Crystal structures sphalerite

Ziegler-Natta catalyst A stereospecific catalyst for polymerization reactions, consisting of titanium tetrachloride and triethylaluminum. zinc-blende structure A crystal structure in which the cations occupy half the tetrahedral holes in a nearly close packed cubic lattice of anions also known as sphalerite 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))...

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... 

FIGURE 1.37 The crystal structure of zinc blende or sphalerite, ZnS. Zn, blue spheres S, grey spheres (or vice versa). 

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). 

Figure 10.2 Crystal structures of (a, b, c) natural iron and (d) zinc sulphides (a) greigite, (b) mackinawite, (c) smythite, and (d) sphalerite...

Sphalerite (p-ZnS) has a cubic crystal structure in which both Zn and S occur in regular tetrahedral coordination. Pure sphalerite is a diamagnetic semiconductor with a large band gap (—3.6 eV Shuey, 1975). On the basis of the observed structure and properties, the simple MO energy-level diagram shown in Fig. 6.1 can be proposed to describe the bonding in a ZnS4 cluster molecular unit. Overlaps between outermost s and p... 

The copper, copper-iron, and the silver sulfides are more complex than the sulfides discussed previously, containing several cations or cation sites in their structures. Thus chalcopyrite (CuFeSj), although having a fairly simple structure based on that of sphalerite, but with Cu and Fe alternately replacing Zn atoms, contains both Cu+ and Fe + in regular tetrahedral coordination (as indicated by neutron diffraction and Moss-bauer studies see Vaughan and Craig, 1978). A family of more than thirty synthetic compounds with the chalcopyrite structure is known, and their properties have been studied because of potential applications as semiconductors. Miller et al. (1981) have reviewed the crystal structures, vibrational properties, and band structures of these materials. 

When the hard-sphere cation-anion radius ratio exceeds 0.732, as it does for the cesium halides, a different crystal structure called the cesium chloride structure, is more stable. It may be viewed as two interpenetrating simple cubic lattices, one of anions and the other of cations, as shown in Figure 21.17. When the cation-anion radius ratio is less than 0.414, the zinc blende, or sphalerite, structure (named after the structure of ZnS) results. This crystal consists of an fee lattice of... 

Figure 4.5 The crystal structure of zinc-blende (sphalerite) ZnS. The compounds Agl, BC, BN,...

Cubic BN is less stable than h-BN, but the conversion rate between these forms is negligible at room temperature. c-BN is prepared by annealing h-BN powder at higher temperatures, under pressures above 5GPa. The cubic form has the sphalerite crystal structure (the B and N atoms are tetrahedrally coordinated), as in the diamond structure. Every boron atom is surrounded by four nitrogen atoms, and vice versa in such an arrangement the boron and nitrogen atoms have sp hybridization. c-BN is often also referred to as P-BN or sometimes z-BN (zinc-blende) [140, 141]. 

BN has at least four crystal structures, namely, hexagonal (h-BN), cubic or zinc blende or sphalerite (c-BN), wurtzite (w-BN), and rhombohedral (r-BN). Among these, the... 

Silicon carbide occurs in two sUghtly different crystal structures the cubic pSiC, and a large number of hexagonal rhombohcdral varieties known collectively as aSiC.1 11 1 The single cubic form, pSiC, is obtained vdien the carbide is synthesized below 2100"C. It is a fiice-centered cubic (fee) structure of the zincblende type shown in Fig. 7.1. Zincblende is a mineral of zinc sulfide also known as sphalerite. In this illustration, the zincblende structure is represented with the cube diagonals vertical and appears as series of identical (although translated) puckered sheets of atoms widi an AA layer sequence. Another view of the pSiC crystal is shown in Fig. 7.2 (the carbon atoms, all located in the 4/ sites, are omitted for clarity] The pSiC structure has no polytype (see Table 7.3 for crystal structure data). 

The history of coordination chemistry may in a sense be said to have begun with the work of Werner. The early crystal-structure determinations by W. L. and W. H. Bragg showed that in crystals such as sphalerite, ZnS, there is tetrahedral coordination around both zinc and sulfur, and in crystals such as sodium chloride there is octahedral coordination about both the anion and the cation. The modem period may be said to have begun in 1921, with the determination of a cryst containing an octahedral complex by Wyckoff and of crystals containing tetrahedral and square planar complexes (1922) by Dickinson. Later developments include application of quantum mechanics, discussion of hybrid orbitals especially suited to bonding, and detailed interpretation of interatomic distances found by careful X-ray diffraction studies. 

Zinc sulfide, ZnS, crystallizes in either of two structures. The zinc sulfide mineral having cubic structure is called zinc blende or sphalerite. Zinc sulfide also exists as hexagonal crystals. (The mineral is called wurtzite.) A sohd substance that can occur in more than one crystal structure is said to be polymorphic. We will discuss only the zinc blende structure (cubic ZnS). 

Interestingly, zinc sulfide (p-ZnS) may also crystallize in a cubic lattice, which consists of a fee array of S , with Zn occupying 1/2 of the available tetrahedral sites. This structure is known as sphalerite or zincblende, and is shared with other compounds such as a-AgI, p-BN, CuBr, and p-CdS. When the same atom occupies both the fee and tetrahedral interstitials of the sphalerite structure, it is described as the diamond lattice, shared with elemental forms (allotropes) of silicon, germanium, and tin, as well as alloys thereof. Important semiconductors such as GaAs, p-SiC, and InSb also adopt the sphalerite crystal structure. 

Silicon carbide exists in a large number of structural forms called polytypes (more than 140), which represent modifications of hexagonal (wurtzite) and cubic (sphalerite) close-packed crystal structures. 

A third AX structure is one in which the coordination number is 4—that is, all ions are tetrahedrally coordinated. This is called the zinc blende, or sphalerite, structure, after the mineralogical term for zinc sulfide (ZnS). A unit cell is presented in Figure 12.4 all corner and face positions of the cubic cell are occupied by S atoms, whereas the Zn atoms fill interior tetrahedral positions. An equivalent structure results if Zn and S atom positions are reversed. Thus, each Zn atom is bonded to four S atoms, and vice versa. Most often the atomic bonding is highly covalent in compounds exhibiting this crystal structure (Table 12.1), which include ZnS, ZnTe, and SiC. 

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