Wurtzite structure type

Slowly dissolved by hot concentrated H3SO4 and hot concentrated NaOH, but not by concentrated HGl, HNO3 and aqua regia. Stable in air, sublimes undecomposed at 800°G. d 6.10. B4 structure type (wurtzite). 

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). 

Equiatomic tetrahedral structure types. (Carborundum structure types). To this group pertain the diamond-type structure, the wurtzite (h) and the sphalerite (c) types, and the large family of SiC polytypes (such as he, hcc, hccc, hcchc,. .. (hcc)5(hccc)(hcc)5hc. .. (hchcc)17(hcc)2,. .. (hcc)43hc...). 

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... 

The sphalerite- and wurtzite-type structures belong to a homeotect structure type set. The layer stacking sequence symbols (triangular nets) of the two structures are ... 

Gallium nitride (GaN) has a hexagonal structure of wurtzite type with a wide direct bandgap of 3.4 eV at 300 K. GaN-based III—V nitrides with wide band gaps are the potential candidates for device applications in the blue and ultraviolet wavelengths... 

Fig. 3.4 The Phiilips-Van Vechten structure map ( c, ) for the sp-valent octet AB compounds. The four-fold coordinated zinc blende and wurtzite structure types are separated from the six-fold coordinated NaCI structure type by the straight line corresponding to the degree of ionicity a = 0.785. (After Phillips and Van Vechten (1969).)...

Cadmium Sulfide. CdS [1306-23-6] is dimorphic and exists in the sphalerite (cubic) and wurtzite (hexagonal) crystal structures (40). At very high pressures it may exist also as a rock-salt structure type. It is oxidized to the sulfate, basic sulfate, and eventually the oxide on heating in air to 700°C, especially in the presence of moisture (9). 

Probably we are dealing here with an intrinsic difference between these two structure types. Their solid state chemistry shows the wurtzite structure to be the more rigid one (in terms of substitution, for example). This could imply a difference in Cd-X (X=S,Se,Te) bond strengths. These bond strengths are, according to Pauling s ionicity concept, influenced by the... 

The majority of unipolar ionic conductors identified to date are polymorphic compounds with several phase transitions, where the phases have different ionic conductivities owing to modifications in the substructure of the mobile ions [28], One of the first studied cationic conductors was a-Agl [21]. Silver iodide exhibits different polymorphic structures. Agl has a low-temperature phase, that is, [3-Agl, which crystallizes in the hexagonal wurtzite structure type, and a high-temperature cubic phase, a-Agl, which shows a cubic CsCl structure type [20,22] (see Section 2.4.5). 

AIN, GaN and InN crystallise in the wurtzite structure which is characterised by lattice parameters a and c, as well as by u-value (u = b/c, where b is a bond-length in the c-direction). For the ideal wurtzite structure, c/a = 1.633 and u = 0.375. In contrast to the cubic sphalerite structure, the wurtzite structure offers two possibilities to deviate from the ideal arrangement, by changing the c/a ratio and by changing the u value. Such deviations are often observed in wurtzite-type structures [1] but there exists a strong correlation between the c/a ratio and the u parameter if c/a decreases, then u increases in such a way that the four tetrahedral distances remain nearly constant and the tetrahedral angles are distorted [2]. The bond lengths would be equal if ... 

William B. Pearson (1921-2005) developed a shorthand system for denoting alloy and intermetallic structure types (Pearson, 1967). It is now widely used for ionic and covalent solids, as well. The Pearson symbol consists of a small letter that denotes the crystal system, followed by a capital letter to identify the space lattice. To these a number is added that is equal to the number of atoms in the unit cell. Thus, the Pearson symbol for wurtzite (hexagonal, space group PS mc), which has four atoms in the unit ceU, is hPA. Similarly, the symbol for sodium chloride (cubic, space group Fm3m), with eight atoms in the unit cell, is cF8. 

Fig. 7.3. The relationship between number of shared edges between cation tetra-hedra and both the electrostatic Madelung energy and the one-electron covalent band-structure energy for 2 real observed and 21 hypothetical polymorphs of BeO. The structure types plotted are the wurtzite (a-BeO) type (-I-), the p-BeO type (X), and the 21 hypothetical dipolar tetrahedral structures with (1,1) or (2,1) unit cells. The small numbers by some of the points show how many points are represented by the single symbol (after Burdett and McLarnan, 1984 reproduced with the publisher s permission).

Cardona, M., and G. Harbeke (1965). Optical properties and band structure of wurtzite-type crystals and rutile. Phys. Rev. 137, A1467-76. 

Other Metal Sulphides. A survey of lattice data and structure types of 40 compounds of the type Cu2ABS4, where A = Mn, Fe, Co, Ni, Zn, Cd, or Hg and B = Si, Ge, or Sn, has shown that three tetrahedral structure types, differing in symmetry and unit-cell size, exist.254 All of the compounds were found to adopt one of the following structure types the stannite structure, an orthorhombic superstructure of wurtzite, or a hitherto unknown structure based on slightly distorted sphalerite cells of tetragonal, orthorhombic, or monoclinic symmetry. 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

Pure white powder. Decon oses in damp air, forming HgTe, which then oxidizes to Te and appears on the surface of the telluride as a black deposit. d 3.85. Crystallizes in structure type B4 (wurtzite type). 

Lemon-yellow powder. Soluble in fuming hydrochloric acid with evolution of HaSe. d (pycn.) 5.30. Crystal structure type B3 (zincblende type) or B4 (wurtzite type). 

In Chapter 5 we will see that packing is the most important consideration in determining the structure adopted by predominantly ionically bonded crystals. The difference in between some crystal structures is very small. In such cases, for example, the zinc blende and wurtzite structures (named after the two crystalline forms of ZnS), the difference in the resulting electrostatic energy is small. For zinc blende and wurtzite it is -0.2%. When the energy difference between structure types of the same stoichiometry is small, we often encounter polymorphism the compound can form with more than one structure. We will examine this useful complication in Chapter 7. 

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