Wurtzite form

A smaller class of type II alloys of II-VI binaries also exists, including the (CdS) ,(ZnSe)i (CdS) ,(ZnTe)i (CdSe) ,(ZnSe)i (CdS) ,(CdTe)i-. (CdSe)x(CdTe)i i , and (CdS) c(ZnS)i i systems, which transform at some critical composition from the W to the ZB structure. Importantly, the transition temperatures are usually well below those required to attain a thermodynamically stable wurtzite form for the binary constituents (e.g., 700-800 °C for pure CdS and > 1,020 "C for pure ZnS). The type 11 pseudobinary CdxZni jcSe is of considerable interest in thin film form for the development of tandem solar cells as well as for the fabrication of superlattices and phosphor materials for monitors. The CdSe Tei-x alloy is one of the most investigated semiconductors in photoelectrochemical applications. 

The arrangement of oxygen atoms in ice I is isomorphous with the wurtzite form of zinc sulphide, and also with the silicon atoms in the tridymite form of silicon dioxide. Hence, ice I is sometimes referred to as the wurtzite or tridymite form of ice (Eisenberg Kauzmann, 1969). 

In the wurtzite form of ZnS the sulfur atoms are arranged in hexagonal close packing, with the metal atoms in one-half of the tetrahedral positions. There are two layers of tetrahedra in the repeat distance, c, and these point in the same direction. This gives the materials a unique axis, the c axis, and these compounds show piezoelectricity. 

Zinc sulfide is white to gray-white or pale yellow powder. It exists in two crystalline forms, an alpha (wurtzite) and a beta (sphalerite). The wurtzite form has hexagonal crystal structure refractive index 2.356 density 3.98 g/cm3 melts at 1,700°C practically insoluble in water, about 6.9 mg/L insoluble in alkalis soluble in mineral acids. The sphalerite form arranges in cubic crystalline state refractive index 2.368 density 4.102 g/cm changes to alpha form at 1,020°C practically insoluble in water, 6.5 mg/L soluble in mineral... 

While referring to precipitation in solution and not CD of films, it is of interest to mention that V-allylthiourea was used as a sulphur source in a Zn /am-monia bath [133]. Pure ZnS (as detected by XRD) was obtained at a pH of ca. 11.0 at 90°C. At lower pH values (and at 80°C), only ZnO was obtained at higher values, a mixture of ZnO and ZnS was formed. The ZnS was the wurtzite form in all cases. The ZnS crystal size was ca. 5 nm at pH = 11.0 and slightly smaller (ca. 4 nm) at higher pH values, which gave a mixed phase of ZnS and ZnO. The ZnO crystal size was much larger (ca. 200 nm). The ZnS fraction increased as the ammonia concentration (at constant pH) increased (lower [Zn ]) or as the pH decreased (at constant ammonia concentration) from 13 to 11 (lower [OH ]). Such experiments should help in choosing optimum conditions for CD. 

The cubic and wurtzitic forms have similar chemical properties (11), but their reaction rates tend to be slower because of the denser structure. The... 

The cubic zinc blende form of boron nitride is usually prepared from the hexagonal or rhombohedral form at high (4—6 GPa (40—60 kbar)) pressures and temperatures (1400—1700°C). The reaction is accelerated by lithium or alkaline-earth nitrides or amides, which are the best catalysts, and form intermediate liquid compounds with BN, which are molten under synthesis conditions (11,16). Many other substances can aid the transformation. At higher pressures (6—13 GPa) the cubic or wurtzitic forms are obtained without catalysts (17). 

The wurtzite form differs only slighdy from the cubic form, but it is not quite as stable. It is most easily obtained by static or dynamic compression of hBN or rBN at high pressures (17). In the presence of a liquid catalyst at high pressures, the wurtzite form changes rapidly to the cubic form. The change occurs more slowly without a catalyst above 6 GPa (60 kbar) (18). 

A compound that has one color at room temperature and a different color at a high temperature is known as a thermochromic compound. Zinc oxide is such a compound because it is white at low temperature but yellow at high temperature. The structure of ZnO is like that of the wurtzite form of ZnS shown in Figure 3.5. 

The other polymorph of ZnS is wurtzite (Fig. 3.13). The zinc atoms are tetrahedrally coordinated as in zinc blende, but the anions in wurtzite form an HCP-hke array instead of a CCP-like array. Indeed, the wurtzite structure is often thought of as an HCP-like array of anions with one-half the tetrahedral sites occupied by Zn " cations. Hence, the... 

The wurtzite form of ZnS, Figure 7-8(b, c), is much rarer than zinc blende, and is formed at higher temperatures than zinc blende. It also has zinc and sulfide each in a tetrahedral hole of the other lattice, but each type of ion forms a hexagonal close-packed lattice. As in zinc blende, half of the tetrahedral holes in each lattice are occupied. 

Silver iodide exhibits an unusual property. In addition to a y-(blende) form and a -(wurtzite) form it has an a-form stable between 146° and 552° (the m.p.). In this the iodide ions are arranged in a body-centred cubic lattice but the Ag+ ions form what may be called an interstitial fluid, being apparently free to move through the rigid network of 1 ions. The variation of conductance with temperature in silver iodide (Table 23) is particularly interesting. 

Derive simplified expressions for F for the wurtzite form of ZnS, including the rules governing observed reflections. This crystal is hexagonal and contains 2 ZnS per unit cell, located in the following positions ... 

From an optical viewpoint, on the other hand, the difference between semiconductors and insulators lies in the value of Eg. The admitted boundary is usually set at 3 eV (see Appendix A for the energy units) and materials with Eg below this value are categorized as semiconductors, but crystals considered as semiconductors like the wurtzite forms of silicon carbide and gallium nitride have band gaps larger than 3 eV, and this value is somewhat arbitrary. The translation into the electrical resistivity domain depends on the value of Eg, and also on the effective mass of the electrons and holes, and on their mobilities. The solution is not unique moreover, the boundary is not clearly defined. Semi-insulating silicon carbide 4H polytype samples with reported room temperature resistivities of the order of 1010flcm could constitute the... 

Many IIA-sulphides and -oxides as well as IIIA-nitrides crystallize in the wurtzite form, but some of them (ZnS, of course, but also CdS, GaN and others) can also be found in the sphalerite form. SiC can adopt the wurtzite form (2//-SiC) or less frequently the sphalerite form (3C -SiC), with a notable difference in the band gap (3.3 or 2.3 eV, respectively), but when grown by vapour-phase epitaxy, SiC is usually obtained in the form of polytypes with... 

Figure 11.13 (a) The structure of hexagonal ZnO (zincite) a Zn04 tetrahedron is outlined, (h) An electric dipole, p, parallel to the c axis, arises in the unsymme-trical Zn04 tetrahedron the wurtzite form of ZnS is isostructural... 

Nine forms of ice are known. Ordinary ice (referred to as hexagonal ice, denoted as 4) has a structure isomorphous to the wurtzite form of ZnS. A detailed description of the geometry and properties of the various forms of ice can be found in Eisenberg and Kauzmann (1969), Fletcher (1970, 1971), and Franks (1972). 

Leonidov VYa, Timofeev IV, Lazarev VB, Bozhko AB (1988) Enthalpy of formation of the wurtzite form of boron nitride. Russ J Inorg Chem 33 906-908... 

Figure 6.1. Pseudolayer representation of (a) Wurtzite form of SiC (the a-form in which horizontal six-membered rings have a chair configuration and vertical puckered hexagonal rings have a boat configuration) and (b) p or zinc blende structure of SiC containing all chair-form rings.

Common AB lattices include the sodium chloride, or rock salt, structure the CsCl structure and the zinc blende and wurtzite forms of zinc sulfide. AB2 lattices... 

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