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ISSN: 2053-2733

November 2014 issue

Highlighted illustration

Cover illustration: The X-ray constrained extremely localized molecular orbital (XC-ELMO) technique combines the quantum-mechanical rigour of wavefunctions with the easy chemical interpretability of multipole models. Here the effects of constraining ELMO wavefunctions to X-ray structure-factor amplitudes is shown for two basis sets (left: ELMO, right: XC-ELMO deformation densities) [Dos Santos et al. (2014). Acta Cryst. A70, 532-551].

advances

scientific commentaries


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The latest generation of scanning transmission electron microscopes equipped with high-efficiency energy-dispersive X-ray detectors are breaking new ground with respect to high-resolution elemental imaging of materials. In this issue, Paulauskas et al. [Acta Cryst. (2014), A70, 524–531] demonstrate impressive results when applying this technique to improve understanding of CdTe dislocation structures.

research papers


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Using atomic-column-resolution high-angle annular dark-field imaging and X-ray spectrum imaging in an aberration-corrected scanning transmission electron microscope, dislocation core structures in zinc-blende CdTe are examined. The formation mechanisms of the polar core structures is described using Thompson's tetrahedron adapted to reactions of polar dislocations as they appear in CdTe and other zinc-blende solids.

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Experimentally constrained wavefunctions are calculated for α-glycylglycine using high-resolution X-ray structure factors and the extremely localized molecular orbital formalism.

foundations

research papers


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The Large Area Medipix-Based Detector Array (Lambda) has been used in a ptychographic imaging experiment on solar-cell nanowires. By using a semi-transparent central stop, the high flux density provided by nano-focusing Kirkpatrick–Baez mirrors can be fully exploited for high-resolution phase reconstructions.

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An approach has been developed for the first time to determine the displacive and substitutional modulation functions of an incommensurate modulated structure from a limited-size high-resolution transmission electron microscope image and electron-diffraction data.

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An algorithm for calculating the 2θ diffraction profiles is developed based on the dynamical theory of X-ray diffraction for multi-beam cases. The connection between the 2θ-scanned profiles and the excitation of modes is revealed.

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The symmetry of complex nanodomain structures in lead-based perovskite crystals is determined using convergent-beam electron-diffraction patterns along different zone axes based on dynamic diffraction simulations.

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All homogeneous sphere packings were derived that refer to the trivariant orthorhombic lattice complexes of the space groups belonging to crystal class 222.

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Twelvefold symmetric quasicrystallographic structures are obtained from the projections of the lattices described by the Coxeter–Weyl groups W(F4), W(B6) and W(E6). The strip-projection technique leads to results compatible with some experimental data.

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The one-to-one correspondence between a set of cosets of the Mathieu group M11, block sets of the Steiner system S(4, 5, 11) and 11-vertex equi-edged triangulated clusters determines symmetry-allowable transformations between different 11-vertex triangulated clusters as automorphisms of the Steiner system S(4, 5, 11). The transformations can describe local reconstructions corresponding to polymorphic transformations in metals.

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Diffuse scattering from PZN (PbZn1/3Nb2/3O3) has been analysed by combining the average structure revealed by Bragg diffraction with the requirements of the bond-valence sum. This goes beyond previous models, which are more prescriptive and artificial in nature, although not all key aspects of the short-range order can be shown to arise `naturally' out of the modelled interactions.

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An efficient procedure and computer program have been developed to perform an accurate parameterization for scattering factors with the correct inclusion of all physical constraints.

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The structure of a particular type of hollow cage fullerenes (C60+N18) is described in detail, and their existence explained from a symmetry-breaking mechanism starting from the perfect icosahedral symmetry of C60 to specific subgroups A2. This mechanism expands previous results describing the existence of other groups of fullerenes (C60+N10) based on the breaking of the icosahedral symmetry of C60 to the subgroup H2. The mechanism is extended to describe the cases that generate carbon nanotubes, as well as stereoisomers of the C78 molecule.

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The coincidence problem for lattices, shifted lattices and crystallographic point packings is considered. The results are illustrated using a shifted square lattice and the diamond packing.

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A new diffraction data analysis method for serial femtosecond crystallography is proposed. This method is found to obtain satisfactory results.

short communications


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Revised Seitz notation is provided for the symmetry operations blocks of International Tables for Crystallography Vol. E, Subperiodic Groups.

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Accurate atomic displacement parameters obtained from time-of-flight single-crystal diffraction neutron data, collected at the newly commissioned SNS beamline TOPAZ, are presented for two organic crystals and compared to those obtained from four alternative methods: experimental charge-density modelling, high-order independent-atom models, estimates from combined TLS analysis and literature values as implemented in SHADE, and Hirshfeld atom refinement based on X-ray diffraction data.

addenda and errata


Acta Cryst. (2014). A70, 682
doi: 10.1107/S2053273314016842

international union of crystallography


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