journal

# Acta Crystallographica. Section A, Foundations and Advances

journal
#1
Marc de Boissieu
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#2
Hans Grimmer
A correction and additions concerning the limiting point groups are made to the article by Grimmer [Acta Cryst. (2017), A73, 333-345].
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#3
Prashant Kumar, Barbara Gruza, Sławomir Antoni Bojarowski, Paulina Maria Dominiak
The transferable aspherical pseudoatom data bank, UBDB2018, is extended with over 130 new atom types present in small and biological molecules of great importance in biology and chemistry. UBDB2018 can be applied either as a source of aspherical atomic scattering factors in a standard X-ray experiment (dmin ≃ 0.8 Å) instead of the independent atom model (IAM), and can therefore enhance the final crystal structure geometry and refinement parameters; or as a tool to reconstruct the molecular charge-density distribution and derive the electrostatic properties of chemical systems for which 3D structural data are available...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#4
Mikhail V Talanov
The quadruple perovskites AA'3 B4 X12 are characterized by an extremely wide variety of intriguing physical properties, which makes them attractive candidates for various applications. Using group-theoretical analysis, possible 1:3 A-site-ordered low-symmetry phases have been found. They can be formed from a parent Pm{\bar 3}m perovskite structure (archetype) as a result of real or hypothetical (virtual) phase transitions due to different structural mechanisms (orderings and displacements of atoms, tilts of octahedra)...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#5
Viktor N Serezhkin, Ekaterina F Rogaleva, Anton V Savchenkov, Denis V Pushkin, Larisa B Serezhkina
Using the parameters of Voronoi-Dirichlet (VD) polyhedra the authors have verified the maximum space-filling principle in substructures constructed of actinide atoms (from thorium to einsteinium) in all crystal structures from the Inorganic Crystal Structure Database (ICSD) and Cambridge Structural Database (CSD). It is shown that most of the actinide atoms in such substructures are surrounded by 14 or 12 neighboring atoms. It was discovered that U substructures with greater than or equal to 20 crystallographically independent U atoms in the unit cell feature 15-faceted VD polyhedra as the most common type...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#6
Daniel C Van Hoesen, James C Bendert, Kenneth F Kelton
Expressions for X-ray absorption and secondary scattering are developed for cylindrical sample geometries. The incident-beam size is assumed to be smaller than the sample and in general directed off-axis onto the cylindrical sample. It is shown that an offset beam has a non-negligible effect on both the absorption and multiple scattering terms, resulting in an asymmetric correction that must be applied to the measured scattering intensities. The integral forms of the corrections are first presented. A small-beam limit is then developed for easier computation...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#7
Ireneusz Buganski, Radoslaw Strzalka, Janusz Wolny
The origin of the characteristic bias observed in a logarithmic plot of the calculated and measured intensities of diffraction peaks for quasicrystals has not yet been established. Structure refinement requires the inclusion of weak reflections; however, no structural model can properly describe their intensities. For this reason, detailed information about the atomic structure is not available. In this article, a possible cause for the characteristic bias, namely the lattice phason flip, is investigated. The derivation of the structure factor for a tiling with inherent phason flips is given and is tested for the AlCuRh decagonal quasicrystal...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#8
K V Nikolaev, S N Yakunin, I A Makhotkin, J de la Rie, R V Medvedev, A V Rogachev, I N Trunckin, A L Vasiliev, C P Hendrikx, M Gateshki, R W E van de Kruijs, F Bijkerk
A structural characterization of W/Si multilayers using X-ray reflectivity (XRR), scanning transmission electron microscopy (STEM) and grazing-incidence small-angle X-ray scattering (GISAXS) is presented. STEM images revealed lateral, periodic density fluctuations in the Si layers, which were further analysed using GISAXS. Characteristic parameters of the fluctuations such as average distance between neighbouring fluctuations, average size and lateral distribution of their position were obtained by fitting numerical simulations to the measured scattering images, and these parameters are in good agreement with the STEM observations...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#9
Xiaojing Huang, Hanfei Yan, Yan He, Mingyuan Ge, Hande Öztürk, Yao Lung L Fang, Sungsoo Ha, Meifeng Lin, Ming Lu, Evgeny Nazaretski, Ian K Robinson, Yong S Chu
Multi-slice X-ray ptychography offers an approach to achieve images with a nanometre-scale resolution from samples with thicknesses larger than the depth of field of the imaging system by modeling a thick sample as a set of thin slices and accounting for the wavefront propagation effects within the specimen. Here, we present an experimental demonstration that resolves two layers of nanostructures separated by 500 nm along the axial direction, with sub-10 nm and sub-20 nm resolutions on two layers, respectively...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#10
Heidrun Sowa
All homogeneous sphere packings were derived that refer to the trivariant lattice complexes of monoclinic space-group types P2/c and P21 /c. In total, sphere packings of 55 types have been found. The maximal inherent symmetry is monoclinic for 17 types while the other types comprise at least one sphere packing with cubic (four cases), hexagonal (six cases), tetragonal (eight cases) or orthorhombic (20 cases) symmetry.
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#11
Philippe Scheid, Corentin Chatelier, Julian Ledieu, Vincent Fournée, Émilie Gaudry
Clusters, i.e. polyhedral geometric entities, are widely used to describe the structure of complex intermetallic compounds. However, little is generally known about their physical significance. The atomic and electronic structures of the Al13 TM4 complex intermetallic compounds (TM = Fe, Co, Ru, Rh) have been investigated using a wide range of ab initio tools in order to examine the influence of the chemical composition on the pertinence of the bulk structure description based on 3D clusters. In addition, since surface studies were found to be a relevant approach to address the question of cluster stability in complex phases, the interplay of the cluster substructure with the 2D surface is addressed in the case of the Al13 Co4 (100) and Al13 Fe4 (010) surfaces...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#12
Sebastian Schenk, Eva Maria Zollner, Oliver Krahn, Berit Schreck, René Hammer, Stefan Förster, Wolf Widdra
The atomically resolved real-space structure of a long-range-ordered dodecagonal quasicrystal is determined based on scanning tunnelling microscopy. For the BaTiO3 -derived oxide quasicrystal which spontaneously forms on a Pt(111) surface, 8100 atomic positions have been determined and are compared with an ideal Niizeki-Gähler tiling. Although the Niizeki-Gähler tiling has a complex three-element structure, the abundance of the triangle, square and rhomb tiling elements in the experimental data closely resembles the ideal frequencies...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#13
Anastasiya I Vinokur, Katerina P Hilleke, Daniel C Fredrickson
Many complex intermetallic structures feature a curious juxtaposition of domains with strict 3D periodicity and regions of much weaker order or incommensurability. This article explores the basic principles leading to such arrangements through an investigation of the weakly ordered channels of Fe2 Al5 . It starts by experimentally confirming the earlier crystallographic model of the high-temperature form, in which nearly continuous columns of electron density corresponding to disordered Al atoms emerge. Then electronic structure calculations on ordered models are used to determine the factors leading to the formation of these columns...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#14
Insung Han, Xianghui Xiao, Haiping Sun, Ashwin J Shahani
Quasicrystals and their approximants have triggered widespread interest due to the challenge of solving their complex crystal structures as well as their possibly exceptional properties. The structural motifs of approximants are similar to those of the corresponding quasicrystals, but to what extent are their crystallization pathways the same? Unfortunately, there have been very few in situ experimental investigations to answer this question. Here, by leveraging the high penetrating power of hard X-rays, synchrotron-based X-ray tomography was conducted in order to capture the nucleation and growth of a decagonal quasicrystal and its related approximant...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#15
Marc de Boissieu
This article reviews some of Ted Janssen's (1936-2017) major contributions to the field of aperiodic crystals. Aperiodic crystals are long-range ordered structures without 3D lattice translations and encompass incommensurately modulated phases, incommensurate composites and quasicrystals. Together with Pim de Wolff and Aloysio Janner, Ted Janssen invented the very elegant theory of superspace crystallography that, by adding a supplementary dimension to the usual 3D space, allows for a deeper understanding of the atomic structure of aperiodic crystals...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#16
Paul Benjamin Klar, Iñigo Etxebarria, Gotzon Madariaga
The benefit of computational methods applying density functional theory for the description and understanding of modulated crystal structures is investigated. A method is presented which allows one to establish, improve and test superspace models including displacive and occupational modulation functions from first-principles calculations on commensurate structures. The total energies of different configurations allow one to distinguish stable and less stable structure models. The study is based on a series of geometrically optimized superstructures of mullite (Al4+2x Si2-2x O10-x ) derived from the superspace group Pbam(α0½)0ss...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#17
J P J Chen, J J Donatelli, K E Schmidt, R A Kirian
Diffraction patterns from small protein crystals illuminated by highly coherent X-rays often contain measurable interference signals between Bragg peaks. This coherent shape transform' signal introduces enough additional information to allow the molecular densities to be determined from the diffracted intensities directly, without prior information or resolution restrictions. However, the various correlations amongst molecular occupancies/vacancies at the crystal surface result in a subtle yet critical problem in shape transform phasing whereby the sublattices of symmetry-related molecules exhibit a form of partial coherence amongst lattice sites when an average is taken over many crystal patterns...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#18
Zirui Gao, Manuel Guizar-Sicairos, Viviane Lutz-Bueno, Aileen Schröter, Marianne Liebi, Markus Rudin, Marios Georgiadis
The recent advent of tensor tomography techniques has enabled tomographic investigations of the 3D nanostructure organization of biological and material science samples. These techniques extended the concept of conventional X-ray tomography by reconstructing not only a scalar value such as the attenuation coefficient per voxel, but also a set of parameters that capture the local anisotropy of nanostructures within every voxel of the sample. Tensor tomography data sets are intrinsically large as each pixel of a conventional X-ray projection is substituted by a scattering pattern, and projections have to be recorded at different sample angular orientations with several tilts of the rotation axis with respect to the X-ray propagation direction...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#19
Eleonora Conterosito, Luca Palin, Rocco Caliandro, Wouter van Beek, Dmitry Chernyshov, Marco Milanesio
The increasing efficiency of detectors and brightness of X-rays in both laboratory and large-scale facilities allow the collection of full single-crystal X-ray data sets within minutes. The analysis of these crystallographic big data' requires new tools and approaches. To answer these needs, the use of principal component analysis (PCA) is proposed to improve the efficiency and speed of the analysis. Potentialities and limitations of PCA were investigated using single-crystal X-ray diffraction (XRD) data collected in situ on Y zeolite, in which CO2 , acting as an active species, is thermally adsorbed while cooling from 300 to 200 K...
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
#20
Gloria Borgstahl, Alan I Goldman, Patricia A Thiel
March 1, 2019: Acta Crystallographica. Section A, Foundations and Advances
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