Crystals cesium chloride

FIGURE 5.42 The repetition of the cesium chloride unit cell recreates the entire crystal. This view is from one side of the crystal and shows several unit cells stacked together. 

Are the following statements true or false (a) Because cesium chloride has chloride ions at the corners of the unit cell and a cesium ion at the center of the unit cell, it is classified as having a body-centered unit cell, (b) The density of the unit cell must be the same as the density of the bulk material, (c) When x-rays are passed through a single crystal of a compound, the x-ray beam will be diffracted because it interacts with the electrons in the atoms of the crystal, (d) All the angles of a unit cell must be equal to 90°. 

Huggins, who has particularly emphasized the fact that different atomic radii are required for different crystals, has recently [Phys. Rev., 28, 1086 (1926)] suggested a set of atomic radii based upon his ideas of the location of electrons in crystals. These radii are essentially for use with crystals in which the atoms are bonded by the sharing of electron pairs, such as diamond, sphalerite, etc. but he also attempts to include the undoubtedly ionic fluorite and cesium chloride structures in this category. 

Inter-Atomic Distances for Cesium Chloride Type Crystals... 

The Sodium Chloride and Cesium Chloride Structures.—The agreement found between the observed inter-atomic distances and our calculated ionic radii makes it probable that the crystals considered are built of only slightly deformed ions it should, then, be possible, with the aid of this conception, to explain the stability of one structure, that of sodium chloride, in the case of most compounds, and of the other, that of cesium chloride, in a few cases, namely, the cesium and thallous halides. 

The elucidation of the factors determining the relative stability of alternative crystalline structures of a substance would be of the greatest significance in the development of the theory of the solid state. Why, for example, do some of the alkali halides crystallize with the sodium chloride structure and some with the cesium chloride structure Why does titanium dioxide under different conditions assume the different structures of rutile, brookite and anatase Why does aluminum fluosilicate, AljSiCV F2, crystallize with the structure of topaz and not with some other structure These questions are answered formally by the statement that in each case the structure with the minimum free energy is stable. This answer, however, is not satisfying what is desired in our atomistic and quantum theoretical era is the explanation of this minimum free energy in terms of atoms or ions and their properties. 

Figure 9.2 is schematic diagram of the crystal structure of most of the alkali halides, letting the black circles represent the positive metal ions (Li, Na, K, Rb, and Cs), and the gray circles represent the negative halide ions (F, Cl, Br, and I).The ions lie on two interpenetrating face-centered-cubic lattices. Of the 20 alkali halides, 17 have the NaCl crystal structure of Figure 9.1. The other three (CsCl, CsBr, and Csl) have the cesium chloride structure where the ions lie on two interpenetrating body-centered-cubic lattices (Figure 9.3). The plastic deformation on the primary glide planes for the two structures is quite different.

Figure 9.3 Cluster of unit cells of the cesium chloride crystal structure. This figure shows that ions of the same sign in this structure line up along the 100 directions. Thus the three rows are orthogonal to one another. Translation of a (100) plane of ions over its nearest (100) neighboring plane keeps ions of opposite sign adjacent to one another. This is also the case on the (110) planes, but the translation vector is V2 larger than for the the (100) planes.

In most ionic crystals, the anion is larger than the cation and, therefore, the packing of the anions determines the arrangement of ions in the crystal lattice. There are several possible arrangements for ionic crystals in which the anions are larger than cations, and cations and anions are present in equal molar amounts. For example. Figure 4.22 shows two different arrangements found in the structures of sodium chloride, NaCl, and cesium chloride, CsCl. 

In the sodium chloride crystal, each chloride ion (Cl ) is surrounded by six sodium ions (Na" ). Similarly, each Na" ion is surrounded by six Cl ions. In CsCl, each cesium ion (Cs" ) ion is surrounded hy eight Cl ions. 

Cesium chloride is prepared by the treatment of cesium oxide or any cesium salt with hydrochloric acid followed by evaporation and crystallization of the solution. 

If self-forming cesium salt gradients are used, take care to be within the solubility of the salt at the given temperature to avoid crystallizations during centrifugation, since salt crystals may destroy the rotor during run (solubilities of cesium chloride and sulfate are given in Table 5.4). 

The alkali halogenides all crystallize with the sodium chloride arrangement (Figs. 1-1, 13-4) except cesium chloride, bromide, and... 

Fig. 13-5.—The arrangement of cesium ions and chloride ions in the cesium chloride crystal.

The observed interionic distances for the cesium and rubidium halogenides (the latter bemg at high pressure) with the cesium chloride structure are compared with the crystal radius sums in Table 13-8. 

Table 13-8.—Interionic Distances fob Crystals with the Cesium Chloride Structure...

Fig. 4.1 Crystal structures of two 1 1 ionic compounds (a) unit cell of sodium chloride, cubic, space group Fm3m (b) unit cell of cesium chloride, cubic, space group Fm3m. [From Ladd, M.F C Structure and Bonding in Solid State Chemistry, Wiley New York, 1979. Reproduced with permission.]...

The cesium thlontle structure. Cesium chloride crystallizes in the cubic arrangement shown in Fig. 4.1b. The cesium or chloride ions occupy the eight comers of the cube and the counterion occupies the center of the cube.1 Again,... 

Fig. 4. Computer-generated crystal structure models nop row. left to right) Cuprite, zinc-blende, rutile, perovskite. iridymite (second row) Cristobalite. potassium dihydrogen phosphate, diamond, pyrites, arsenic (third rowt Cesium chloride, sodium chloride, wurtzite. copper, niccolite (fourth row) Spinel, graphite, beryllium, carbon dioxide, alpha i uanz. [AT T Bel Laboratories ...

Estimate the density of cesium chloride from its crystal structure. 

Cesium chloride crystallizes in a cubic unit cell with Cl- ions at the corners and a Cs+ ion in the center. Count the numbers of + and — charges, and show that the unit cell is electrically neutral. 

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