Diffraction General Structure Analysis

A.C. Larson and R.B. Von Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratory Report, LAUR 86-748 (2000). Although GSAS is suitable for treatment of both powder diffraction and single crystal data, in the context of this book we are chiefly concerned with its capabilities to processing powder data. 

Applications The general applications of XRD comprise routine phase identification, quantitative analysis, compositional studies of crystalline solid compounds, texture and residual stress analysis, high-and low-temperature studies, low-angle analysis, films, etc. Single-crystal X-ray diffraction has been used for detailed structural analysis of many pure polymer additives (antioxidants, flame retardants, plasticisers, fillers, pigments and dyes, etc.) and for conformational analysis. A variety of analytical techniques are used to identify and classify different crystal polymorphs, notably XRD, microscopy, DSC, FTIR and NIRS. A comprehensive review of the analytical techniques employed for the analysis of polymorphs has been compiled [324]. The Rietveld method has been used to model a mineral-filled PPS compound [325]. 

Abstract Electron diffraction Structure Analysis is generally used to study thin films... 

X-ray crystal structure analysis showed no crystallinity. Fructan is amorphous. X-ray analysis was performed on a General Electric X-ray diffraction refractometer. 

For solid surfaces with crystalline structure, we can apply diffraction techniques for analysis. In a diffraction experiment the sample surface is irradiated with electrons, neutrons, atoms, or X-rays and the angular distribution of the outgoing intensity is detected. The analysis of diffraction patterns is a formidable task and in the first subsection we only introduce a simple case, which, nevertheless, contains the main features. A more general formalism for the interested reader is described in the Appendix. 

The program just described, for Rietveld analyses using generalized coordinates, has been used in the structural analysis of isotactic polypropylene recently undertaken both with x-ray and with neutron powder diffraction data. We believe this analysis (Immirzi, in preparation) to be the first Rietveld analysis of a polymer done from x-ray data. Rietveld analyses of polymers from neutron data have been done but, at least in the polyethylene case reported by Willis and co-workers (15), there was no use of generalized coordinates. 

Diffraction experiments by X-rays, electrons, and neutrons are the main methods that fall to be considered under this heading. These methods can all be strictly quantitative, but their power in structural analysis and the kind of information that can be obtained from them are very different. It is true that some of the most accurate measurements of bond length can be made by spectroscopic methods, particularly in the microwave region, but these methods can generally be applied only to very simple molecules. For structures of the complexity considered in this article, diffraction methods are by far the most important. 

X-ray structural analysis or, more generally, structure determination based on the analysis of diffraction pictures, is very a reliable method delivering us enormous amount of chemical information. These techniques are commonly considered objective, that is, independent of preliminary assumptions. In this chapter, some criticism of this common belief is given, though it is not the intention of the authors to question the importance and validity of the diffraction techniques. On the contrary, our aim is to stress the importance and value of structural data derived from diffraction, but we would also emphasize the important role of creating and developing structural models by other approaches and subsequent use of the models in structure analysis. Molecular modeling is one, if not the most important, possibility to do this. 

Exactly 10 years after the previous statement appeared, the first lithium enolate crystal structures were published as (5) and (6). Thus, structural information derived from X-ray diffraction analysis proved the tetrameric, cubic geometry for the THF-solvated, lithium enolates derived from r-butyl methyl ketone (pinacolone) and from cyclopentanone. Hence, the tetrameric aggregate characterized previously by NMR as (7) was now defined unambiguously. Moreover, the general tetrameric aggregate (7) now became embellished in (5) and (6) by the inclusion of coordinating solvent molecules, i.e. THE. A representative quotation from this 1981 crystal structure analysis is given below. 

Bruker AXS, TOPAS V2.1 General Profile and Structure Analysis Software for Powder Diffraction Data.-User s Manual, Bruker AXS, Karlsruhe, Germany, 2003. 

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