Crystalline materials structural analysis

The classical approach for determining the structures of crystalline materials is through diflfiaction methods, i.e.. X-ray, neutron-beam, and electron-beam techniques. Difiiaction data can be analyzed to yield the spatial arrangement of all the atoms in the crystal lattice. EXAFS provides a different approach to the analysis of atomic structure, based not on the diffraction of X rays by an array of atoms but rather upon the absorption of X rays by individual atoms in such an array. Herein lie the capabilities and limitations of EXAFS. 

Solid state NMR is a relatively recent spectroscopic technique that can be used to uniquely identify and quantitate crystalline phases in bulk materials and at surfaces and interfaces. While NMR resembles X-ray diffraction in this capacity, it has the additional advantage of being element-selective and inherently quantitative. Since the signal observed is a direct reflection of the local environment of the element under smdy, NMR can also provide structural insights on a molecularlevel. Thus, information about coordination numbers, local symmetry, and internuclear bond distances is readily available. This feature is particularly usefrd in the structural analysis of highly disordered, amorphous, and compositionally complex systems, where diffraction techniques and other spectroscopies (IR, Raman, EXAFS) often fail. 

ReflEXAES can be used for near-surface structural analysis of a wide variety of samples for which no other technique is appropriate. As with EXAES, ReflEXAES is particularly suited for studying the local atomic structure around particular atomic species in non-crystalline environments. It is, however, also widely used for the analysis of nanocrystalline materials and for studying the initial stages of crystallization at surfaces or interfaces. ReflEXAES was first proposed by Barchewitz [4.135], and after several papers in the early nineteen-eighties [4.136, 4.168-4.170] it became an established (although rather exotic) characterization technique. Most synchrotron radiation sources now have beam-lines dedicated to ReflEXAES experiments. 

In spite of the slow development of crystal structure analysis, once it did take olT it involved a huge number of investigators tens of thousands of crystal structures were determined, and as experimental and interpretational techniques became more sophisticated, the technique was extended to extremely complex biological molecules. The most notable early achievement was the structure analysis, in 1949, of crystalline penicillin by Dorothy Crowfoot-Hodgkin and Charles Bunn this analysis achieved something that traditional chemical examination had not been able to do. By this time, the crystal structure, and crystal chemistry, of a huge variety of inorganic compounds had been established, and that was most certainly a prerequisite for the creation of modern materials science. 

Polycrystalline and well-oriented specimens of pure amylose have been trapped both in the A- and B-forms of starch, and their diffraction patterns84-85 are suitable for detailed structure analysis. Further, amylose can be regenerated in the presence of solvents or complexed with such molecules as alcohols, fatty acids, and iodine the molecular structures and crystalline arrangements in these materials are classified under V-amylose. When amylose complexes with alkali or such salts as KBr, the resulting structures86 are surprisingly far from those of V-amyloses. 

Recent developments and prospects of these methods have been discussed in a chapter by Schneider et al. (2001). It was underlined that these methods are widely applied for the characterization of crystalline materials (phase identification, quantitative analysis, determination of structure imperfections, crystal structure determination and analysis of 3D microstructural properties). Phase identification was traditionally based on a comparison of observed data with interplanar spacings and relative intensities (d and T) listed for crystalline materials. More recent search-match procedures, based on digitized patterns, and Powder Diffraction File (International Centre for Diffraction Data, USA.) containing powder data for hundreds of thousands substances may result in a fast efficient qualitative analysis. The determination of the amounts of different phases present in a multi-component sample (quantitative analysis) is based on the so-called Rietveld method. Procedures for pattern indexing, structure solution and refinement of structure model are based on the same method. 

Thus the analysis of texture patterns expands the present possibilities of electron crystallography for accurate structure characterization of crystalline materials, which can form microcrystalline textured samples. 

How modeling has been useful in the crystal structure analysis of polysaccharides—and how it could lead to a better understanding of other condensed j)hase states—can be illustrated with structural worK done on cellulose. It is one of the world s most important and widely used raw materials whose structure, properties, derivatives, and transformations remain under continuous study. Some of the results, problems, and indications of future directions resulting from the study of its crystalline structure—and the attendant roles for molecular modeling—are briefly described in the following. 

Many substances can rotate the plane of polarization of a ray of plane polarized light. These substances are said to be optically active. The first detailed analysis of this phenomenon was made by Biot, who found not only the rotation of the plane of polarization by various materials (rotatory polarization) but also the variation of the rotation with wavelength (rotatory dispersion). This work was followed up by Pasteur, Biot s student, who separated an optically inactive crystalline material (sodium ammonium tartrate) into two species which were of different crystalline form and were separately optically active. These two species rotated the plane of polarized light equally but in opposite directions and Pasteur recognized that the only difference between them was that the crystal form of one was the mirror image of the other. We know to-day, in molecular terms, that the one necessary and sufficient condition for a substance to exhibit optical activity is that its molecular structure be such that it cannot be superimposed on its image obtained by reflection in a mirror. When this condition is satisfied the molecule exists in two forms, showing equal but opposite optical properties and the two forms are called enantiomers. 

Transmission electron microscopy (tern) is used to analyze the structure of crystals, such as distinguishing between amorphous silicon dioxide and crystalline quartz. The technique is based on the phenomenon that crystalline materials are ordered arrays that scatter waves coherently. A crystalline material diffracts a beam in such a way that discrete spots can be detected on a photographic plate, whereas an amorphous substrate produces diffuse rings. Tern is also used in an imaging mode to produce images of substrate grain structures. Tern requires samples that are very thin (10—50 nm) sections, and is a destructive as well as time-consuming method of analysis. 

The characteristics of the process of X-ray crystal structure analysis have led to an undue emphasis on classically crystalline materials to the neglect of organized structures which do not give simple diffraction patterns with sharp spots. 

A specialized method for sample preparation is to fracture or cleave the sample inside the vacuum system, thus creating a fresh surface for immediate analysis. Some crystalline materials (semiconductors, anisotropic structures such as graphite, etc.) have preferred cleavage planes that can be sectioned inside the chamber using a blade or chisel (operated through bellows... 

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