Alternative unit layers

Besides the M layer, other unit layers were introduced, in most cases to simplify the description of some features, such as the diffraction pattern. 

Despite that, the terms Franzini-type A and B have found their way into the literature. As Franzini (1969) noted, owing to ditrigonalization, the basal-oxygen atoms in the type A approach the cations in the adjacent octahedral sheet, whereas they move 

1 Griffen (1992) described the direction of the ditrigonalization of the T sheets with respect to the triangular bases of the octahedra in terms of the rotations O (opposite, corresponding to Franzini-type A of layer) and S (same, corresponding to Franzini-type B of layer). This terminology, borrowed from pyroxenes, is commonly not adopted for micas. 

Furthermore, both Ml and M2 mica layers within the same family consist of the same kind of OD packet. 

The crystal chemical reason for polytypism is that adjacent layers (two-dimensionally-periodic units) can be linked to each other in many translationally nonequivalent ways. However, the nearest-neighbor relationships remain preserved. Translated into the language of symmetry, this means that the pairs of adjacent layers remain geometrically equivalent in all polytypes of the same family. 

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This type of chain arrangement was observed only in the case of a-[Fe(Cp )2][Pt (mnt)2]. In the crystal structure of this salt, layers of parallel type II chains, with a net charge (—) per repeat unit, [A2]2 D+, alternate with layers presenting a D+D+ A-... 

The crystal structure of NaFeCb is rhombohedral. If the oxide ions are assigned to P layers, the Na+ and Fe3+ ions fill alternate O layers thus, all O layers are filled by metal ions, but because of the alternation of C>Na and C>Fe layers there are 12 layers in the repeating unit. As shown in Figure 5.7, the relative positions and heights of all packing layers are given below. 

Cuproscheelite, CuWCh, is triclinic, Cj, PI, a = 4.7026, b = 5.8389, c = 4.8784 A, a = 91.677°, (3 = 92.469°, and y = 82.805°, with two molecules in the unit cell. In Figure 5.12a, the oxygen atoms are in AB close-packed layers with the packing direction along b. Cu and W occupy half of alternate O layers. O sites occur only at C layers, but the metal ions occupy two sites alternating with two vacancies along b. The occupied sites for W are above and below the vacancies in the Cu layer. The CuC>6 and WC>6 octahedra share edges in... 

Brucite, Mg(OH)2, has a structure very similar to that of Cdl2. The trigonal unit cell contains one molecule, D d, P3ml, aQ = 3.142, and cQ = 4.766 A. Figure 5.59 shows the layers. Hydrogen atoms are on the outside of the POP sandwiches. Mg2+ ions are only at C positions, but alternate O layers are vacant. 

Thallium(I) oxide, T120, has a monoclinic modification, Q h, C2/m, a = 6.082, b = 3.52, c = 13.24 A, and p = 108.2°, with four molecules in the unit cell. Figure 5.63 shows the T1 atoms in close-packed layers in an AB sequence. Alternate O layers are filled by oxygen atoms giving the notation 2 3/2POP(m). The octahedra share edges. This is an unusual structure with oxygen atoms in octahedra. The Tl-O distances are 2.511-2.531 A. The Tl-Tl distances within a plane are 3.514 A, within... 

The structure of a 2 1 complex between antimony trichloride and phenanthrene consists of alternating double layers of SbCl3 and arene molecules (276). Unlike complex Cl and SbCl3 itself, Sb—Cl contacts between neighboring SbCl3 units are observed in this complex. In both structures mentioned here, vacant coordination sites are observed, which might be occupied by a stereochemically active lone pair at the antimony center. 

The surface coverage of an adsorbate is another important parameter in ordering. We shall use the common definition of coverage where one monolayer corresponds to one adsorbate atom or molecule for each unit cell of the clean, unreconstructed substrate surface. Thus, if an adsorbed undissociated carbon monoxide molecule bonds to alternating top-layer metal atoms exposed at the Ni(100) surface, we have a coverage of a half monolayer. 

Biissem (B12) determined the approximate crystal structure of C4AF. Subsequent determinations or refinements were reported for preparations with. Y = 0 (C F) (B13,C4,B11),. v = 0.285 and 0..36 (C2) and, v = 0.5 (C3). Fig. 1.8 shows the structure for the compositional range with 0.33 <. Y < 0.7. It is built from layers of corner-sharing octahedra similar to those in perovskite (CaTiOj), alternating with layers composed of chains of tetrahedra, together with Ca " ions. The layers are perpendicular to the h axis and the chains are parallel to c. The composition of an individual octahedral layer in the ac cross-section of the unit cell is MiO. and that of an individual chain is where M and T denote octahedral and... 

Finally, it may also be pointed out that the structure of baddeleyite itself may be described in a similar manner very shghtly distorted (100) unit layers of fluorite-type (ZrO layers of edge-shared OZt4 tetrahedra) alternate with commensurate 3 4 3 4 layers of anions. Small rotations of the triangles in the latter convert 3 4 3 4 to 3 (the anion-only layers in Y 0 iF 4.2) similarly, small rotations of the squares convert 3 4 3 4 to 4 (the anion-only layers in the fluorite type) . ... 

FIG. 1.4. (a) Alternative unit cells of layer structure of AS2O3. (b) Projection of unit cell of a... 

Fig. 10.3 (a) The solid state structure of Li3N consists of layers of N and Li+ ions (ratio 1 2) alternating with layers of Li+ ions the latter are arranged such that they lie over the ions. Each N centre is in a hexagonal bipyramidal (8-coordinate) environment there are two types of Li ion, those in layer 1 are 2-coordinate, and those in layer 2 are 3-coordinate with respect to the N centres (see problem 10.12 at the end of the chapter), (b) The unit cell of sodium nitride Na3N adopts an anti-Re03 structure. Colour code N, blue Li, red Na, orange. 

Careful examination of the unit cell packing illustration (Figure 2), together with crude placement of the benzene molecules, affords a microscopic picture of the crystalline solid in which there are layers of solid material alternating with layers of liquid (Structure 11). (There are two... 

The structure of the a form was early established. Natta and Corradini (Natta Corradini, 1960) proposed a model based on the monoclinic unit cell. According to this model, polypropylene helices of the same hand are arranged in layers parallel to the ac plane. Layers of isochiral helices alternate with layers formed from helices of the opposite hand. The model was commonly accepted and confirmed by subsequent studies of Menok (Mencik, 1972), Hikosaka (Hikosaka Seto, 1973 ) and Corradini (Corradini et al., 1980). 

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