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A structure model for τ(μ) phase in Al-Cr-Si alloys deduced from λ phase by the strong reflections approach
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
Stockholm University, Faculty of Science, Department of Physics.
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2006 (English)In: Acta Crystallographica Section B: Structural Science, ISSN 0108-7681, E-ISSN 1600-5740, Vol. 62, no 1, 16-25 p.Article in journal (Refereed) Published
Abstract [en]

There are very obvious common features in the electron diffraction patterns of the λ and τ(μ) phases in the Al–Cr–Si system. The positions of the strong reflections and their intensity distributions are similar for the two structures. The relation of the reciprocal lattices of the λ and τ(μ) phases is studied. By applying the strong-reflections approach, the structure factors of τ(μ) are deduced from the corresponding structure factors of the known λ phase. Rules for selecting reflections for the strong-reflections approach are described. Similar to that of λ, the structure of τ(μ) contains six layers stacked along the c axis in each unit cell. There are 752 atoms in each unit cell, 53 of them are unique. The corresponding composition of the τ(μ) model is Al3.82  −  xCrSix. Simulated electron diffraction patterns from the structure model are in good agreement with the experimental ones. The arrangement of interpenetrated icosahedral clusters in the τ(μ) phase is discussed.

Place, publisher, year, edition, pages
2006. Vol. 62, no 1, 16-25 p.
Keyword [en]
quasicrystal approximants • strong-reflections approach • electron crystallography.
National Category
Physical Sciences
Research subject
URN: urn:nbn:se:su:diva-25270DOI: 10.1107/S0108768105035779OAI: diva2:199276
Part of urn:nbn:se:su:diva-791Available from: 2006-01-05 Created: 2006-01-05 Last updated: 2010-08-19Bibliographically approved
In thesis
1. Exploiting reciprocal space: Electron diffraction, textures and precession
Open this publication in new window or tab >>Exploiting reciprocal space: Electron diffraction, textures and precession
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Electron crystallography emerges as a new tool for accurate structure determination of very small crystals. In order to exploit the full potential of electron diffraction, the methods for data acquisition and processing have to be developed towards the high standards of X-ray crystallography. Spot diffraction patterns give complete separation of reflections and provide two-dimensional data sets. Separation of reflections occurs also in texture patterns; the whole or almost whole reciprocal lattice is represented on them in two dimensions. In order to speed up texture pattern analysis and improve the accuracy, the program TexPat was developed for quantification of texture patterns. It introduces new approaches for automated detection of centre and symmetry axes and simplifies the process of indexing and estimating the intensities of all reflections.

The precession technique, developed by Vincent and Midgley at the University of Bristol in 1994, has become available to the TEM users because of recent hardware implementations that allow data acquisition of sufficient quality for structure determination. Dynamical interactions can be effectively reduced using the precession method, and the intensity data can be treated within the kinematical approximation. However, a deeper understanding of the theory is necessary. A program eMap has been developed which performs kinematical and dynamical simulations of precession electron diffraction patterns. A set of algorithms was introduced for indexing of precession patterns with HOLZ present, estimation of three-dimensional lattice parameters and crystal symmetry deduction using both ZOLZ and HOLZ. All developed algorithms were implemented in the diffraction simulation program eMap and electron diffraction analysis program ELD. Both oblique texture and precession electron diffraction patterns provide intensities of high quality, which can be treated kinematically for further crystal structure solution.

However, there is also the fundamental phase problem in crystallography. For quasicrystal approximants, the strong reflections approach allows overcoming this problem and deducing structural models from already known structures. In order to perform the analysis of quasicrystal approximant structures in real and reciprocal space, the eMap program was used. The operations included in the program allow the calculation of three-dimensional electron density or potential maps; peak search, calculation of theoretical structure factors both for X-rays and electrons (kinematical and dynamical); visualization of reciprocal space and phase values of reflections in specified colouring scheme.

Place, publisher, year, edition, pages
Stockholm: Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi, 2006. 96 p.
electron crystallography, electron diffraction, texture patterns, precession electron diffraction
National Category
Inorganic Chemistry
urn:nbn:se:su:diva-791 (URN)91-7155-188-3 (ISBN)
Public defence
2006-01-27, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 10:00
Available from: 2006-01-05 Created: 2006-01-05Bibliographically approved

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Zhang, HongqiangOleynikov, PeterHovmöller, Sven Erik
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Department of PhysicsDepartment of Physical, Inorganic and Structural Chemistry
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