Metal hexagonal closest packing

The symbols Al, A2, and A3 represent the three simple metal structures cubic closest packed, body centered, and hexagonal closest packed, respectively. 

Table 14.2 The element structures of the metals at ambient conditions h = hexagonal closest-packing c = cubic closest-packing...

Even smaller c/a ratios are observed for the more electron-rich arsenides and antimonides (e.g. 1.39 for NiAs). Since the ideal c/a ratio of hexagonal closest-packing is 1.633, there is a considerable compression in the c direction, i.e. in the direction of the closest contacts among the metal atoms. 

The same atom-centered polyhedra can be used to describe interstitial diffusion in all the many metal structures derived from both face-centered cubic and hexagonal closest packing of atoms. In these cases the polyhedra are centered upon a metal atom and all the tetrahedral and octahedral interstitial sites are empty. The hardening of metals by incorporation of nitrogen or carbon into the surface layers of the material via interstitial diffusion will use these pathways. 

The suggestion that in metal crystals the atoms are arranged in closest packing was made by Barlow before the development of the x-ray technique, in order to account for the observations that many metals crystallize with cubic or hexagonal symmetry and that in the latter case many of the observed values of the axial ratio lie near the ideal value 2y/2/ v 3 = 1.633 for hexagonal closest packing. 

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 small difference in nature of the cubic and hexagonal closest-packed arrangements is verified by the approximate equality of interatomic distances for the two structures of metals that exist in the corresponding allotropic forms, as given in Table 11-2. 

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... 

The cubic closest-packed arrangement (Figure 10.21b) has three alternating layers, a-b-c-a-b-c. The a-b layers are identical to those in the hexagonal closest-packed arrangement, but the third layer is offset from both a and b layers. Silver, copper, and 16 other metals crystallize with this arrangement. 

Finally, LEED, TDS and UPS studies on the interaction of carbon monoxide with the hexagonally closest packed faces of the group VIII metals show numerous similarities. This is not... 

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. 

Berkelium metal exhibits two stable crystallographic modifications, double hexagonal closest packed (dhcp) and face-centered cubic (fee). Thus it is isostructural with the two preceding elements, all of which exhibit the fee structure at high temperature. The room-temperature lattice constants of the dhcp form are ao = 0.3416 0.0003 nm and c0 = 1.1069 0.0007 nm, yielding a calculated density of 1.478 x 104 kg/m3 and a metallic radius (CN = 12) of 0.170 nm (119). The room-temperature fee lattice parameter is a0 = 0.4997 0.0004 nm from which the... 

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

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 alkaline-earth hydrides all have a PbCl2 type of structure in which the metal atoms are arranged approximately in hexagonal closest packing. Of the two sets of non-equivalent H atoms one occupies tetrahedral holes while the other H atoms have (3 + 2)-coordination ... 

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