issue contents

Journal logoJOURNAL OF
ISSN: 1600-5767

February 2001 issue

Highlighted illustration

Cover illustration: Diffuse scattering around a 200 Bragg peak from a Cu-Ti single crystal (the intensities are in ln scale, the plane is (001), with the [100] direction horizontal, and the Bragg peak is masked by the blue beam stop). Courtesy of O. Lyon, C. Servant & J. P. Simon [J. Appl. Cryst. (2000), 33, 928-937].

research papers

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A redetermination of the structure of KGd(WO4)2, a complete description of the thermal linear expansion tensor and a qualitative morphological study are presented.

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High-pressure investigations of TiP2O7, ZrP2O7 and ZrV2O7 have revealed a cubic to pseudo-tetragonal (2 × 3 × 3 orthorhombic supercell) phase transition and subsequent X-ray amorphization of ZrV2O7. The pyrophosphates TiP2O7 and ZrP2O7 show no such amorphization and compress up to high pressures without transitions.

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The influence of the instrumental resolution on two-dimensional reflection profiles of epitaxic YBa2Cu3O7−δ films on SrTiO3 (001) has been studied in order to investigate the strain in the superconducting films.

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An alternative algorithm for the correction of preferred orientation in Rietveld refinement is introduced.

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Dislocation contrast associated with superscrew dislocations in X-ray topographs of hexagonal SiC crystals that seems to violate the g·b = 0 criterion is shown to result from a high density of a different type of dislocation occurring as individual dislocations in close association with the superscrew dislocations, below the resolution of the X-ray topography.

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The concept of a general four-circle diffractometer has been realised by introducing axial misalignments into mathematical formulae describing the diffractometer geometry. The advantage of using the described diffractometer formalism for routine measurements is discussed and the magnitudes of errors resulting from the neglect of axial misalignments are calculated.

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An algorithm is presented for automated best-matching alignment of three-dimensional models represented by ensembles of points. The method is implemented in a computer program that allows the superposition of high- and low-resolution structural models obtained by different methods.

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Multiple-data-set Rietveld analysis of powder neutron diffraction data using isotopically substituted nickel samples with a single crystallographic model has been found to reduce parameter correlation effects dramatically. This has allowed accurate determination of the nickel dopant level in NixMg1−xO to 0.5%, a level not achievable from single-data-set analysis.

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The prediction that aminoguanidinium hexafluorozirconate is ferroelectric, based on a previous demonstration that it satisfies the structural criteria required in order to exhibit this property, is confirmed experimentally. With a Curie temperature Tc = 383 (1) K and a spontaneous polarization of 0.45 (9) × 10−2 C m−2 at 298 K that decreases to zero above Tc, the two symmetry-independent CN4H82+ ions are shown to undergo the tilts and rotations necessary to achieve symmetry about the planes at z = 0 or 1/2 at a rate of the order of 100 Hz.

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Synchrotron X-ray and neutron diffraction data have been collected for pure (Ca2FeAlO5) and lightly doped (Ca2Fe0.95Al0.95Mg0.05Si0.05O5) brownmillerite so that the structures can be refined using, simultaneously, both diffraction data sets and known compositional information; this overcomes the problem of under-determinacy resulting from multi-occupation of the tetrahedrally and octahedrally coordinated sites in the structure.

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Simultaneous small- and wide-angle X-ray scattering (SAXS and WAXS) measurements give evidence of the formation of columns that constitute an ephemeral hexagonal structure in a cooled poly(vinylpyrrolidone) (PVP)–water mixture.

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Dislocation bundles have been induced in a GaAs wafer by means of a standard thermal treatment that accompanied epitaxic growth in a molecular beam epitaxy machine. These dislocation bundles have been classified into different types employing synchrotron-based single-crystal X-ray transmission topography under conditions of high anomalous transmission.

cryocrystallography papers

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Details of a flow system used for collection of a number of large data sets at a temperature of ∼16 K are described, including the equipment, temperature calibration and a typical result. Instability problems encountered when using a helium cryostat for three-dimensional data collection are eliminated.

short communications

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Based on the results of refinements of the atomic parameters of 3564 amino acid residues of bovine heart cytochrome c oxidase at 2.0 Å resolution by using X-PLOR, a strong bond energy constant for Sδ—C of 500.0 kcal mol−1 Å−2 is recommended instead of the default value of 170.066 kcal mol−1 Å−2.

computer programs

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CHOOCH is a program for deriving anomalous-scattering factors from X-ray fluorescence data measured prior to an optimized anomalous-scattering or a multiple-wavelength anomalous diffraction (MAD) experiment.

addenda and errata


international union of crystallography

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