Close-packed layer

The systematic study of alkanetliiols (CH2(CH2) -SH) witli different chain lengtlis revealed tliat for = 11 or greater closely packed layers are obtained witli a tilt angle of tire alkyl chains between 30 and 35° from tire surface... 

In a pure metal the atoms of the solid are arranged in closely packed layers. There is more than one way of achieving close packing but it... 

In Table 5.3, is compared with the total hydroxyl concentration (Ni, + N ) of the corresponding fully hydroxylated, sample. The results clearly demonstrate that the physical adsorption is determined by the total hydroxyl content of the surface, showing the adsorption to be localized. It is useful to note that the BET monolayer capacity n JH2O) (= N ) of the water calculated from the water isotherm by the BET procedure corresponds to approximately 1 molecule of water per hydroxyl group, and so provides a convenient means of estimating the hydroxyl concentration on the surface. Since the adsorption is localized, n.(H20) does not, of course, denote a close-packed layer of water molecules. Indeed, the area occupied per molecule of water is determined by the structure of the silica, and is uJH2O) 20A ... 

The most important compound of Mo (TV) is molybdenum disulfide [1317-33-5] M0S2 (21). The layered stmcture of M0S2 is reflected in the flat plate-like hexagonal gray-black crystaUites found in natural and synthetic samples. The stmcture consists of pairs of close-packed layers of sulfur which are echpsed with respect to each other. The close-packed sulfur surfaces are naturally hydrophobic, which faciUtates the extraction of M0S2 ore by flotation. 

The fdr studies reveal that the alkyl chains in SAMs of thiolates on Au(lll) usually are tilted 26-28° from the surface normal, and display 52-55° rotation about the molecular axis. This tilt is a result of the chains reestabUshing VDW contact in an assembly with - 0.5 nm S—S distance, larger than the distance of - 0.46 nm, usually quoted for perpendicular alkyl chains in a close-packed layer. On the other hand, thiolate monolayers on Ag(lll) are more densely packed owing to the shorter S—S distance. There were a number of different reports on chain tilt in SAMs on Ag(lll), probably owing to different amounts of oxide, formed on the clean metallic surface (229,230,296,297). In carefully prepared SAMs of alkanethiolates on a clean Ag(lll) surface, the alkyl chains are practically perpendicular to the surface. 

Titanium Trichloride. Titanium trichloride [7705-07-9] exists in four different soHd polymorphs that have been much studied because of the importance of TiCl as a catalyst for the stereospecific polymerization of olefins (120,124). The a-, y-, and 5-forms are all violet and have close-packed layers of chlorines. The titaniums occupy the octahedral interstices between the layers. The three forms differ in the arrangement of the titaniums among the available octahedral sites. In a-TiCl, the chlorine sheets are hexagonaHy close-packed in y-TiCl, they are cubic close-packed. The brown P-form does not have a layer stmcture but, instead, consists of linear strands of titaniums, where each titanium is coordinated by three chlorines that act as a bridge to the next Ti The stmctural parameters are as follows ... 

On silicon carbide, it is easier to see and measure step heights than in crystals like beryl, because SiC has polytypes, first discovered by the German crystallog-rapher Baumhauer (1912). The crystal structure is built up of a succession of close-packed layers of identical structure, but stacked on top of each other in alternative ways (Figure 3.24). The simplest kind of SiC simply repeats steps ABCABC, etc., and the step height corresponds to three layers only. Many other stacking sequences... 

ScCl, made up of close-packed layers of Sc and Cl atoms in the sequence Cl-Sc-Sc-Cl has, like analogous Y and La materials with Cl and Br, since been shown to have been stabilised by interstitial H impurity. ... 

The holes in the close-packed structure of a metal can be filled with smaller atoms to form alloys (alloys are described in more detail in Section 5.15). If a dip between three atoms is directly covered by another atom, we obtain a tetrahedral hole, because it is formed by four atoms at the corners of a regular tetrahedron (Fig. 5.30a). There are two tetrahedral holes per atom in a close-packed lattice. When a dip in a layer coincides with a dip in the next layer, we obtain an octahedral hole, because it is formed by six atoms at the corners of a regular octahedron (Fig. 5.30b). There is one octahedral hole for each atom in the lattice. Note that, because holes are formed by two adjacent layers and because neighboring close-packed layers have identical arrangements in hep and ccp, the numbers of holes are the same for both close-packed structures. 

FIGURE 5.30 The lot ations of (a) tetrahedral and (b) octahedral holes Note that both types of holes are defined by two neighboring close-packed layers, so they are present with equal abundance in both hep and ccp structures. 

Consider a metallic element that crystallizes in a cubic close-packed lattice. The edge length of the unit cell is 408 pm. If close-packed layers are deposited on a flat surface to a depth (of metal) of 0.125 mm, how many close-packed layers are present ... 

There are three close-packed layers from which all of the infinite number of close-packed structures can be built namely, A, with an atom at J=0, Y — 0 (X, Y, Z being hexagonal coordinates) B, with an atom at X = Y= and G,... 

Close-packed layers of halogen (and interstitials) run horizontally. One pair of the short II 12 inter-cluster contacts is marked at the lower left. 

At saturation coverage, the Pb oxidation process is split into three peaks, and the charge under the Pb redox process amounts to approximately 300 iJiC/cm, indicating the formation of a close-packed layer. 

The hardest of the transition-metal borides are the diborides. Their characteristic crystal structure (Figure 10.6) consists of plane layers of close-packed metal atoms separated by plane openly-patterned layers of boron atoms ( chicken-wire pattern). If the metal atoms in the hexagonal close-packed layer have a spacing, d, then the boron atoms have a spacing of d/V3. 

In most oxides, the oxygen atoms are present as close-packed layers stacked either to produce cubic symmetry or hexagonal symmetry. Some of the cubic cases have already been discussed. Now some hexagonal cases will be considered. 

Figure 3.11 Cubic close-packed structure of face-centered cubic crystals such as copper as a packing of atom layers (a) a single close-packed layer of copper atoms (b) two identical layers, layer B sits in dimples in layer A (c) three identical layers, layer C sits in dimples in layer B that are not over atoms in layer A. The direction normal to these layers is the cubic [111] direction.

The difference between the fee and hep structures is best seen if one considers the sequence of close-packed layers. For fee lattices this is the (111) plane (see Fig. A.l), for hep lattices the (001) plane. The geometry of the atoms in these planes is exactly the same. Both lattices can now be built up by stacking close-packed layers on top of each other. If one places the atoms of the third layer directly above those of the first, one obtains the hep structure. The sequence of layers in the <001> direction is ababab, etc. In the fee structure, it is every atom of the fourth layer that is above an atom of the first layer. The sequence of layers in the <111> direction is abcab-cabc... 

Uptake measurements were made [16] at several oxide/solution ratios, reported as surface loading (SL) or m2 oxide surface/liter of solution, as PdCl, 2 concentration was increased and pH was held constant at the optimal value (Figure 6.10a). Each SL indeed indicated a plateau near the steric value [16], For Pt and Pd ammine cations, the maximum surface density over many oxides appears to be a close-packed layer, which retains two hydration sheaths representative results for PTA uptake over silica from a recent paper [19] are shown in Figure 6.10b. The physical limit of cationic ammine surface density thus appears to be 0.84 pmol/m2, or about 1 cationic complex/2 nm2. Cationic uptake, therefore, is inherently half of anion uptake in many cases. 

Coordinative unsaturation arises from the fact that because of steric and electronic reasons, only a limited number of ligands or nearest neighbors can be within bonding distance of a metal atom or ion. In most transition metal oxides, the oxygen anions in the bulk form closed-packed layers and the metal cations occupy holes among the anions as schematically depicted in Fig. 2.1. In this picture, the oxide ion ligands appear to have saturated the coordination sphere of the bulk cation. 

The tI10-MoNi4 type is another superstructure based on face-centred cubic pseudo-cells. In the projection shown in Fig. 3.36, inside the true cell, the pseudo-cubic subcell (aps = 362 pm, cps = 356.4 pm) has been evidenced by dotted lines. Close-packed layers can be identified in this structure they are stacked in a 15 close-packed repeat sequence. 

In the Mg structure the atoms are arranged in close-packed layers stacked in the sequence ABAB... (equivalent to BCBC..., etc) the corresponding layer symbol (triangular nets) is Mgo.25Mgo.75. Compare this symbol with those relevant to Cu, La, Sm. 

The atoms are arranged in close-packed layers stacked in the ABC sequence (see the triangular net stacking symbol). 

The As atoms form close-packed layers in the hexagonal stacking sequence. .. BC. The Ni atoms (on sites A) are placed in the centres of all the octahedral interstices in the As layers. Ni has 6 As neighbours and As is surrounded by 6 Ni (forming a right trigonal prism). 

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