Structural study of monoclinic KGd(WO₄)₂ and effects of lanthanide substitution

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.

High-pressure properties of TiP₂O₇, ZrP₂O₇ and ZrV₂O₇

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.

Anisotropic strain in YBa₂Cu₃O_7−δ films analysed by deconvolution of two-dimensional intensity data

The influence of the instrumental resolution on two-dimensional reflection profiles of epitaxic YBa₂Cu₃O_7−δ films on SrTiO₃ (001) has been studied in order to investigate the strain in the superconducting films.

Alternative algorithm for the correction of preferred orientation in Rietveld analysis

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

X-ray topographic dislocation contrast visible in reflections orthogonal to the Burgers vectors of axial screw dislocations in hexagonal silicon carbide

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.

Towards general diffractometry. III. Beyond the normal-beam geometry

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.

Automated matching of high- and low-resolution structural models

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.

Multiple-data-set Rietveld analysis using isotopes in powder neutron diffraction. I. Accurate determination of the doping level in the ternary system Ni_xMg_1−xO, 0.005 < x < 0.1

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 Ni_xMg_1−xO to 0.5%, a level not achievable from single-data-set analysis.

Aminoguanidinium hexa­fluorozirconate: a new ferroelectric

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 T_c = 383 (1) K and a spontaneous polarization of 0.45 (9) × 10⁻² C m⁻² at 298 K that decreases to zero above T_c, 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.

The site occupancy of Mg in the brownmillerite structure and its effect on hydration properties: an X-ray/neutron diffraction and EXAFS study

Synchrotron X-ray and neutron diffraction data have been collected for pure (Ca₂FeAlO₅) and lightly doped (Ca₂Fe_0.95Al_0.95Mg_0.05Si_0.05O₅) 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.

Small-angle X-ray scattering by PVP–water mixtures

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.

Analysis of thermal-treatment-induced dislocation bundles in GaAs wafers by means of X-ray transmission topography and complementary methods

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.

A modified Pinkerton-type helium gas-flow system for high-accuracy data collection at the X3 SUNY synchrotron beamline at NSLS

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.

Optimization of the energy constant of the me­thionine S^δ—C^∊ bond for X-PLOR refinement of protein structure

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 S^δ—C^∊ of 500.0 kcal mol⁻¹ Å⁻² is recommended instead of the default value of 170.066 kcal mol⁻¹ Å⁻².

CHOOCH: a program for deriving anomalous-scattering factors from X-ray fluorescence spectra

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.

A new version of the method for analysing peak broadening caused by local tilt distribution in double-crystal X-ray diffraction measurements. Erratum

Erratum to J. Appl. Cryst. (2000), 33, 1376–1385.

André Guinier (1911–2000)

Harry Brumberger (1926–2000)

Notes for Authors

Forthcoming meetings and short courses