Wurtzite ZnS

Betyllium, because of its small size, almost invariably has a coordination number of 4. This is important in analytical chemistry since it ensures that edta, which coordinates strongly to Mg, Ca (and Al), does not chelate Be appreciably. BeO has the wurtzite (ZnS, p. 1209) structure whilst the other Be chalcogenides adopt the zinc blende modification. BeF2 has the cristobalite (SiOi, p. 342) structure and has only a vety low electrical conductivity when fused. Be2C and Be2B have extended lattices of the antifluorite type with 4-coordinate Be and 8-coordinate C or B. Be2Si04 has the phenacite structure (p. 347) in which both Be and Si... 

Fig. 1.2 Crystal structures of the major sulfides (metal atoms are shown as smaller or black spheres) (A) galena (PbS) structure (rock salt) (B) sphalerite (ZnS) structure (zinc blende) (C) wurtzite (ZnS) strucmre (D) pyrite structure and the linkage of metal-sulfur octahedra along the c-axis direction in (/) pyrite (FeSa) and (//) marcasite (FeSa) (E) niccolite (NiAs) structure (F) coveUite (CuS) structure (layered). (Adapted from Vaughan DJ (2005) Sulphides. In Selley RC, Robin L, Cocks M, Plimer IR (eds.) Encyclopedia of Geology, MINERALS, Elsevier p 574 (doi 10.1016/B0-12-369396-9/00276-8))...

Similar considerations may be made with reference to the other simple close-packed structure, that is to the hexagonal Mg-type structure. In this case two basic derived structures can be considered the NiAs type with occupied octahedral holes and the wurtzite (ZnS) type with one set of occupied tetrahedral holes (compare with the data given with an origin shift in 7.4.2.3.2). For a few more comments about interstices and interstitial structures see 3.8.4. See Fig. 3.35. 

Some of the discharged sulfide particles settle onto the chimney s exterior, where they are buried by the outward growth of anhydrite. Sulfide precipitation within the chimneys, causes copper, zinc, and iron sulfides to deposit and partially replace the anhydrite. Chimneys can build to several meters in height and their orifices range in diameter from 1 to 30 cm. Both the smoke and the chimneys are composed of polymetallic sulfide minerals, chiefly pyrrhotite (FeS), pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite or wurtzite (ZnS). 

FIGURE 1.38 The crystal structure of wurtzite, ZnS. Zn, blue spheres S, grey spheres. 

Half tetrahedral every alternate site occupied Zinc blende ZnS, CuCl, y-AgI Wurtzite ZnS, PAgI... 

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. 

A variant of CD has been described where ZnS was precipitated as a gel by adding concentrated S to a concentrated solution of a Cd salt. The pH was then reduced by HNO3 to a value between 5 and 7, when a semitransparent sol formed. Heating this sol between 100 and 200°C (in an autoclave) resulted in the formation of zincblende ZnS films [134]. If the S was not in excess (twice the Cd concentration), some ZnO, together with some wurtzite ZnS, also formed. Addition of a CuCli solution to the pH-adjusted sol and heating at 140°C resulted in Cu-doped ZnS (particle size 60 X 10 nm) that showed several photoluminescence bands [135]. 

Figure 6.5. (a) The 2 2PT structure of wurtzite (ZnS). The P layers (S) are large dark balls, (b) Open channels at C positions in wurtzite. 

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.

There are various polymorphs of silicon carbide made by high temperature interaction some have wurtzite (ZnS) or diamond structures. It is exceedingly hard and inert it finds uses in polishing products, furnace linings, and semiconductor technology. 

The preparation, manufacture, and reactions of SiC have been discussed in detail in Gmelin, as have the electrical, mechanical, and other properties of both crystalline and amorphous of SiC. Silicon carbide results from the pyrolysis of a wide range of materials containing both silicon and carbon but it is manufactured on a large scale by the reduction of quartz in the presence of an excess of carbon (in the form of anthracite or coke), (Scheme 60), and more recently by the pyrolysis of polysilanes or polycarbosUanes (for a review, see Reference 291). Although it has a simple empirical formula, silicon carbide exists in at least 70 different crystalline forms based on either the hexagonal wurtzite (ZnS) structme a-SiC, or the cubic diamond (zinc blende) structme /3-SiC. The structmes differ in the way that the layers of atoms are stacked, with Si being fom-coordinate in all cases. 

Figure 2.15. Model of the wurtzite (ZnS) crystal structure. The framework is based oir air hep lattice of anions (yellow the unit cell consists of A and B ions) with zinc ions occupying tetrahedral interstitial sites (white, labeled as X and Y ions).

Figure 7.19 The anion-centred polyhedron found in the hexagonal closest-packed structure (a) oriented with the hexagonal c-axis vertical (b) cation positions occupied in the ideal corundum, AI2O3, structure (c) cation positions filled in the idealised rutile, Ti()2, structure (d) cation positions occupied in the wurtzite, ZnS, structure, the central anion is omitted for clarity. Cations in tetrahedral sites are small and cations in octahedral sites are medium-sized. Adapted from E. W. Gorter, Int. Cong, for Pure and Applied Chemistry, Munich, 1959, Butterworths, London, 1960, p 303...

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