issue contents

ISSN: 2053-2733

May 1998 issue

Highlighted illustration

Cover illustration: Power spectrum of a high-resolution electron-microscopy image of NiAl. The image calculation includes a realistic phonon spectrum by coupling the `molecular dynamics' and `multislice' simulation techniques. The diffuse background, satellite peaks and bands mirror the effect of thermal diffuse scattering and correlated atom vibrations. Courtesy of G. Möbus, T. Gemming and P. Gumbsch.

research papers

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The meaning of the Opechowski–Guccione magnetic space-group symbols depends on information contained in International Tables for Crystallography. The impact of changes in International Tables for Crystallography on the meaning of these symbols is discussed.

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The eigenvalues of large Karle–Hauptman matrices can be predicted using the rank of the matrix and the value of the diagonal elements. The specific properties of the eigenvalues and eigenvectors can be used in solving the phase problem.

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The consequences for the phase problem of the theory developed in an earlier paper, where the properties of the eigenvalue spectrum of a large Karle–Hauptman matrix were derived, are discussed. Practical applications are given.

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An algebraic method plus a determinant technique permit the determination of one-dimensional centrosymmetric point-atom structures. In favourable cases, a solution may be proved to be unique.

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A general formula for the absorptive form factors accounting for anisotropic thermal vibrations is given using the Einstein model. A case study shows that even slight deviations from isotropy lead to a strong directional dependence of the absorptive form factors.

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When globular density, e.g. a projected α-helix, is treated as a pseudo atom for calculating |Eh|, a protein structure can be determined at low resolution by direct methods. The orthorhombic form of bacteriorhodopsin is treated as an example.

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Development of a minimum-entropy direct-methods approach.

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The crystal structure of a metastable phase AlmFe has been determined by application of Patterson analysis and direct methods to a three-dimensional electron diffraction intensity data set of 598 reflections collected by a precession technique.

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14-coordinated packings of symmetry-related spheres in which all the holes are tetrahedral are described. The dual structures with vertices in the holes of the packings form 4-connected nets that divide space into congruent simple polyhedra.

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The Fourier transformation to calculate structure factors was applied to a crystal simulated by the molecular-dynamics method to clarify the crystallographic symmetry of the crystal and its change during phase transition.

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Triplet phases from three-beam diffraction experiments have been used for structure verification and discrimination between two similar crystal structures of the title complex. This is a novel and potentially powerful application of physically estimated triplet phases.

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Some new definitions, which allow a systematic approach to the indexing of diffraction patterns, are introduced to classify mica polytypes with any number of layers and their axial settings. The transformation rules between settings are given.

short communications

international union of crystallography

Acta Cryst. (1998). A54, 358
doi: 10.1107/S0108767398004243
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