issue contents

ISSN: 2053-2733

July 2002 issue

Highlighted illustration

Cover illustration: Electrostatic equilibrium of simultaneous Coulomb attraction and core repulsion for an ensemble {\cal F}=(X_1^-,X_2^-); the conjoint centroids, shown in yellow, characterize force-free positions of the fragment inside the unit cell. For comparison, the pure Coulomb equilibrium is shown in the background. See Mädler, Behrends & Knorr [Acta Cryst. (2001). A57, 20-23].


research papers

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Energy-filtered Debye–Scherrer electron powder data have been succesfully employed to determine the structure of nanocrystalline anatase (TiO2). The results obtained in the present study show for the first time that small-unit-cell structures of symmetry lower than hexagonal are directly accessible from non-textured electron powder data.

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A possible mechanism for the phase transition between the high-temperature polymorphs of quartz and tridymite is described by a deformation of a homogeneous sphere packing with contact number 3 formed by the Si atoms in the common subgroup P6122 of P6422 and P63/mmc.

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Morphological features observed in snow crystals, like a macroscopic hexagonal lattice and pinned growth boundaries, are analyzed in the framework of a general crystallography that allows point groups of infinite order.

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The topologically stable odd-line defects are found for a continuous random network model of amorphous silicon. From these, the topological entropy of disorder is found.

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The average structure of a quasilattice is calculated using the dual generalized method.

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Normalized structure-factor algebra is presented for few-atoms structure models with N = 1, 2, 3, … equal atoms or polyatomic globs per unit cell.

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The article presents a method for calculation of X-ray susceptibility and the Debye–Waller factor for crystals. The method improves the precision of calculated values in comparison with the presently used algorithms. The results are illustrated by numerical examples and compared with experimental data.

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Several X-ray reflections from an icosahedral quasicrystal Al–Pd–Mn have been measured with great accuracy on an absolute basis by making use of Bragg-case diffraction. Anomalous transmission was found to be strong in the `good' regions of the quasicrystalline specimen and it was measured on an absolute basis, but the small residual strains present in the specimen prevented an accurate comparison between theory and experiment.

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The crystal structure determination of the 1/1 cubic approximant of the icosahedral phase i-AlCuFe is performed by single-crystal X-ray diffraction. The refined structure is shown to agree reasonably well with the theoretical one obtained from the ideal parent quasicrystal structure by the perpendicular shear method. The 1/1 cubic approximant shares many features of the quasicrystal including large atomic clusters that extend significantly farther than the unit cell of the approximant.

short communications

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The symmetry point groups for all combinatorially non-isomorphic 9- and 10-hedra (2606 and 32300, respectively) are contributed for the first time.

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Lateral shift of the Bragg diffracted beam was measured for a spectrally narrow beam.

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Form factors are calculated for the core electrons of the first-row atoms B, C, N, O and F.

book reviews

Acta Cryst. (2002). A58, 412
doi: 10.1107/S0108767302009960

Acta Cryst. (2002). A58, 414
doi: 10.1107/S0108767302009972

books received

Acta Cryst. (2002). A58, 414
doi: 10.1107/S0108767302005561
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