The wurtzite ZnS structure

The rutile lattice is adopted by Sn02 cassiterite, the most important tin-bearing mineral), Mn02 (pyrolnsite) and Pb02. 

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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...

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... 

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. 

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). 

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... 

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 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).

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. 

Our own cluster calculations were performed at different levels of approximation. We started with a free Za O cluster with the wurtzite-hke structure of bulk ZnO [128]. This cluster is shown in Fig. 4a. It has the advantage that its dipole moment is still so small (in contrast to the Zn O cluster shown in Fig. lb) that it will not affect the results too much. Furthermore, it contains threefold (3C) and twofold (2C) coordinated Zn and O atoms and can therefore serve as a model for threefold coordinated Zn atoms on the terraces of the (0001) plane or at edges on the (10-10) plane as well as for twofold coordinated Zn atoms at various types of corners. The results for all those configurations of CO on Zn404 which exhibit an attractive interaction are collected in Table 6. They show without any doubt that CO will only bind to Zn atoms, with... 

Pal et al. have in a follow-up study used the same calculational method in studying the relative stability of different ZnS nanoparticles. They considered stoichiometric clusters that were constructed either as spherical cut-outs of the zincblende or the wurtzite crystal structure as well as stoichiometric, hollow cages. As shown in Fig. 21, the hollow cages are indeed stabler as the zincblende or wurtzite derived clusters. Despite this theoretical finding, most experimental studies on similar systems find structures that resemble those of the zincblende or wurtzite crystal structure. 

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))...

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). 

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-... 

The zincblende (ZB), or sphalerite, structure is named after the mineral (Zn,Fe) S, and is related to the diamond structure in consisting entirely of tetrahedrally-bonded atoms. The sole difference is that, unlike diamond, the atoms each bond to four unlike atoms, with the result that the structure lacks an inversion center. This lack of an inversion center, also characteristic of the wurtzite structure (see below), means that the material may be piezoelectric, which can lead to spurious ringing in the free-induction decay (FID) when the electric fields from the rf coil excite mechanical resonances in the sample. (Such false signals can be identified by their strong temperature dependence due to thermal expansion effects, and by their lack of dependence on magnetic field strength). 

In the schemes of Fig. 7.10, typical sections of a few adjacent cells of this structure are shown these are also compared with those of a number of related hexagonal structures, some of which are described in the following paragraphs. Notice that important filled-up derivatives can be considered among the ordered structures derived from Mg. Typical examples are the hP4-NiAs type with occupied octahedral holes and the wurtzite (hP4-ZnS) type with one set of occupied tetrahedral holes. 

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... 

Sphalerite and wurtzite structures general remarks. Compounds isostructural with the cubic cF8-ZnS sphalerite include AgSe, A1P, AlAs, AlSb, BAs, GaAs, InAs, BeS, BeSe, BeTe, BePo, CdS, CdSe, CdTe, CdPo, HgS, HgSe, HgTe, etc. The sphalerite structure can be described as a derivative structure of the diamond-type structure. Alternatively, we may describe the same structure as a derivative of the cubic close-packed structure (cF4-Cu type) in which a set of tetrahedral holes has been filled-in. This alternative description would be especially convenient when the atomic diameter ratio of the two species is close to 0.225 see the comments reported in 3.7.3.1. In a similar way the closely related hP4-ZnO... 

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