Wurtzite, ZnS, structure

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.

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...

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 polycarbosilanes (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) structure a-SiC, or the cubic diamond (zinc blende) structure /i-SiC. The structures differ in the way that the layers of atoms are stacked, with Si being four-coordinate in all cases. 

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))...

Perhaps the most depressing fact associated with the consequences of the above division is the lack of consistency often found in treatments of compounds which are essentially isostructural. Take, for instance, the different descriptions of the bonding situation in B2H6 on the one hand, and the isostructural (e.g. AI2CI6) molecules on the other while the latter may be treated by the conventional bonding principles expressed in Hyps. III.l to III.5, the treatment of the former (in terms of 3-centre bonds) breaks with Hyps. III.l to III.4. A similar conclusion is in fact reached in the majority of abnormal cases. Other simple examples are provided by the alkali-metal hydrides (with NaCl-type structure), CuH (with ZnS-wurtzite type structure), etc. These examples are typical in that it is only when a scarcity of electrons and/or orbitals enforces a search for extraordinary bonding principles that Hyps. III.l to III.4 are reluctantly (partly or completely) replaced by alter-... 

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. 

Simple binary tetrahedral structures and polytypes (ZnS-sphalerite, cF8-ZnS and ZnS-wurtzMe, hP4-ZnO, structural types). The sphalerite- and wurtzite-type structures (together with C diamond) are well-known examples of the... 

Figure 7.24. Section sequence parallel to the base plane of the hP4-ZnO (ZnS wurtzite) type structure.

Several superstructures and defect superstructures based on sphalerite and on wurtzite have been described. The tI16-FeCuS2 (chalcopyrite) type structure (tetragonal, a = 525 pm, c = 1032 pm, c/a = 1.966), for instance, is a superstructure of sphalerite in which the two metals adopt ordered positions. The superstructure cell corresponds to two sphalerite cells stacked in the c direction. The cfla ratio is nearly 1. The oP16-BeSiN2 type structure is another example which similarly corresponds to the wurtzite-type structure. The degenerate structures of sphalerite and wurtzite (when, for instance, both Zn and S are replaced by C) correspond to the previously described cF8-diamond-type structure and, respectively, to the hP4-hexagonal diamond or lonsdaleite, which is very rare compared with the cubic, more common, gem diamond. The unit cell dimensions of lonsdaleite (prepared at 13 GPa and 1000°C) are a = 252 pm, c = 412 pm, c/a = 1.635 (compare with ZnS wurtzite). 

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

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. 

MnTe, AIN, GaN, InN, SiC and NH4F. The wurtzite structure is, in a sense, intermediate between NaCl and ZnS structures. 

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. 

The structure of Agl varies at different temperatures and pressures. The stable form of Agl below 409 K, y-Agl, has the zinc blende (cubic ZnS) structure. On the other hand, /3-AgI, with the wurtzite (hexagonal ZnS) structure, is the stable form between 409 and 419 K. Above 419 K, ft-Agl undergoes a phase change to cubic a-Agl. Under high pressure, Agl adopts the NaCl structure. Below room temperature, y-Agl obtained from precipitation from an aqueous solution exhibits prominent covalent bond character, with a low electrical conductivity of about 3.4 x 10-4 ohm 1cm 1. When the temperature is raised, it undergoes a phase change to a-Agl, and the electrical conductivity increases ten-thousandfold to 1.3 ohm-1 cm-1. Compound a-Agl is the prototype of an important class of ionic conductors with Ag+ functioning as the carrier. 

The chemisorption of water on ZnO has been investigated by Nagao and Morimoto (147). Dent and Kokes (148) explained H2 chemisorption and ethylene hydrogenation on the basis of a model in which the (0001), (0001), and non-close-packed faces such as (1010) planes of the wurtzite crystal structure are assumed to form the external crystal surface. Chemisorption of H2 was suggested to occur on Zn—O pair sites, since the Zn—H and O—H stretching... 

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. 

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).

If an ionic crystal has two opposite polar faces, then, in order for the total crystal to be charge neutral, the two opposing faces must be of opposite types. This effect can readily be observed macroscopically a well known case is ZnO. ZnO has a tetrahedrally coordinated wurtzite crystal structure in the [0001] direction, it consists of alternating planes of Zn and O ions [1]. When a sample... 

Wurtzite ZnO structure with four atoms in the unit cell has a total of 12 phonon modes (one longitudinal acoustic (LA), two transverse acoustic (TA), three longitudinal optical (LO), and six transverse optical (TO) branches). The optical phonons at the r point of the Brillouin zone in their irreducible representation belong to Ai and Ei branches that are both Raman and infrared active, the two nonpolar 2 branches are only Raman active, and the Bi branches are inactive (silent modes). Furthermore, the Ai and Ei modes are each spht into LO and TO components with different frequencies. For the Ai and Ei mode lattice vibrations, the atoms move parallel and perpendicular to the c-axis, respectively. On the other hand, 2 modes are due to the vibration of only the Zn sublattice ( 2-low) or O sublattice ( 2-high). The expected Raman peaks for bulk ZnO are at 101 cm ( 2-low), 380 cm (Ai-TO), 407 cm ( i-TO), 437 cm ( 2-high), and 583 cm ( j-LO). 

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