Zinc-blende = sphalerite

C, b.p. 907"C, d 713. Transition element occurring as zinc blende, sphalerite (Zn,Fe)S calamine or smithsonite (ZnCO j), willemite (Zo2Si04), franklinite (ZnFe204). Extracted by roasting to ZnO and reduction with carbon. The metal is bluish-white (deformed hep) fairly hard and brittle. Burns... 

Zink-harz, n. zinc blende, sphalerite, -hutte,/. zinc works, zinc smeltery, -hiittearauch, m. zinc (smelter) fume. 

FIGURE 5.43 Hie zinc-blende (sphalerite) structure, rhe tour zinc ions (pink) form a tetrahedron within a face-centered cubic unit cell composed of sulfide ions (vellow).The zinc ions occupy half the tetrahedral holes between the sulfide ions, and the parts or the unit cell occupied by zinc ions are shaded blue. The detail shows how each zinc ion is surrounded by four sulfide ions each sulfide ion is similarly surrounded by four zinc ions. 

For compounds of the composition MX (M = cation, X = anion) the CsCl type has the largest Madelung constant. In this structure type a Cs+ ion is in contact with eight Cl-ions in a cubic arrangement (Fig. 7.1). The Cl- ions have no contact with one another. With cations smaller than Cs+ the Cl- ions come closer together and when the radius ratio has the value of rM/rx = 0.732, the Cl- ions are in contact with each other. When rM/rx < 0.732, the Cl- ions remain in contact, but there is no more contact between anions and cations. Now another structure type is favored its Madelung constant is indeed smaller, but it again allows contact of cations with anions. This is achieved by the smaller coordination number 6 of the ions that is fulfilled in the NaCl type (Fig. 7.1). When the radius ratio becomes even smaller, the zinc blende (sphalerite) or the wurtzite type should occur, in which the ions only have the coordination number 4 (Fig. 7.1 zinc blende and wurtzite are two modifications of ZnS). 

By substituting alternately the carbon atoms in cubic diamond by zinc and sulfur atoms, one obtains the structure of zinc blende (sphalerite). By the corresponding substitution in hexagonal diamond, the wurtzite structure results. As long as atoms of one element are allowed to be bonded only to atoms of the other element, binary compounds can only have a 1 1 composition. For the four bonds per atom an average of four electrons per atom are needed this condition is fulfilled if the total number of valence electrons is four times the number of atoms. Possible element combinations and examples are given in Table 12.1. 

Structure of cubic (left) and hexagonal (right) diamond. Top row connected layers as in a-As. Central row the same layers in projection perpendicular to the layers. Bottom unit cells when the light and dark atoms are different, this corresponds to the structures of zinc blende (sphalerite) and wurtzite, respectively... 

Sodium chloride structure crystals have all octahedral sites filled, and so cation diffusion will be dependent upon vacancies on octahedral sites. In the zinc blende (sphalerite) structure, adopted by ZnS, for example, half of the tetrahedral sites are empty, as are all of the octahedral sites, so that self-diffusion can take place without the intervention of a population of defects. 

Figure 5.15 Anion-centered polyhedron (rhombic dodecahedron) found in the cubic closest-packed structure (a) oriented with respect to cubic axes, the c axis is vertical (b) oriented with [111] vertical (c) cation positions occupied in the sodium chloride, NaCl, structure and (id) cation positions occupied in the zinc blende (sphalerite) cubic ZnS structure.

Wurtzite structure. Zinc sulfide can also crystallize in a hexagonal form called wurtzite that is formed slightly less exothermically than the cubic zinc blende (sphalerite) modification (Afff = —192.6 and —206.0 kJ mol-1, respectively) and hence is a high temperature polymorph of ZnS. The relationship between the two structures is best described in terms of close packing (Section 4.3) in zinc blende, the anions (or cations) form a cubic close-packed array, whereas in wurtzite they form hexagonal close-packed arrays. This relationship is illustrated in Fig. 4.13 note, however, that this does not represent the actual unit cell of either form. 

The structure of cubic zinc sulfide (zinc blende, sphalerite) may be described as a ccp of S atoms, in which half of the tetrahedral sites are filled with Zn atoms the arrangement of the filled sites is such that the coordination numbers of S and Zn are both four, as shown in Fig. 10.1.7. The crystal belongs to space group 7 2 — / 43m. Note that the roles of the Zn and S atoms can be interchanged by a simple translation of the origin. 

Sphalerite group Sphalerite (zinc-blende)- Sphalerite (p-ZnS)... 

Zinc blende (sphalerite, ZnS) has a diamond-type structure. The space group is F43m for a cubic unit cell with a = 5.42 A. The structure is illustrated in Figure 14.20. Parallel to the (100) face of zinc blende... 

N = NaCl structure W = wurtzite structure Z = zinc-blende (sphalerite) structure... 

Figure 7.22 Three-dimensional nets (a) the cubic diamond structure (b) the net equivalent to (a) (c) the cubic zinc blende (sphalerite) structure (d) the net equivalent to (c), which is identical to that in (b) (e) the hexagonal wurtzite structure (f) the net equivalent to (e)...

The structure of both the SiC and ZnS polytypes can be illustrated with reference to the crystalline forms of ZnS. Zinc sulphide crystallises in either of two structures, one of which is cubic and given the mineral name zinc blende (sphalerite) while the other is hexagonal and given the mineral name wurtzite. The relationship... 

Silicon carbide, carborundum, also crystallises in two forms, of which /(-SiC has the cubic zinc blende (sphalerite) structure (Figure 8.8a). When viewed along the cube face-diagonal [110] direction, the layers of both silicon and carbon are packed in the cubic closest packing arrangement. .. aAbBcCaAbBcC. .., where the uppercase and lowercase letters stand for layers of Si and C. The other form of silicon carbide, a-SiC, is a collective name for the various silicon carbide polytypes, which consist of complex arrangements of zinc blende and wurtzite slabs. Some of these are known by names such as carborundum I, carborundum II, carborundum III, and so on. One of the simplest structures is that of carbo-... 

Figure 32. The wurtzite type structure of a-BeO, space group Ptjmc. The three-dimensional network of condensed Be04-tetrabedra is outlined. The stacking of these tetrahedra has the sequence AB, AB. The corresponding stacking ABC, ABC is known to occur for the zinc blende (sphalerite) structure (Fig. 3).

Figure 4.5 The crystal structure of zinc-blende (sphalerite) ZnS. The compounds Agl, BC, BN,...

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