issue contents

ISSN: 2053-2733

July 2023 issue

Highlighted illustration

Cover illustration: Interest in so-called bilayer structures has grown since the discovery of strong electronic correlations and superconductivity in twisted bilayer graphene. In this issue, Gratias and Quiquandon [Acta Cryst. (2023), A79, 301–317] investigate the symmetry properties of twisted bilayers. Top left: These two identical cups share the same mirror (the blue frame) and are transformed into each other by another mirror (the red frame) perpendicular to it. Alone, each cup has the point symmetry m, but the pair of cups, taken as a whole, has point symmetry 2mm. Bottom left and right: Applied to homophase bilayer structures, such as two superimposed graphene planes disoriented by a rotation [\hat\alpha], the newly created mirrors are the elements of the exchange set [{\cal E}_\alpha] whereas the rotations (binary, ternary and hexagonal) belong to the original set of symmetry elements and define the common set [{\cal I}_\alpha]. This space-group composition reconstructs the original hexagonal symmetry as long as the rotation angle generates a coincidence lattice, whatever its size. In their article Gratias and Quiquandon discuss the existence and characterization of such coincidence lattices and induced space-group symmetries for any 2D layer structures.


research papers

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A general scheme is proposed to classify and determine the crystallographic properties of twisted bilayers of any homophase 2D structures using complex numbers.

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Improved analytical representations of X-ray atomic form factors are put forward based on the inverse Mott–Bethe formula. Applying these representations, the mean absolute errors calculated for the complete set of form factors given in Table in International Tables for Crystallography, Vol. C, 3rd ed., are reduced by a factor of ∼50 from previous published analyses. Various form factor compilations are examined to record the applicability of the approach outlined.


research papers

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Four deep learning architectures were applied and SqueezeNet scored best. It was combined with the grid programming system BOINC to realize automatic real-time scoring of crystallization well images. Scores are written to a database and displayed to facilitate image inspection for users.

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Indexing of diffraction patterns for application to automatic orientation mapping of quasicrystalline materials is described.

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A multi-component description of the unit-cell content is introduced. Efficient algorithms to define the contribution of these components to structure factors are described and implemented in CCTBX and Phenix.

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The double-slit dynamical diffraction of X-rays in curved crystals is theoretically investigated. It is shown that interference fringes similar to Young's fringes are formed which can be used for determination of the curvature radius.

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Neural networks were trained for robust classification of narrow electron beam diffraction patterns and may significantly decrease the need for storage space.

crystal lattices

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Starting from a Niggli-reduced cell, a crystallographic lattice may be characterized by seven parameters describing the Dirichlet cell: the three shortest non-coplanar lattice vector lengths, the three shorter of each pair of face-diagonal lengths and the shortest body-diagonal length, from which the Niggli-reduced cell may be recovered.

short communications

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It is shown that there are 51 types of physical quantities in arbitrary dimensions with distinct transformations by wedge reversion symmetry. In the paper by Gopalan [(2020). Acta Cryst. A76, 318–327] only 41 types were enumerated.


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