Hexagonal closest packed structure

Figure 5.16 Anion-centered polyhedron found in the hexagonal closest-packed structure (a) oriented with the hexagonal c axis vertical (b) cation positions occupied in the ideal corundum, A1203, structure (c) cation positions occupied in the idealized rutile, Ti02, structure and (d) cation positions occupied in the wurtzite hexagonal ZnS structure the central cation is omitted for clarity.

The close approximation of metal atoms in these crystals to mutually attracting spheres is further shown by the values observed for the axial ratio c/aot the hexagonal closest-packed structures, as tabulated below. 

The maximum number of rigid spheres that can be brought into contact with another sphere with the same radius is twelve. The corresponding coordination polyhedra, seen in the cubic and hexagonal closest-packed structures, have eight triangular faces and six square faces. 

This brings us to a class of compounds too often overlooked in the discussion of simple ionic compounds the transition metal halides. In general, these compounds (except fluorides) crystallize in structures that are hard to reconcile with the structures of simple ionic compounds seen previously (Figs. 4.1-4.3). For example, consider the cadmium iodide structure (Fig. 7.8). It is true that the cadmium atoms occupy octahedral holes in a hexagonal closest packed structure of iodine atoms, but in a definite layered structure that can be described accurately only in terms of covalent bonding and infinite layer molecules. 

Mg crystallizes in the hexagonal closest packed structure and has a density 1.74 g/cm3. (a) What is the volume of the unit cell (b) What is the distance between nearest neighbors (c) How many nearest neighbors does each atom have ... 

The 17 rare-earth metals are known to adopt five crystalline forms. At room temperature, nine exist in the hexagonal closest packed structure, four in the double c-axis hep (dhep) structure, two in the cubic closest packed structure and one in each of the body-centered cubic packed and rhombic (Sm-type) structures, as listed in Table 18.1.1. This distribution changes with temperature and pressure as many of the elements go through a number of structural phase transitions. All of the crystal structures, with the exception of bep, are closest packed, which can be defined by the stacking sequence of the layers of close-packed atoms, and are labeled in Fig. 18.1.1. 

Use the diagram of the unit cell for the hexagonal closest packed structure in Fig. 16.14 to determine the net number of atoms in the hep unit cell. 

The densest ABAB close-packing structure is also known as the hexagonal closest packing structure (hep) because of its hexagonal elementary unit. It can be found in metals like magnesium and zinc Magnesium Type (see Fig. 5.6). 

Table 3 gives the important properties of elemental cobalt. The metal itself is lustrous and silvery-blue at room temperature, and exists in two allotropic forms see Allotrope), a and /3. The a-form has a hexagonal closest-packed structure... 

HGURE 11.56 Closest-Packed Crystal Structures in Metals Nickel crystallizes in the cubic closest-packed structure. Zinc crystallizes in the hexagonal closest-packed structure. 

Some crystal lattices can also be depicted as closest-packed structures, including the hexagonal closest-packing structure (not cubic) and the cubic closest-packing structure (which has a face-centered cubic unit cell). 

Study Figure 12-40 and you will see that the cubic closest packed structure has a face-centered cubic unit cell. The unit cell of the hexagonal closest packed structure is shown in Figure 12 1. In both the hep and fee structures, holes account for only 25.96% of the total volume. For the simple cubic and bcc unit cells, holes account for a higher percentage of the total volume. In the bcc unit cell, holes account for 31.98% of the total volume. The best examples of... 

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