Neutron powder diffraction experimental methods

Diffraction studies have been used in a small number of cases to determine the minimum energy positions of adsorbed moleeules within cationic zeolites. These difficult and relatively expensive experiments have been used to establish computational simulation as a reliable method to predict such interactions without recourse to experimentation. Crystallographic X-ray studies of the adsorption of small moleeules sueh as CO and NO on cationic forms of zeolite are of particular relevance to study of eation-molecule interactions, as are neutron powder diffraction studies of the location of pyridine, coordinatively bound to potassium ions in zeolite K-L, and of benzene, bound to sodium ions in zeolite Na-X. ... 

The most important experimental task in structural chemistry is the structure determination. It is mainly performed by X-ray diffraction from single crystals further methods include X-ray diffraction from crystalline powders and neutron diffraction from single crystals and powders. Structure determination is the analytical aspect of structural chemistry the usual result is a static model. The elucidation of the spatial rearrangements of atoms during a chemical reaction is much less accessible experimentally. Reaction mechanisms deal with this aspect of structural chemistry in the chemistry of molecules. Topotaxy is concerned with chemical processes in solids, in which structural relations exist between the orientation of educts and products. Neither dynamic aspects of this kind are subjects of this book, nor the experimental methods for the preparation of solids, to grow crystals or to determine structures. 

In a paper by Albinati and Willis (1982) the application of the Rietveld method in neutron and X-ray powder diffraction was discussed considering the different experimental techniques of obtaining the diffraction patterns. For a detailed description of the method and its applications see a reference publication (Young 1995). See also Jenkins and Snyder (1996). A frequently used calculation program for the... 

Powder diffraction techniques have developed enormously since 1990. This is partly due to improved X-ray and neutron sources and instrumentation and partly because of improved methods, algorithms and software for structure solution and refinements of experimental data. The more effective computers are also important. 

The next entry in this column refers to the extent of the experimental methods. The terms Einkristalle (= single crystal) or Pulver (= powder) define the method X , N or E indicate the type of rays used (X-rays, neutron or electron diffraction). If only an X-ray examination was carried out, X is omitted. 

The experimental diffraction data were analyzed by a combined technique involving Rietveld analysis, the maximum entropy method (MEM), and MEM-based pattern fitting (MPF) [10-15]. Rietveld analysis, which is used to refine the crystal structure from the powder diffraction data by a least squares method, was carried out using the RIETAN-2000 program [27], which yields structure factors and their errors after structural refinement. It is known that MEM can be used to obtain a nuclear density distribution map based on neutron structure factors and their errors [5, 6, 8, 10-15, 26-29] any type of complicated nuclear density distribution is allowed so long as it satisfies the symmetry requirements. MEM calculations were carried out using the PRIMA program [29]. To reduce the bias imposed by the simple structural model in the Rietveld refinement, an iterative procedure known as the REMEDY cycle [29] was applied after MEM analysis (Fig. 6.3). In this procedure, structure factors... 

VMRIA VMRIA a program for a full profile analysis of powder multiphase, neutron diffraction time of ffight (direct and Fourier) spectra, V. B. Zlokazov, V. V. Chernyshev, J. Appl. Crystallogr., 1992, 25, 447 451 and DELPHI based visual object oriented programming for the analysis of experimental data in low energy physics, V. B. Zlokazov, Nucl. Instrum. Methods Rhys. Res. A, 2003, 502(2 3), 723 724 Rietveld refinement of TOF data... 

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