chloride structure and it is not a bcc structure because the ion in the middle is not the same as those on the comers. The space group is Pm3m. 

Figure 12.3 shows a unit cell for the cesium chloride (CsCl) crystal structure the coordination number is 8 for both ion types. The anions are located at each of the corners of a cube, whereas the cube center is a single cation. Interchange of anions with cations, and vice versa, produces the same crystal structure. This is not a BCC crystal structure because ions of two different kinds are involved. 

---