Super-structures

The compounds characterized by X Me = 3.5 have a common formula of M2Me205F2 and crystallize either in a pyrochlore [192] or a veberite [229] type structure. According to X-ray powder diffraction patterns, the structure of Na2Nb205F2 can be regarded as a super-structure of pyrochlore, which is made up of octahedrons connected in layers and arranged in the (111) direction. The layers are linked via octahedrons so that each octahedron in one layer shares three vertexes with an octahedron in the adjacent layer. 

Fig. 8-63 After removal from the mold, work continued on the ship to which the super structure was added.

Fig. 11 Formation of crystalline 3D super-lattices of tin nanoparticles a TEM view of a facetted super-crystal b SEM image showing particles included into a super-crystal as well as the organic surrounding c High resolution micrograph showing the alignment of the tin atomic planes inside the super-structure...

This study could be extended to the synthesis of iron nanoparticles. Using Fe[N(SiMe3)2]2 as precursor and a mixture of HDA and oleic acid, spherical nanoparticles are initially formed as in the case of cobalt. However, a thermal treatment at 150 °C in the presence of H2 leads to coalescence of the particles into cubic particles of 7 nm side length. Furthermore, these particles self-organize into cubic super-structures (cubes of cubes Fig. ) [79]. The nanoparticles are very air-sensitive but consist of zerovalent iron as evidenced by Mossbauer spectroscopy. The fact that the spherical particles present at the early stage of the reaction coalesce into rods in the case of cobalt and cubes in the case of iron is attributed to the crystal structure of the metal particles hep for cobalt, bcc for iron. 

The defect structure of Fei O with the NaCl-type structure had been estimated to be a random distribution of iron vacancies. In 1960, Roth confirmed, by powder X-ray diffraction, that the defect structure of wiistite quenched from high temperatures consists of iron vacancies (Vp ) and interstitial iron (Fcj) (there are about half as many FCj as Vpe). This was a remarkable discovery in the sense that it showed that different types of crystal defects with comparable concentrations are able to exist simultaneously in a substance, Roth also proposed a structure model, named a Roth cluster, shown in Fig. 1.84. Later this model (defect complex = vacancy -F interstitial) was verified by X-ray diffraction on a single crystal and also by in-situ neutron diffraction experiments. Moreover, it has been shown that the defect complex arranges regularly and results in a kind of super-structure, the model structure of which (called a Koch-Cohen model) is shown in Fig. 1.85 together with the basic structures (a) and (b). 

Ca and Cg) for simplicity. In this case two types of vernier structure are considered as shown in Fig. 2.32. Structure [a] is composed of A- and B-planes, which are stacked alternatively along the h-axis as. .. ABAB. .. Due to the difference between the dimensions of the unit cells of each plane, the unit length of the supercell structure along the c-axis is expressed by eqn (2.7). In structure [b], the structures A and B are mutually interpenetrated. The dimension of the super structure along the c-axis is also expressed by eqn (2.7). In each case, the relation between structures A and B is very similar to that between the main scale and vernier scale in vernier calipers. 

At the present time, there are many examples of super-structured hemes, highly stable in interactions with molecular oxygen at room temperature [16-19],... 

Fig. 11. A view of the ribbon-like hydrogen-bonded super-structure of the 6-hydroxy- ra ,S -3-hexenoic acid 107 in the crystal.

Many of the alignment media presented so far possess stereogenic centres, but only media with a large chiral super structure are referred to as chiral alignment media, since only those are capable of distinguishing enantiomers (see also Section 6.3). 

In a 1 1 complex between alcohols and primary amines, the usual zig-zag chains of H-bonds may be joined together alternately by N(H)0 interactions to generate sheets of trans-fused super-cyclohexane rings with chair-like conformations. From a topological point of view, these super-structures resemble the sheet structure of the stable allotrope of arsenic as super-arsenic sheets in which 0(H)0, N(H)N, and N(H)0 interactions are involved (Figure 4) [28],... 

Other super-structures for 1 1 alcohol-amine complexes from polyhedral H-bond clusters with the shape of a tetrahedron, cube, pentagonal dodecahedron, trigonal-prismatic, truncated-icosahedral, or even extremely hollow truncated-icosahedral are theoretically possible [28] (Figure 8). 

In Figures 10-13 are illustrated the stereopacking diagrams of complexes 3 5, 3 6, 4 5, and 4 6. In these cases, a super-adamantane unit may be cut out as a characteristic building block of the super-diamond architecture. The space group of the former super-structure was found to be P2 /c, which is a subgroup of that of cubic diamond, Fd im. 

The H-bonded sheets create distorted super-cyclohexane boats (and not chairs ) that are neither zincblende-nor wurtzite-like super-structures. A careful revision of the literature reveals other interesting examples of super-structures formed by alcohols and amines. For example, during their studies dealing with the preparation of a stable form of hydrazine, Toda and co-workers [41] reported the formation of the supramolecular 1 1 amine-alcohol complex 1 3 between hydrazine (1) and hydroquinone (3) (Scheme 3). 

The space-filling representation of the complex (R,R)-29 (R,R)-44 in which the triple-stranded helicate super-structure is more evident is shown in Figure 54. 

We will consider below self-assembled superstructures, like collagen, plant cell walls, starch and the contractile macromolecular complex. These super structures are also liquid crystals, evident from their birefringence and rheological properties. With increasing knowledge of the molecular mechanisms behind function, however, there has been an unfortunate tendency to neglect the role of overall structure. 

Fig. 17.7 (a) STM image (40 nm x40 nm) and LEED pattern, and (b) top and side view structural model of the Nb Oj oxide film grown on Cu AuflOO). The STM image in (a) shows two domains rotated by 90°. The rhombic j3x-j3 R30° nit cell the niobia layer and the (2x7) super structure between oxygen and metal atoms are indicated. Adapted in part from [44] with permission. Copyright (2005) Elsevier... 

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