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ISSN: 2053-2733

September 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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A new reverse Monte Carlo (RMC) method for modelling both lattice and magnetic disorder in powder crystalline materials is presented.

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Rigorous and universal rules for qualitative and quantitative estimation of chirality have been developed.

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The structure of the R phase of the alloy Ti50Ni48.7Co1.3 was determined by the crystallogeometric analysis of martensitic transformations in titanium nickelide. This analysis is based on the concept of cooperative thermal vibrations of the (110)-like planes in the b.c.c. and B2 structures and their freezing (transformation to static displacements) at the point of the phase transition.

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It is demonstrated how to extract information on the structure-forming properties of atoms through the analysis of atomic structures via the Voronoi tessellation.

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Calculation of the interaction energy of urea from the experimental and Hartree–Fock X-ray structure amplitudes has been performed using a modified atom–atom approximation. Similar electron-density distributions yield widely different atomic multipole moments, resulting in unphysical energy differences.

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The structures of α-Ba3RAl2O7.5 and β-Ba3RM2O7.5 (R = rare-earth elements, M = Al, Ga) complex oxides have been studied by X-ray diffraction, electron diffraction and high-resolution electron microscopy. The order–disorder phase transition between the orthorhombic high-temperature α form and the monoclinic low-temperature β form results from sequential migration of oxygen atoms along the strings of vertex-sharing tetrahedral Al2O7 pairs.

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The theory for rocking curves in perfect rectangular t\times l crystals is outlined using the Takagi–Taupin equations. Calculated examples are presented for different scattering geometries in germanium and silicon.

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A large Si1−xGex gradient crystal was investigated by means of high-energy synchrotron radiation in the energy range 100–200 keV. The experimental data are explained by geometrical optics theory.

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A new method has been developed in which the intensity of Bragg reflections is measured while changing continually the radiation wavelength over a range in the vicinity of the absorption edge. The intensity gradient with respect to the wavelength is in a simple relation to the real and imaginary parts of the structure factor and its phase can be derived by solving two simultaneous linear equations.

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Algebraic expressions for the reduced projection operators for the irreducible representation of the icosahedral group are found by using the double-induced technique and eigenfunction method.

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Domain pairs arising in ferroic transitions are classified. Physical property tensors are tabulated according to their ability to distinguish between domains of each class of domain pairs.

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The cosine minimal function used in Shake-and-Bake has been replaced by a function of exponential type. With proper choice of parameter values, the exponential function yielded significantly higher success rates for several P1 structures.

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The breakdown of Friedel's law in electron diffraction patterns from a polar polymer structure is readily explained by multiple-beam dynamical scattering calculations. While not restricting an ab initio structure analysis, the Bijvoet differences can provide additional structural information.

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The fundamental notions associated with the determination and reporting of absolute structure and absolute configuration from crystal structure analysis are treated in detail.

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The suggested procedure of Fourier-series decomposition provides a new tool for analysis of low-resolution macromolecular images. Together with the suggested connectivity-based criterion, the decomposition allows a search for the macromolecular position in a crystal by phasing several very low resolution reflections.

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Parameterization has been made for the temperature dependence of the Debye–Waller factors of 68 elemental crystals and 17 compounds with the zinc-blende structure.

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Without invoking anomalous dispersion and heavy-atom derivatives, it is demonstrated that it is possible to directly determine the phases of a large number of reflections collected in a short time from macromolecular crystals using a stereoscopic oscillation-crystal imaging technique in a multibeam diffraction geometry.

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A new method to refine crystal structural parameters using energy-filtered convergent-beam electron diffraction (CBED), which is applicable to nanometre-size crystal structure analysis, is proposed.

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A transmission-type X-ray polarizer is realised with a thin Bragg reflector as a polarizing filter. This device gave a polarization ratio, defined as I_H/I_V, higher than 105.

short communications


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The magnetic subperiodic groups are defined and given symbols based on the symbols of non-magnetic subperiodic groups in Volume E of International Tables for Crystallography.

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The origin dependence of the errors in an X-ray molecular dipole moment is discussed and a suitable choice of origin is defined.

book reviews



books received


Acta Cryst. (1999). A55, 973
doi: 10.1107/S0108767399001889

Acta Cryst. (1999). A55, 973
doi: 10.1107/S0108767399001567

Acta Cryst. (1999). A55, 973
doi: 10.1107/S0108767399004791

Acta Cryst. (1999). A55, 973
doi: 10.1107/S0108767399001555
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