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ISSN: 2053-2733

September 1997 issue

Highlighted illustration

Cover illustration: A structural model of the decagonal Al-Pd-Mn quasicrystal, which consists of clusters represented by decagons and ten pentagons surrounding each decagon. This is obtained from a periodic structure in five-dimensional space by taking a three-dimensional section. Courtesy of A. Yamamoto.

obituaries



research papers


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A numerical algorithm is described for calculating the TDS correction to Bragg peaks that are observed in time-of-flight neutron diffraction studies on single crystals.

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Four different types of twinning in crystals are recognized, based on a tensor distinction of their properties and on a new and rigorous definition of prototype symmetry.

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A new method for the determination of atomic moments fitted to theoretical or XX experimental electrostatic potential is described. It is also shown that a reduced set of moments respecting local symmetry and chemical equivalence reproduces accurately V(r) and then can be used in force-field calculations.

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A method is presented that improves the estimation of structure-factor difference amplitudes by using Bayesian statistics. The method is shown to give significant improvement for poor data and is straightforward to use.

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Three different multislice formulations for large beam tilt are compared and evaluated. The tested multislice formulations are then applied to calculate higher-order Laue-zone reflections in non-orthogonal crystals.

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This paper presents a rigorous theoretical framework for understanding X-ray scattering from dislocations. This theory is then applied to the specific case of screw dislocations.

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Electron crystallography was combined with quantum-mechanical calculations in order to estimate quadratic nonlinear optical coefficients of two-dimensional organic molecules.

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The scaling symmetry observed in the morphology of snow crystals is analysed on the basis of the point group of a multimetrical space group of ice.

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High-resolution X-ray synchrotron radiation study of the supersatellites in the feldspar mineral labradorite. Data analysis shows a transverse displacement modulation with a large amplitude.

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A maximum-entropy algorithm for deconvoluting intensities of multiple Laue diffraction spots is described. Test results have shown that this method is capable of producing high-quality data without the requirement of data redundancy.

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Extra reflectivity curves, calculated numerically for various periodic multilayers using the optical-matrix method, are used to test the dynamical theory of diffraction.

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General expressions are derived for the elastic and absorptive atomic scattering amplitudes and for the general case of anisotropic thermal parameters.

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The application of normal probability methods to a comparison of structural parameters recently derived from independent X-ray and neutron diffraction refinement identifies systematically underestimated uncertainties in addition to significantly differing atomic displacement parameters that previously had been overlooked.

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A method is proposed for the calculation of combined standard uncertainties for interatomic distances taking into account covariance terms involved by linear relations of symmetry constraints in one-dimensionally modulated crystals.

short communications


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The uniformity characteristics R of crystallographic orbits are studied by the geometrical analysis of the fundamental regions for the corresponding space groups.

international union of crystallography


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