Hexagonal-closest packing crystal 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. 

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. 

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. 

Many simple minerals, especially simple salts like halite, NaCl, sulfides, sulfosalts and oxides, have structures based upon cubic or hexagonal closest-packed arrays of either cations or anions. Coordination geometries of metal ions in many of these kinds of minerals are thus confined to more or less regular octahedra and tetrahedra. The occupancy of the two types of sites is dictated by the stoichiometry of the mineral, the radius of the ions involved and their preferred coordination geometries. Coordination of cations in mineral species in terms of bonding and crystal field effects has been extensively reviewed.16-21 Comprehensive lists of ionic radii relevant to cation coordination geometries in minerals have also been compiled.16,21... 

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 ... 

Fullerene-C6o is a brown-black crystal, in which the nearly spherical molecules rotate continuously at room temperature. The structure of the crystal can be considered as a stacking of spheres of diameter 1000 pm in cubic closest packing (a = 1420 pm) or hexagonal closest packing (a = 1002 pm, c = 1639pm). Figure 14.1.4 shows the crystal structure of fullerene-C6o. 

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. 

Slip occurs along specific crystal planes (slip planes) and in specific directions (slip directions) within a crystal structure. Slip planes are usually the closest-packed planes, and slip directions are the closest-packed directions. Both face-centered-cubic (FCC) and hexagonal-close packed (HCP) structure are close packed structures, and slip always occurs in a close packed direction on a closepacked plane. The body-centered-cubic (BCC) structure is not, however, close packed. In a BCC system, slip may occur on several nearly close packed planes or directions. Slip planes and directions, as well as the number of independent slip systems (the product of the numbers of independent planes and directions), for these three structures are listed in Table 7.2. 

Translation of ions within crystals is less frequently observed than is rotation. Perhaps one of the most interesting cases is that of silver iodide which may actually be said to melt in halves. When this solid is heated to 145.8° C, the crystal structure then changes and the ionic conductivity increases tremendously the iodide ions are hexagonally closest-packed below the transition temperature but at this temperature they rearrange to form a more open structure, and the silver atoms are allowed to move within the lattice. At 555° C, the network of iodide ions collapses, and the compound becomes a liquid. The solids Cul and Ag2Se show similar behavior. 

Examples of metals that are cubic closest packed are aluminum, iron, copper, cobalt, and nickel. Magnesium and zinc exhibit hexagonal closest packing. Calcium and certain other metals can crystallize in either structure. 

The Au(lll) face, which is the closest packed fee structure, is known to have the lowest surface energy among all possible fee crystal faces. The clean Au( 111) face surface was first approximated to be a /3 X >/3.R30° unit cell. A model with a uniaxially contracted top hexagonal layer was proposed and a charge-density-wave (CDW) structure was also proposed as an explanation of the LEED observations. ... 

Examples of metals that form cubic closest packed solids are aluminum, iron, copper, cobalt, and nickel. Magnesium and zinc are hexagonal closest packed. Calcium and certain other metals can crystallize in either of these structures. Some metals, however, assume structures that are not closest packed. For example, the alkali metals have structures characterized by a body-centered cubic (bcc) unit cell (see Fig. 10.9), where the spheres touch along the body diagonal of the cube. In this structure, each sphere has 8 nearest neighbors (count the number of atoms around the atom at the center of the unit cell), as compared with 12 in the closest packed structures. Why a particular metal adopts the structure it does is not well understood. 

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