issue contents

Journal logoJOURNAL OF
APPLIED
CRYSTALLOGRAPHY
ISSN: 1600-5767

December 1998 issue

Highlighted illustration

Cover illustration: Powder diffraction pattern of LaB6 obtained using a 200 × 400 mm flat imaging plate mounted at an angle of 45° with the incoming X-ray beam. Experimental conditions: [lambda] = 0.8884 Å, D0 = 104 mm, exposure time 60 s. Courtesy of Poul Norby.

obituaries


research papers


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A new expression of the X-ray incidence parameter is derived in order to evaluate the lattice strain by high-resolution X-ray diffraction rocking curves of heterostructures made of materials with different crystallographic symmetry. In particular, high-Tc perovskite superconductors deposited on substrates of cubic symmetry are considered.

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The thermal and cold neutron transmission characteristics of high-grade single-crystal sapphire show negligible degradation after irradiation. The results are analyzed in terms of single-photon scattering. Poorer grade sapphire has similar properties.

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By reducing most of the single-photon scattering, a liquid-nitrogen-cooled single-crystal magnesium oxide filter has thermal and cold neutron transmission characteristics superior to those of room-temperature single-crystal sapphire.

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A combination of X-ray diffraction and molecular-mechanics simulations in the Cerius2 modelling environment has been used in the structure analysis of α-zirconium phosphate intercalated with ethanol.

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The theory is developed for generating the aberration profiles for axial divergence in a conventional diffractometer in terms of the dimensions of the X-ray source, the sample, the receiving slit and the Soller slits in the incident and/or diffracted beams.

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Profiles for a conventional powder diffractometer are generated and fitted using a fundamental-parameters approach which incorporates an accurate axial divergence function. The fit and the self-consistency of the approach are evaluated for different instrument configurations and a range of reference samples.

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Microcrystals of cardiolipin (tetraacyldiphosphatidylglycerol) prepared according to different procedures have been studied by transmission electron microscopy and selected-area electron diffraction. The radiation damage has been assessed.

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The refinement of powder diffraction data of partly crystalline organic compounds is described. Calculation of the structural data was performed using a Rietveld algorithm, taking the results of other methods into consideration.

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Small-angle X-ray scattering is combined with simultaneous rheology to yield a wealth of information on the effect of shear on block copolymer melts and gels. The focus is on cubic micellar phases formed by amphiphilic block copolymers.

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X-ray absorption near-edge structure (XANES) spectra taken at the Al K edge on a pure synthetic α-Al2O3 sample, a natural corundum (ruby/sapphire), and five artificial alumina made for abrasives have been interpreted quantitatively on the basis of difference spectra. Three of the artificial alumina contain β-Al2O3 as an admixed phase, the tetrahedral Al of which interferes with the predominant octahedral Al, while ruby/sapphire and the remaining two artificial alumina are very pure α-phases with minor substitutions of foreign atoms for their octahedral Al.

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A simulation program for macromolecular diffraction patterns is presented which considers all essential parameters of the source, the sample and the detector. Tests and possible applications of the program are described.

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Why do the current crystallite models show a larger intensity in the small-angle region than the experimental values? This paper attempts to answer this question by providing a new model of the stacking structure of disordered carbon layers.

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An accurate structure analysis of a spherical LaB6 single crystal has been performed with the use of short-wavelength X-rays (WKα1) in order to estimate the termination effect in Fourier synthesis on the electron-density distribution (EDD). It is concluded that the effect on the EDD is negligible if data are taken up to \sin \theta/\lambda=2.0\;Å−1, because the negative values disappear completely from the map.

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Two algorithms are presented that determine the translational symmetry (TRS) and space group (RGS) of a crystal structure, both for synthesized compounds and computer-generated structures.

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A method that allows the calculation of single-crystal elastic constants from the diffraction elastic constants is presented. Values for the diffraction elastic constants have been obtained from diffraction measurements on polycrystals.

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An X-ray tube with a focal spot diameter between 10 and 20 µm is described which can be operated at about 20 W and which is used with ellipsoidal focusing mirrors to deliver an intensity at the sample that is comparable with the intensity achieved with rotating-anode generators and non-focusing collimators.

cryocrystallography papers


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A technique for direct cooling of crystals in liquid nitrogen has been developed. A set of implements has been designed and tested.

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Cooling rates in liquid nitrogen, liquid propane and nitrogen gas have been compared. Liquid nitrogen affords the most rapid cooling, nitrogen gas the slowest.

short communications


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Silver behenate powder (AgBE), one of the very few materials featuring Bragg reflections in the angular range accessible to small-angle neutron scattering (SANS) instruments, is suggested as a possible new wavelength calibration standard.

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Two new and two renewed procedures for structure determination based on symmetry minimum and/or superposition functions were applied to equal-atom structures. Fourier recycling is able to extract structure from noisy superposition maps.

computer programs


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A graphical computer program for the preparation of SHELXL instruction files is described.

CIF applications


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A software package has been developed to read user-specified data from a CIF with a single C or Fortran function call. This approach involves simple additions to a CIF dictionary and significantly reduces the effort involved in creating a CIF interface. The procedure is applicable to both DDL1 and DDL2 dictionaries.

laboratory notes


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A simple device for preparation of Guinier–Hägg-camera samples and easy filling of capillary tubes is described.

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A simple and inexpensive system for producing video images of collimated X-ray beams is described and is suitable for use as an alignment tool.

computer program abstracts



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A command interpreter for instrument control with macro file input is described. It is flexible, extensible and portable and a wide variety of experimental areas are supported.

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General position/symmetry diagrams of the 80 layer groups are available in VRML format for viewing on a World Wide Web browser with a VRML viewer plug-in.

crystallographers


new commercial products



international union of crystallography


books received





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