Structural Analyses X-ray Diffraction

Abstract Structural analyses of materials are presented using x-ray diffraction techniques. Rietveld method was applied for determining cell parameters. 

Keywords Crystallite sizes Rietveld Stmcture X-ray parameters 

Few scientific events have had such an impact on humanity as the discovery of X-rays by Rontgen in November 1895. At that time, even before the nature of these rays was fully xmderstood, there were immediate industrial and medical applications. The first radiographs appeared as early as 1896. The exact nature of X-radiation was only established in 1912, when Max von Laue discovered the phenomenon of diffraction by crystals. He proved that X-rays are in fact electromagnetic waves and, at the same time, he discovered a rather powerful method for studying the structure of materials. In practice, diffraction is applicable to a vast array of scientific problems and technologies. All structures known to date have been determined by diffraction data from X-rays, neutrons, or electrons. Some notable examples are the double-helix structure of DNA, the structures of hemoglobin, vitamins, proteins, minerals, polymers, metals, and ceramics [1]. 

X-ray diffraction is not only applicable to determining material structures. It is frequently used in the laboratory to identify phases in samples of unknown materials, to quantitatively analyze phases, and to determine the size of crystallites and the crystallinity of a material. It may also be applied to measure micro-deformation and stress in steel parts, to characterize metallic substrates and deposits in microelectronics, and to study property variations of materials with regard to temperature, pressure, and atmosphere. 

All the potential for X-ray diffraction for the analysis of materials is due to one simple fact the wavelength of X-radiation is of the same order of magnitude of separation between atoms in matter in a condensed state. X-rays are electromagnetic waves with wavelengths of the order of magnitude of 10 ° m, 1 A, or 0.1 nm. 

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The techniques of determining molecular structure by X-ray diffraction analyses have been initiated, developed, and expanded during the twentieth century, and in this book we have tried to show how the method works and what type of results can be expected. 

Illustration of the closepacking of two types of motifs, such as the Na" " and cr ions in NaCI as proposed in 1898 just before it became possible to analyse crystal structures by X-ray diffraction. 

Markova et al. (1967) constructed a phase diagram for the dysprosium-yttrium system utilizing thermal analyses, X-ray diffraction, metallography, hardness and electrical resistance measurements. Their starting materials were distilled yttrium and dysprosium of 99.6 to 99.7(wt )% purity. Both metals contained gaseous impurities as well as the metals Ca, Fe, Cu, Y, Gd, Dy and Ta. Alloys were prepared at approximately 10 at% intervals, arc-melted under purified helium and annealed at 850°C for 70 hr. The structure of all specimens was single phase with the hep lattice. A continuous series of solid solutions between isomorphic modifications was observed. 

Modern knowledge of cell wall structure comes from detailed chemical analyses, X-ray diffraction studies and work with the electron microscope. The dry primary cell wall contains hemicelluloses (up to 50%), cellulose (up to 25%) and smaller amounts of pectic substances, fats and protein. Hemicelluloses, cellulose and pectins are all polysaccharides with molecules built up from the linking together in chains of sugars or uronic acid residues. The morphology of cell walls is, however, not destroyed by extracting the hemicelluloses and pectins, indicating that the structural framework of the wall is... 

A preliminary least-squares refinement with the conventional, spherical-atom model indicated no disorder in the low-temperature structure, unlike what had been observed in a previous room-temperature study [4], which showed disorder in the butylic chain at Cl. The intensities were then analysed with various multipole models [12], using the VALRAY [13] set of programs, modified to allow the treatment of a structure as large as LR-B/081 the original maximum number of atoms and variables have been increased from 50 to 70 and from 349 to 1200, respectively. The final multipole model adopted to analyse the X-ray diffraction data is described here. 

The structures of the bryostatins were determined by a combination of singlecrystal X-ray diffraction analysis and/or a series of detailed spectroscopic analyses. 

The techniques of X-ray diffraction analyses of crystals of compounds of interest can be used to determine, with high precision, the three-dimensional arrangement of atoms, ions and molecules in such crystals (14) in each case the result is referred to as the "crystal structure." X-ray diffraction by crystals was discovered by von Laue, Friedrich and Knipping (15) and the technique was applied by the Braggs to the determination of the structures of... 

Introduce instrumental techniques used in analysis of the bioinorganic systems I will lecture on (Chapter 3 Instrumental and Computer-Based Methods). Typically, these would be electron paramagnetic resonance (EPR) and Mossbauer spectroscopies not often covered in undergraduate instrumental analysis courses plus X-ray diffraction and NMR techniques used for structural analyses of metalloproteins and their small molecule model compounds. 

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