issue contents

ISSN: 2053-2733

March 1999 issue

Highlighted illustration

Cover illustration: One quadrant of each of four fibre diffraction patterns from disordered polynucleotide fibres. The patterns were calculated for fibres with disorder as follows. Upper left: crystalline (no disorder); upper right: noncrystalline (completely disordered); lower left: discrete rotational disorder and correlated lattice disorder; lower right: random screw disorder. See Stroud & Millane [Acta Cryst. (1996), A52, 812-829].

research papers

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The experimental verification of the statistical dynamical theory of diffraction has been carried out by means of short-wavelength highly monochromatic γ-ray diffractometry. Dislocation-free silicon and low-dislocation quartz single crystals have been used as the experimental objects.

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A study of the forms of X-ray standing waves (XSW) when the real part of the scattering factor is zero shows that the effect of the anomalous transmission and absorption are connected with the phase of the XSW.

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The role of partial beam coherence and its interplay with finite scattering volumes in diffraction experiments is discussed. Under certain conditions, the analysis of diffraction profiles in terms of a sample response function convolved with an instrumental resolution function may fail and, further, under high absorption, new information of the source of scattering below the sample surface may become available.

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Scattering of a quasi-monochromatic electron beam by a crystal with a defect is described with the use of the mutual coherency function and the formalism of quasi-Bloch waves. As an example of applying the new approach, the LACBED patterns of a crystal with a dislocation have been simulated.

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Formation of bonds between atoms shifts the electron distribution from that of the free atoms, thus changing the Coulomb potential. The consequences of this effect are investigated in the context of electron crystallography of proteins, where chemical bonding effects are shown to significantly increase R factors that are calculated using free-atom scattering factors.

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Joint probability distribution functions of structure factors with rational indices are derived in P1¯. Phase relationships are obtained.

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57 three-dimensional nets were obtained by converting into crankshaft chains all edges of the simpler two-dimensional nets in the catalog of the Consortium of Theoretical Frameworks; nine occur in known structures.

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138 3D nets were generated by converting some horizontal edges of three-connected 2D nets into zigzag chains, and 19 of them are represented by known structures.

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Seven of the 175-plus three dimensional nets based on parallel saw chains occur in known structures: edingtonite, `nepheline hydrate', offretite, Linde Type L, ATT, SZF and ZST.

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Algorithms are presented for three-dimensional crystallographic space groups, handling tasks such as generation of symmetry operations, the characterization of symmetry operations, the determination of space-group type and the generation of structure-seminvariant vectors and moduli.

short communications

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A formula for the estimation of four-phase structure invariants involving four pairs of isomorphously related structure factors is described.

book reviews

Acta Cryst. (1999). A55, 399
doi: 10.1107/S0108767398016900
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