European Powder Diffraction EPDIC 8

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Authors: Matteo Leoni
Abstract: The role of grain surfaces and interfaces in nanocrystalline materials is investigated by X-ray diffraction (XRD). Two apparently distinct aspects show the importance of the interaction of a single grain with the environment in which it is placed: the role of grain-grain interaction in determining the residual stress in a polycrystalline thin film and the surface effects in single powder grains on the macroscopic average cell parameter determined by powder diffraction. A brief theoretical introduction of the phenomena and their possible modelling is presented together with a discussion of practical examples.
Authors: Scott Habershon, David Albesa-Jové, Eugene Y. Cheung, Giles W. Turner, Roy L. Johnston, Kenneth D.M. Harris
Abstract: Solids that can be prepared only as microcrystalline powders are not suitable for structural investigation using single crystal diffraction techniques, and it is necessary instead to carry out structure determination using powder diffraction data. In this paper, we focus on a direct-space strategy for solving crystal structures directly from powder diffraction data in which a hypersurface based on the powder profile R-factor Rwp is searched using a Genetic Algorithm, and we highlight some recent fundamental developments relating to this methodology.
Authors: Angela Altomare, Corrado Cuocci, Carmelo Giacovazzo, Anna Grazia, Anna Grazia Giuseppina Moliterni, Rosanna Rizzi
Abstract: The ab-initio crystal structure solution via powder diffraction data is often uncomplete. A recent procedure POLPO [1] aims at completing a partial structure model provided by Direct Methods by exploiting the prior information on the polyhedral coordination of the located atoms (tetrahedral or octahedral) and their connectivity has been developed. The POLPO procedure requires that all the cations are correctly labelled and rightly located. This condition does not always occur, particularly when the data quality is poor. A new method is described which is able to locate missing cations and surrounding anions when the cation coordination is tetrahedral or octahedral.
Authors: Giovanni Berti, U. Bartoli, M. D'Acunto, F. De Marco
Abstract: Reproducibility of diffraction experiments provides an indication of data consistency with respect to different variables. The level of reproducibility and accuracy of the diffraction measurements is increased when using calibrated instruments. The paper presents most significant results of the 'Calibration Monitoring of Diffractometers', a project using a systematic approach to the Round Robin tests. At the present stage, the results suggest that instrumental performances and other related experimental effects in a diffraction the experiment can be summarised through 'characteristic curves'.
Authors: Giovanni Berti, Rob Delhez, S. Norval, B. Peplinski, E. Tolle, J. Verollet
Abstract: This paper outlines the standardisation process for the XRPD method that is currently being considered by a Working Group (WG10) of Technical Committee 138 "Non-destructive Testing" of the European Committee for Standardisation CEN. Several Standard Documents are on the verge of being released. These documents concern the general principles of (X-ray) diffraction, its terminology, and the basic procedures applied. Another document concerns the instruments used and it offers procedures to characterise and control the performance of an X-ray diffractometer properly. It is intended to issue Standard Documents on specific methods, e.g. determination of residual stresses. In fact work is in progress on this subject. The Standard Documents can be used by industry, government organisations, and research centres with activities related to safety, health and the environment, as well as for educational purposes.
Authors: V. Favre-Nicolin, R. Černý
Abstract: Program FOX for ab initio crystal structure determination from powder diffraction uses global optimization algorithms to find the correct structure by making trials in the direct space. It is a modular program, capable of using several criteria for evaluating each trial configuration, e.g. combining neutron and X-ray diffraction patterns. The program describes the structural building blocks (polyhedrons, molecules etc.) with their internal coordinates (Z-matrices), thus allowing natural constraints on interatomic distances and angles. Identical atoms shared between neighboring building blocks, and special positions are handled with an automatic, adaptive method (Dynamical Occupancy Correction). Several algorithms (Simulated Annealing, Parallel Tempering) are currently available. The program is based on an object-oriented crystallographic library ObjCryst++(programming language c++). The program and the library are available for Linux and Windows on Inorganic and organic structures with the complexity up to 26 independents atoms are routinely solved from laboratory X-ray, synchrotron or neutron data.
Authors: Ewa Grzanka, Bogdan F. Palosz, Stanislaw Gierlotka, Svetlana Stelmakh, Roman Pielaszek, Ulrich Bismayer, J. Neuefeind, P. Jóvári, W. Palosz
Abstract: A considerable fraction of atoms in nanosize particles is at the grain surface (due to the small size of the crystals). We assume that the surface atoms form a separate structural phase relative to the bulk of the grain (the core). Therefore, one set of the lattice parameters characterizing a nanocrystal may be inadequate for a unique description of its structure. An alternative evaluation of diffraction data of nanocparticles, based on the 'apparent lattice parameter' is proposed. Based on this new methodology it is shown that real nano-crystals constitiute a complex, more than a one-uniform-phase structure.
Authors: B. Peplinski, R. Kleeberg, J. Bergmann, J. Wenzel
Abstract: On the basis of data gathered in connection with a certification project, problems were investigated that impair the accuracy of quantitative phase analyses (QPA) using the Rietveld method. Some mechanisms were elucidated by which insufficient counting statistics of the diffraction data or inappropriate data handling and refinement strategy influence the QPA results.
Authors: E. Estevez-Rams, R. Lora, A. Penton, B. Aragon-Fernandez, Hartmut Fuess

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