Cesium chloride arrangement

The three structures already described (the zinc blende, sodium chloride, and cesium chloride arrangements) must of necessity be confined to compounds of simplest formula AiCv The two most important type structures for compounds of type, AC2 (or A2C), are the fluorite (CaF2) and rutile (Ti02) structures. 

The Transition to the Sphalerite Structure.—The oxide, sulfide and selenide of beryllium have neither the sodium chloride nor the cesium chloride structure, but instead the sphalerite or the wurzite structure. The Coulomb energy for the sphalerite arrangement is... 

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. 

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 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,... 

Several crystalline ternary intermetallic compounds ate presently used in engineering applications. The ternary phase Ni2Mnln (Pearson symbol cF16) is illustrated in Figure 3.26a. Each atom is located on the site of a cesium-chloride cubic lattice. The unit cell consists of a FCC arrangement of nickel atoms with one additional nickel... 

The structure of sodium chloride, which is the prototype for most of the alkali halides, is best described as a cubic closest packed array of Cl- ions with the Na+ ions in all of the octahedral holes [see Fig. 16.42(b)]. The relative sizes of these ions are such that rua 0.66i ci-> so this solid obeys the guidelines given previously. Note that the CP ions are forced apart by the Na+ ions, which are too large for the octahedral holes in the closest packed array of CP ions. Since the number of octahedral holes is the same as the number of packed spheres, all the octahedral holes must be filled with Na+ ions to achieve the required 1 1 stoichiometry. Most other alkali halides also have the sodium chloride structure. In fact, all the halides of lithium, sodium, potassium, and rubidium have this structure. Cesium fluoride has the sodium chloride structure but because of the large size of Cs+ ions, in this case the Cs ions form a cubic closest packed arrangement with the F ions in all the octahedral holes. On the other hand, cesium chloride, in which the Cs+ and CP ions are almost the same size, has a simple cubic structure of CP ions, with each Cs+ ion in the cubic hole in the center of each cube. The compounds cesium bromide and cesium iodide also have this latter structure. 

When the ions are nearly equal in size, a cubic arrangement of anions with the cation in the body center results, as in cesium chloride with CN = 8. Although a close-packed structure (ignoring the difference between cations and anions) would seem to give larger attractive forces, the CsCl structure separates ions of the same charge, reducing the repulsive forces between them. 

Analysis of many different salts show that the salts all have ordered packing arrangements, such as those described earlier for NaCl and CaF2. Another example is the salt cesium chloride, where the ratio of cations to anions is 1 1 just as it is in sodium chloride. However, the size of a cesium cation is larger than that of a sodium cation. As a result, the structure of the crystal lattice is different. In sodium chloride, a sodium cation is surrounded by six chloride anions. In cesium chloride, a cesium cation is surrounded by eight chloride anions. The bigger cation has more room around it, so more anions can cluster around it. 

Most salts crystallize as ionic solids with ions occupying the unit cell. Sodium chloride (Figure 13-28) is an example. Many other salts crystallize in the sodium chloride (face-centered cubic) arrangement. Examples are the halides of Li+, K+, and Rb+, and M2+X2 oxides and sulfides such as MgO, CaO, CaS, and MnO. Two other common ionic structures are those of cesium chloride, CsCl (simple cubic lattice), and zincblende, ZnS (face-centered cubic lattice), shown in Figure 13-29. Salts that are isomorphous with the CsCl structure include CsBr, Csl, NH4CI, TlCl, TlBr, and TIL The sulfides of Be2+, Cd2+, and Hg2+, together with CuBr, Cul, Agl, and ZnO, are isomorphous with the zincblende structure (Figure 13-29c). 

The ammonium ion can take the place of a monatomic cation in an ionic crystal struaure. For example, the crystal structure of ammonium chloride, NH4CI, which is found in fertilizers, is very similar to the crystal struaure of cesium chloride, CsCl, which is used in brewing, mineral waters, and to make fluorescent screens. In each structure, the chloride ions form a cubic arrangement with chloride ions at the corners of each... 

In fact, only CsCi, CsBr, and Csl, under normal conditions, possess the bcc structure. Cesium chloride crystallizes with the rock-salt structure (cfc) at temperatures above 445°C. This indicates that the bcc and cfc structures have similar energies. One should note in Table 14 the too-weak values obtained for LiC , LiBr, and Lil, which would crystallize in the zinc-blende or wurtzite structures. Finally, let us note that the distances between nearest neighbors in the arrangements bcc and cfc, observed in halides possessing both structures, are practically the same AgF, in spite of the value p 0.9, crystallizes in the cfc structure. 

Three common ionic structure types are shown in FIGURE 12.26. The cesium chloride (CsCl) structure is based on a primitive cubic lattice. Anions sit on the lattice points at the comers of the unit cell, and a cation sits at the center of each cell. (Remember, there is no lattice point inside a primitive unit cell.) With this arrangement, both cations and anions are surrounded by a cube of eight ions of the opposite type. 

Cesium chloride consists of positive ions and negative ions of about equal size arranged in a cubic array. Figure 11.41 shows the unit cell of cesium chloride. It shows both a space-filling model (ions are shown as spheres of approximately the correct relative sizes) and a model in which the ions are shrunk in size in order to display more clearly their relative positions. Note that thrae are chloride ions at each comer of the cube and a cesium ion at the center. Altanatively, the unit cell may be taken... 

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