Wurtzite structure type example compounds

For structures of type D (fiA bridging), three alternatives are possible, all rather rare. Type D1 was first detected in the compound [Co4S2(CO)10].73 More recent examples are [Mo4(NO)4S,3]4 (ll)3 and [Ni9(/i4-S)3(/i3-S)6(PEt3)6]2 (12).74 Type D2 has been found in [SZn4(S2 AsMe2)6]75 and type D3 in [Fe4S(SR)2(CO)12].76 Here the coordination corresponds to that of the zincblende or wurtzite structure. 

Many compounds of the type AB have structures in which each A is tetrahedrally bonded to four B, and vice versa. The zinc blende and wurtzite structures are of this type, and differ in details which need not concern us. Examples include ZnS, Cul, BeO, BN and A1P. The diamond lattice is of the zinc blende type. 

The structures of metallic monoxides are set out in the self-explanatory Table 12.3. The symbols mean that the structure of a particular compound is of the general type Indicated and is not necessarily the ideal structure with maximum symmetry. An interesting point about the wurtzite structure is that unless 4a = (12 - 3)c, where Hi is the z coordinate of 0 referred to Zn at (000), there is a small but real difference between one M—0 distance and the other three, for example, in ZnO, 1-973 A (3) and 1-992 A (1). For the ideal structure with regular tetrahedral bonds u = 3/8, and c/a = 2V2/V3 = 1-633. 

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. 

Compounds with the zincblende or wurtzite structure are commonly formed when one element belongs to the nth. and the other to the (8—rc)th B sub-groups. In these structures each atom is bound to four neighbours by purely covalent bonds and it is necessary that the number of valency electrons available for the formation of these bonds should average four per atom. It is not, however, necessary that these should be contributed equally by the atoms of the two types, and the condition is therefore satisfied if these atoms are related in the manner indicated. Some B-B compounds with the zincblende and wurtzite structures are shown in table 13.09. These structures are not, of course, confined to systems of the type B-B we have encountered numerous examples of their occurrence in compounds of quite other kinds. 

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. 

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-... 

In order to have around each atom in this hexagonal structure four exactly equidistant neighbouring atoms, the axial ratio should have the ideal value (8/3 that is 1.633. The experimental values range from 1.59 to 1.66. This practical constancy of the axial ratio, in contrast with what is observed for other families of isostructural compounds such as those of the NiAs type, may be attributed to a sort of rigidity of the tetrahedral (sp3) chemical bonds. As for the atomic positional parameters, the ideal value of one of the parameters (being the other one fixed at zero by conventionally shifting the origin of the cell) is z = 3/8 = 0.3750. The C diamond, sphalerite- and wurtzite-type structures are well-known examples of the normal tetrahedral structures (see 3.9.2.2). 

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