Superstructures crystallography

By following the discussed experiments, a model for the Ir-Cu(lOO) surface has been suggested and is presented in Fig. 14. As a matter of fact, a two dimensional epitaxial sub-surface alloy has developed and consists of adjacent chains of Ir and Cu atoms along the [Oil] directions to form an ordered (2x1) periodicity. The Ir-Cu sub-surface layer happens to be buried under a monolayer of copper. Remarkably enough, although the surface crystallography of Cu(lOO) expresses four-fold symmetry, a two fold symmetric pattern is showing up for the chains of subsurface Ir to resemble the (2x1) superstructure. 

Even without atomic resolution, AFM has proved its worth as a technique for the local surface structural determination of a number of bio-inorganic materials, such as natural calcium carbonate in clam and sea-urchin shells [123]. minerals such as mica [124] and molybdenite [125] as well as the surfaces of inorganic crystals, such as silver bromide [126] and sodium decatungstocerate [127]. This kind of information can prove invaluable in the understanding of phenomena such as biomineralization, the photographic process or catalysis, where the surface crystallography, especially the presence of defects and superstructures, can play an important role, but is difficult to determine by other methods. AFM has the considerable advantage that it can be used to examine powdered samples, either pressed into a pellet, if the contact mode is employed, or loosely dispersed on a surface, if intermittent or non-contact AFM is available. 

In addition to X-ray crystallography, a number of other techniques have demonstrated the existence of intramolecular hydrogen bonds in the polypeptide chain. Hydrogen bonds form between the carbonyl and amide groups of nearly all the polypeptide chains. They are responsible for the secondary structure of proteins. Thus, intramolecular interactions (hydrogen bonds and others—see below) impose a new superstructure upon the primary structure of the polypeptide, and this first level of suprastructure is referred to as the protein s secondary structure. 

A wide range of instrumental techniques are needed to characterise products fully - X-ray crystallography, mass spectrometry (especially FAB-MS and electrospray MS), H and NMR, UV-Vis spectroscopy, and electrochemistry - in the solid state and in solution. As much information as possible is needed in order to establish both the exact nature and long-range structural features (superstructure) of rotaxanes, catenanes and knots. As noted at appropriate points in the text, there is considerable interest in applications for these classes of compounds, particularly in respect to molecular switching devices. 

For the nomendature, see footnote 2 in Ref. [77], which is reprinted here verbatim for convenience In the mathematical literature, and in some of the crystallography literature, these derivative lattices are referred to as sublattices. Although this nomendature is more correct from a mathematical and/or group theory point of view, we follow the nomenclature typically seen in the physics literature where a lattice (or a structure) whose volume is larger than that of the parent is referred as a superlattice (or a superstructure). ... 

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