Closest-packed crystal structures cubic

FIGURE 11.49 Cubic Closest-Packing Crystal Structure The unit cell of the cubic closest-packed structure is face-centered cubic. 

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 structure of crystals can be investigated by the electron-diffraction method as well as by the x-ray-diffraction method. The electron-diffraction method has been especially useful in studying the strueture of very thin films on the surface of a crystal. For example, it has been shown that when argon is adsorbed on a clean face of nickel crystal the argon atoms occupy only one-quarter of the positions formed by triangles of nickel atoms (in the octahedral face of the cubic closest-packed crystal Figure 2-7). The structure of very thin films of metal oxide that are formed on the surface of metals, and that protect them against further corrosion, has been studied by this method. 

When D = 300 or 350 nm, all the PSMSs were also trapped within the dimple on the patterned substrate as shown in Fig. 28e [L = 2.1 pm and P = 4.0pm] and f[L = 1.0pm and P = 2.0pm]. No PSMSs were left on the terrace within the observed SEM images. PSMS colloidal crystal formed in the dimple is defined as the dimension of the dimple, which took cubic-closest packed (cep) structure. The facet, (100) plane, is evidently parallel to the substrate surface. In other words, the present dimple may act as a template to produce PSMS colloidal crystals, the nucleation and growth of which would be confined and conttolled by the four sidewalls inside the dimples [ 132,133]. Namely, the deposition process of PSMSs on patterned substrate... 

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... 

In the crystal structure of a-R12 boron, the B12 icosahedra are arranged in approximately cubic closest packing and are linked together in a three-dimensional framework. Figure 9.6.21 shows a layer of interlinked icosahedra perpendicular to the 3-axis, and a perspective view of the crystal structure approximately along the a axis (note the linkage between layers of icosahedra at z = 0,1 /3,2/3,1). Further details are given in Section 13.2. 

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. 

Silver crystallizes in a cubic closest packed structure. The radius of a silver atom is 1.44 A (144 pm). Calculate the density of solid silver. 

Density is mass per unit volume. Thus we need to know how many silver atoms occupy a given volume in the crystal. The structure is cubic closest packed, which means the unit cell is face-centered cubic, as shown in the figure at left. 

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. 

Pure, sublimed buckminsterfullerene condenses in a regular face-centered cubic (= cubic closest packed) structure. Complications may occur through the presence of other fullcrcnc impurities or crystallization from a variety of solvents. Sec Hawkins, J. M. Lewis, T. A. Loren, S. D. Meyer, A. Heath, J. R. Saykally, R. J. Hollander, F. J. J. Chem. Soc., Chem. Commun. 1991,175-716, Guo. Y. Kurasawa, N. Goddard, W. A., HI Nature 1991, S5l, 464-467. 

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