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ISSN: 2053-2733

September 2019 issue

Highlighted illustration

Cover illustration: Serial crystallography often records thousands of snapshot (or still) diffraction patterns from individual crystals. The program XGANDALF developed by Gevorkov et al. [Acta Cryst. (2019), A75, 694-704] rapidly and accurately indexes noisy diffraction patterns using a variant of Fourier indexing where Bragg peaks create sinusoidal densities in real space. Noise tolerance is achieved by replacing sine waves with other periodic basis functions. Smooth basis functions provide a large radius of convergence for the peak search and are then replaced by sharper functions in the later stages. The images in the background show real-space distributions that follow this progression from fast convergence to high precision.

advances

research papers


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The atomic pair distribution function (PDF) represents the structure of a material as a list of distances between pairs of atoms. This article presents an algorithm that can extract the distance list from a measured PDF in a highly automated manner without prior knowledge of the structure.

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An algorithmic procedure is proposed for extraction of physically meaningful matching rules directly from the phased diffraction data of a quasicrystal.

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A description and evaluation are given of XGANDALF, extended gradient descent algorithm for lattice finding, an algorithm developed for fast and accurate indexing of snapshot diffraction patterns.

foundations

research papers


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Contributions of different physical effects (thermal smearing, relativity, electron correlation, basis set, environment and/or atomic contributions) to the structure factors of heavy-element compounds are inspected. Relativistic Hirshfeld atom refinement yields an almost perfect agreement with reference geometries in this theoretical pilot study, showing the usefulness of the employed methodology.

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Anomalous X-ray diffraction was used to study the energy dependence of diffuse scattering around the reciprocal-lattice point h = [006]β \equiv [06{\overline 6}3]_\omega of a Ti–15Mo single crystal. The diffuse scattering as well as its energy dependence around an Mo K absorption edge were qualitatively explained by a numerical model of ω particles which elastically deform the surrounding β matrix.

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Given a symmetrical pattern {\cal P} consisting of disjoint congruent symmetric motifs, a method is described for arriving at a coloring of {\cal P} which is perfect and transitive under its global symmetry group G and where the coloring of each motif is also perfect and transitive under its own group of symmetries (local symmetries of {\cal P}). The coloring of {\cal P} is coordinated with the property that the symmetry of {\cal P} that is both a global and local symmetry effects the same permutation of the colors of {\cal P} and the corresponding motif, respectively.

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A growth form and an asymptotic formula for the coordination sequence of the vertex graph of the Ammann–Beenker tiling are obtained.

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A novel method is introduced for in situ X-ray total scattering experiments. Two examples of the method as applied to non-classical nucleation and crystal growth studies are discussed.

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The truncated singular value decomposition is applied to extract the underlying 2D correlation functions from small-angle scattering patterns.

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This article presents an analytical description of the X-ray diffraction peak profile of superabsorbing crystals. For thick crystals, the absorption intrinsic profile is described by a Lorentzian and characterized by an absorption intrinsic width.

addenda and errata


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