Close-Packed Crystalline Structures

A crystal can be defined as a solid in which the unit cells (atoms or molecules) have a three-dimensional periodic arrangement. In many crystalline systems, including interstitial carbides, the packing of atoms is such that they occupy a minimum of space and this is known as close packing. 

In a close-packed structure, the atoms of the close-packed planes fit into the depressions of the adjacent planes and each atom is surrounded by six close neighbors in a hexagonal configuration as shown schematically in 

B Interstices pointing upwards C Interstices pointing downwards 

2 Hexagonal Close-Packed (hep) and Face-Centered Cubic 

The close-packed crystalline structures are either hexagonal close-packed (hep) or face-centered cubic close-packed (fcc) In a hexagonal close-packed structure, the atoms of the first layer are directly over those of the third layer and this planar arrangement is shown in Fig. 3.9a and 3.9b. The layer sequence is expressed as ABAB and the resulting crystal has a hexagonal symmetry. 

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The very heavy shades such as blacks and navies cannot be dyed by conventional methods on polyester fibres, even with the help of carriers. Since the difficulty is the slow diffusion of the larger dyestuff molecules into the closely packed crystalline structure of the fibre, the obvious approach is to present simple molecules and cause them to combine to form a coloured pigment after they have entered. Brenthols and Naphthol AS products are not adsorbed by polyesters from aqueous solutions of their sodium salts. A limited measure of success was achieved by immersing the goods in a suspension of naphthoic acid anilide derivatives and then coupling with a diazotized base. The results were more successful, however, when coupling components with smaller molecules were used and a- and /3-naphthoIs, applied from suspension, were satisfactory but jS-hydroxynaphthoic acid proved to be the most suitable. 

Crystalline copper and magnesium have face-centred-cubic and close-packed-hexagonal structures respectively. 

In either of these close-packed structures, each sphere has 12 nearest neighbors 6 in the same plane, 3 in the dimples above, and 3 in the dimples below. The expanded views in Figure 11-30 show the different arrangements of the hexagonal and cubic close-packed crystalline types, hi the hexagonal close-packed structure, notice that the third layer lies directly above the first, the fourth above the second, and so on. The layers can be labeled ABAB. 

Kawakubo s fluorescence results 86> for methyl- and dimethylnaphthalene solids can be similarly related to the crystal structure. Both 2-and 2,6-substituted naphthalenes retain the same close-packed layer structure as seen in naphthalene. The only effect of the methyl substitution is to increase the crystal dimension along the naphthalene long axis87 . Less is known about the crystal structures of 1- and 1,6-substituted naphthalenes, except that the 1-substituent requires a different packing pattern than naphthalene and that 1- and 1,6-substituted naphthalenes have much lower melting points than the 2-substituted naphthalenes. The absence of sandwich pairs in 2- and 2,6-substituted naphthalene crystals certainly explains the lack of excimer fluorescence in the crystal spectra. Presumably, such pairs are also absent in crystalline 1-methylnaphthylene, but they seem to be present in glassy 1-methyl-naphthalene and in 1,6-dimethylnaphthalene solid. 

When we determined the crystalline structure of solids in Chapter 4, we noted that most transitional metals form crystals with atoms in a close-packed hexagonal structure, face-centered cubic structure, or body-centered cubic arrangement. In the body-centered cubic structure, the spheres take up almost as much space as in the close-packed hexagonal structure. Many of the metals used to make alloys used for jewelry, such as nickel, copper, zinc, silver, gold, platinum, and lead, have face-centered cubic crystalline structures. Perhaps their similar crystalline structures promote an ease in forming alloys. In sterling silver, an atom of copper can fit nicely beside an atom of silver in the crystalline structure. 

In the case of metal clusters, for example, valence electrons show the shell structure which is characteristic of the system consisting of a finite number of fermions confined in a spherical potential well [2]. This electronic shell structure, in turn, motivated some theorists to study clusters as atomlike building blocks of materials [3]. The electronic structure of the metallofullerenes La C60 [4] and K C60 [5] was investigated from this viewpoint. This theorists dream of using clusters as atomlike building blocks was first realized by the macroscopic production of C60 and simultaneous discovery of crystalline solid C60, where C60 fullerenes form a close-packed crystalline lattice [6]. 

Characterization by WAXS showed crystalline nanoparticles displaying the expected hexagonal close packed (hep) structure of bulk ruthenium. Reactivity studies were carried out in particular with CO to show the availability of the nanoparticles surface for reactivity. Recently, the synthesis of these PVP-stabilized RuNPs has been reproduced to perform an exhaustive study of the coordination of CO to their surface by a combination of IR and NMR techniques [62]. It has been observed that (1) the coordination mode of the CO to the NP surface depends on the reaction time and (2) CO is mobile. Low reaction-times give rise to CO adsorption in the bridging mode while longer reaction times result in the adsorption of more CO molecules only adsorbed in the linear or multicarbonyle modes. 

We begin by looking at the smallest scale of controllable structural feature - the way in which the atoms in the metals are packed together to give either a crystalline or a glassy (amorphous) structure. Table 2.2 lists the crystal structures of the pure metals at room temperature. In nearly every case the metal atoms pack into the simple crystal structures of face-centred cubic (f.c.c.), body-centred cubic (b.c.c.) or close-packed hexagonal (c.p.h.). 

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