issue contents

ISSN: 2053-2733

May 2015 issue

Highlighted illustration

Cover illustration: Unique atom hyper-kagome order in Na4Ir3O8 [Talanov et al. (2015). Acta Cryst. A71, 301-318]. Crystals with hyper-kagome atomic order are expected to show unusual physical properties. Image courtesy of V. Talanov.


research papers

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Periodically-poled ferroelectric crystals are studied by observing their superlattice (grating) diffraction profiles with high-resolution X-ray diffraction. In order to successfully model the data, the effects of strain, and sample and beam coherence, must be taken into account.

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The characterization of Bravais types is extended according to metrical, algebraic and geometric properties onto a wide class of primitive lattices (including Buerger-reduced and a substantial part of Delaunay-reduced) related to low-restricted semi-reduced descriptions. There are excellent theoretical and practical reasons for looking at crystal lattice symmetry from an entirely new point of view – the combinatorial set of 960 matrices, their semi-reduced lattice context and their geometric properties.

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A structure factor for an icosahedral quasicrystal with an arbitrary decoration scheme based on a primitive icosahedral tiling model and a statistical approach is derived. The average unit cell concept is used as an alternative to the commonly used higher-dimensional description.

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The application of X-ray phase measurements for absolute identification and improvement of atomic model structures is described.

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The structure of a particular type of hollow-cage higher fullerene (C60+N24) is described and their construction explained from a symmetry-breaking mechanism starting from perfect icosahedral symmetry of C60 to specific subgroups A1 × A1. This mechanism expands and completes previous results describing the existence of other groups of fullerenes (C60+N10, C60+N18) based on the breaking of icosahedral symmetry of C60 to the subgroups H2 and A2. The mechanism is extended to describe the cases that generate carbon nanotubes.

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Group-theoretical and thermodynamic methods of the Landau theory of phase transitions are used to investigate the hyper-kagome atomic order in structures of ordered spinels and a spinel-like Na4Ir3O8 crystal. The existence of hyper-kagome lattices in six types of ordered spinel structures is predicted theoretically.

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A method from numerical algebraic geometry, called homotopy continuation, is used to determine small crystal structures of four or fewer atoms directly from a minimum set of diffraction intensities.

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Combined atomic and magnetic pair distribution function (PDF) refinements have been performed against neutron scattering data from MnO, recovering the expected long-range-ordered antiferromagnetic state and supporting a scenario of monoclinic symmetry on a local length scale of ∼100 Å. This represents the first experimental application of the magnetic PDF method.

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Ternary intermetallics – 13 026 compounds contained in the database Pearson's Crystal Data – are discussed with respect to their components, structure types and stoichiometries.

short communications

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Physical property tensors of materials such as nanotubes or polymers are determined by the material's axial point group. Rank 1, 2, 3 and 4 property tensors are given for a wide variety of tensor types invariant under each point group in all 31 infinite series of axial point groups.
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