Halides Wurtzite structure

For a 1 1 solid MX, a Schottky defect consists of a pair of vacant sites, a cation vacancy, and an anion vacancy. This is presented in Figure 5.1 (a) for an alkali halide type structure the number of cation vacancies and anion vacancies have to be equal to preserve electrical neutrality. A Schottky defect for an MX2 type structure will consist of the vacancy caused by the ion together with two X anion vacancies, thereby balancing the electrical charges. Schottky defects are more common in 1 1 stoichiometry and examples of crystals that contain them include rock salt (NaCl), wurtzite (ZnS), and CsCl. 

V. Oxides and Halides with the Wurtzite Structure Surface Structures, Reactivity, and Catalytic Activity... 

At a low temperature NH Br and NH I have a third form which is tetragonal. Hydrogen bonding imposes a wurtzite structure on NH4F (Fig. 214). The copper(I) halides have the zinc blende structure. 

Ammonium fluoride, NH4F, crystallizes with a structure different from those of the other ammonium (and alkali) halides. The chloride, bromide, and iodide have the CsCl structure at temperatures below 184 3°, 137-8 , and — IT S C respectively, and the NaCl structure at temperatures above these transition points, but NH4F crystallizes with the wurtzite structure, in which each N atom forms N-H—F bonds of length 2-71 to its four neighbours arranged tetrahedrally around it. This is essentially the same structure as that of ordinary ice. 

The structures of a number of ammonium salts have already been described. In many of them the NH4+ ion is in free rotation and can be treated as a spherical ion of radius 1 48 A so that the structures observed are determined by the usual geometrical considerations. Thus most of the ammonium halides have either the sodium chloride or the caesium chloride structure. In ammonium fluoride, however, the cation is not in free rotation and the wurtzite structure results. 

Copper(I) fluoride is not known CuCl, CuBr and Cul are white solids and are made by reduction of a Cu(II) salt in the presence of halide ions, e.g. CuBr forms when SO2 is bubbled through an aqueous solution of CUSO4 and KBr. Copper(I) chloride has a zinc blende structure (see Figure 5.18). The y-forms of CuBr and Cul adopt the zinc blende structure but convert to the jB-forms (wurtzite structure, Figure 5.20) at 660 and 690 K respectively. Values of iCsp(298K) for CuCl, CuBr and Cul are 1.72x10 , 6.27 X 10 and 1.27 x 10. Copper(I) iodide precipitates when any Cu(II) salt is added to KI solution (equation 21.102). 

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. 

A few other monohalides possess the NaCl or CsCl stracture, e.g. AgF, AgQ, and we have already discussed (Section 6.15) that these silver(I) halides exhibit significant covalent character. The same is true for CuCl, CuBr, Cul and Agl which possess the wurtzite structure (Fig. 6.21). 

Figure 4. Structures of ammonium halides (a) CsCl-type of structure shown by NH4CI, NFUBr, and NH4I (b) wurtzite (ZnS) structure, shown by NH4F, and induced by the formation of N-H--F hydrogen bonds.

Rg. 4.18 Actual crystal structures of Ihe alkali halides (as shown by the symbols) contrasted with the predictions of the radius ratio rule. The figure is divided into three regions by Ihe lines r /r — 0.414 and rt/r. — 0.732, predicting coordination number 4 (wurtzite or zinc blende, upper left), coordination number 6 (rock salt, NaO, middle), and coordination number 8 (CsCI. lower right). The crystal radius of lithium, and to a lesser extent that of sodium, changes with coordination number, so both the radii with C.N. = 4 (left) and CN. => 6 (right) have been plotted. 

The most important MX structures involving tetrahedral coordination are the cubic ZnS sphalerite (Fig. 5a) and the hexagonal ZnS wurtzite (Fig. 5b) arrangements. It is striking that halides and sulfides of metal ions with d5 and d10 shells have a tendency to crystallize in the sphalerite structure for example, the cuprous halides, Agl, HgS, MnS, CdS, and ZnS. (The last three also occur in the wurtzite modification, as do the oxides of Zn and Be). (See Table V.) Here again, the simple ionic theory fails to account for the facts for (1) the radius ratios of some of these compounds are compatible with a 6-coordinated structure, and (2) interatomic distances calculated from the usual ionic radii (decreased by 5% to com-... 

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