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Acta Crystallographica. Section A, Foundations and Advances

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https://www.readbyqxmd.com/read/28862168/accelerated-scattering-of-convex-polyhedra
#1
Yury L Voytekhovsky
The formulas for the minimum (minn) and maximum (maxn) names in the classes of convex n-acra (i.e. n-vertex polyhedra) are found for any n. The asymptotic behaviour (as n → ∞) for maxn+1/maxn, minn+1/minn, minn+1/maxn and maxn/minn is established. They characterize in detail the accelerated scattering of [minn, maxn] ranges on a real line.
September 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28862167/real-time-detection-and-resolution-of-atom-bumping-in-crystallographic-models
#2
Yu Liu
A basic principle in crystal structure determination is that there should be proper distances between adjacent atoms. Therefore, detection of atom bumping is of fundamental significance in structure determination, especially in the direct-space method where crystallographic models are just randomly generated. Presented in this article is an algorithm that detects atom bonding in a unit cell based on the sweep and prune algorithm of axis-aligned bounding boxes and running in the O(n log n) time bound, where n is the total number of atoms in the unit cell...
September 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28862166/the-wavevector-substar-group-in-reciprocal-space-and-its-representation
#3
Il Hwan Kim, Jong Ok Pak, Il Hun Kim, Song Won Kim, Lin Li
A new concept of the wavevector substar group is established which, in the study of translational symmetry breaking of a crystal, only considers the particular arms of the wavevector star taking part in the phase transition; this is in contrast with the traditional Landau theory which considers all of the arms of the wavevector star. It is shown that this new concept can be used effectively to investigate the interesting physical properties of crystals associated with translational symmetry breaking. It is shown that studies on complicated phase transitions related to reducible representations, such as those in perovskite KMnF3 multiferroics and the high-temperature superconductor La2/3Mg1/2W1/2O3 (La4Mg3W3O18), are much simplified by the new concept...
September 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28862165/mathematical-aspects-of-molecular-replacement-iv-measure-theoretic-decompositions-of-motion-spaces
#4
Gregory S Chirikjian, Sajdeh Sajjadi, Bernard Shiffman, Steven M Zucker
In molecular-replacement (MR) searches, spaces of motions are explored for determining the appropriate placement of rigid-body models of macromolecules in crystallographic asymmetric units. The properties of the space of non-redundant motions in an MR search, called a `motion space', are the subject of this series of papers. This paper, the fourth in the series, builds on the others by showing that when the space group of a macromolecular crystal can be decomposed into a product of two space subgroups that share only the lattice translation group, the decomposition of the group provides different decompositions of the corresponding motion spaces...
September 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28862164/close-packed-structure-dynamics-with-finite-range-interaction-computational-mechanics-with-individual-layer-interaction
#5
Edwin Rodriguez-Horta, Ernesto Estevez-Rams, Raimundo Lora-Serrano, Reinhard Neder
This is the second contribution in a series of papers dealing with dynamical models in equilibrium theories of polytypism. A Hamiltonian introduced by Ahmad & Khan [Phys. Status Solidi B (2000), 218, 425-430] avoids the unphysical assignment of interaction terms to fictitious entities given by spins in the Hägg coding of the stacking arrangement. In this paper an analysis of polytype generation and disorder in close-packed structures is made for such a Hamiltonian. Results are compared with a previous analysis using the Ising model...
September 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28862163/davide-viterbo-1939-2017
#6
Carlo Mealli, Marco Milanesio
No abstract text is available yet for this article.
September 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28862162/philip-coppens-1930-2017
#7
Pierre Becker
No abstract text is available yet for this article.
September 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28660868/creating-symmetry-the-artful-mathematics-of-wallpaper-patterns-by-frank-a-farris-princeton-university-press-2015-pp-248-price-gbp-27-95-usd-35-00-hardcover-isbn-9780691161730
#8
Gregory McColm
No abstract text is available yet for this article.
July 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28660867/close-packed-structures-with-finite-range-interaction-computational-mechanics-of-layer-pair-interaction
#9
Edwin Rodriguez-Horta, Ernesto Estevez-Rams, Reinhard Neder, Raimundo Lora-Serrano
The stacking problem is approached by computational mechanics, using an Ising next-nearest-neighbour model. Computational mechanics allows one to treat the stacking arrangement as an information processing system in the light of a symbol-generating process. A general method for solving the stochastic matrix of the random Gibbs field is presented and then applied to the problem at hand. The corresponding phase diagram is then discussed in terms of the underlying ℇ-machine, or optimal finite-state machine. The occurrence of higher-order polytypes at the borders of the phase diagram is also analysed...
July 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28660866/hard-sphere-displacive-model-of-deformation-twinning-in-hexagonal-close-packed-metals-revisiting-the-case-of-the-56%C3%A2-a-contraction-twins-in-magnesium
#10
Cyril Cayron
Contraction twinning in magnesium alloys leads to new grains that are misoriented from the parent grain by a rotation (56°, a). The classical shear theory of deformation twinning does not specify the atomic displacements and does not explain why contraction twinning is less frequent than extension twinning. The paper proposes a new displacive model in line with our previous work on martensitic transformations and extension twinning. A continuous angular distortion matrix that transforms the initial hexagonal close-packed (h...
July 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28660865/thermoelectric-transport-properties-in-magnetically-ordered-crystals
#11
Hans Grimmer
The forms of the tensors describing thermoelectric transport properties in magnetically ordered crystals are given for frequently used orientations of the 122 space-time point groups up to second order in an applied magnetic field. It is shown which forms are interchanged for the point groups of the hexagonal crystal family by two different conventions for the connection between the Hermann-Mauguin symbol and the orientation of the Cartesian coordinate system. The forms are given in Nye notation, which conspicuously shows how the forms for different point groups are related...
July 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28660864/reduction-of-small-angle-scattering-profiles-to-finite-sets-of-structural-invariants
#12
Jérôme Houdayer, Frédéric Poitevin
This paper shows how small-angle scattering (SAS) curves can be decomposed in a simple sum using a set of invariant parameters called Kn which are related to the shape of the object of study. These Kn, together with a radius R, give a complete theoretical description of the SAS curve. Adding an overall constant, these parameters are easily fitted against experimental data giving a concise comprehensive description of the data. The pair distance distribution function is also entirely described by this invariant set and the Dmax parameter can be measured...
July 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28660863/a-first-prototype-multi-determinant-x-ray-constrained-wavefunction-approach-the-x-ray-constrained-extremely-localized-molecular-orbital-valence-bond-method
#13
Alessandro Genoni
All the current variants of Jayatilaka's X-ray constrained wavefunction (XCW) approach work within the framework of the single-determinant wavefunction ansatz. In this paper, a first-prototype multi-determinant XCW technique is proposed. The strategy assumes that the desired XCW is written as a valence-bond-like expansion in terms of pre-determined single Slater determinants constructed with extremely localized molecular orbitals. The method, which can be particularly suitable to investigate systems with a multi-reference character, has been applied to determine the weights of the resonance structures of naphthalene at different temperatures by exploiting experimental high-resolution X-ray diffraction data...
July 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28660862/high-resolution-x-ray-diffraction-with-no-sample-preparation
#14
G M Hansford, S M R Turner, P Degryse, A J Shortland
It is shown that energy-dispersive X-ray diffraction (EDXRD) implemented in a back-reflection geometry is extremely insensitive to sample morphology and positioning even in a high-resolution configuration. This technique allows high-quality X-ray diffraction analysis of samples that have not been prepared and is therefore completely non-destructive. The experimental technique was implemented on beamline B18 at the Diamond Light Source synchrotron in Oxfordshire, UK. The majority of the experiments in this study were performed with pre-characterized geological materials in order to elucidate the characteristics of this novel technique and to develop the analysis methods...
July 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28660861/three-dimensional-single-cell-imaging-with-x-ray-waveguides-in-the-holographic-regime
#15
Martin Krenkel, Mareike Toepperwien, Frauke Alves, Tim Salditt
X-ray tomography at the level of single biological cells is possible in a low-dose regime, based on full-field holographic recordings, with phase contrast originating from free-space wave propagation. Building upon recent progress in cellular imaging based on the illumination by quasi-point sources provided by X-ray waveguides, here this approach is extended in several ways. First, the phase-retrieval algorithms are extended by an optimized deterministic inversion, based on a multi-distance recording. Second, different advanced forms of iterative phase retrieval are used, operational for single-distance and multi-distance recordings...
July 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28660860/introducing-the-holo-tie-approach-to-cellular-imaging
#16
Ian Robinson
No abstract text is available yet for this article.
July 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28447598/convex-polyhedra-with-minimum-and-maximum-names
#17
Yury L Voytekhovsky
The paper reports the combinatorial types of convex n-acra (i.e. n-vertex polyhedra) for which the minimum (min.) and maximum (max.) names are attained. Hence, min. and max. names can be independently found without generating the whole combinatorial variety of convex n-acra (e.g. by the routine recurrence Fedorov algorithm) and calculating names for each n-acron.
May 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28447597/resonant-and-non-resonant-magnetic-scatterings-with-circularly-polarized-x-rays-magnetic-scattering-factor-and-electron-density-of-gadolinium-iron-garnet
#18
Yo Sasaki, Maki Okube, Satoshi Sasaki
Theoretical and experimental studies on resonant magnetic scattering have been carried out using circularly polarized X-rays. Polarization dependence of the asymmetric ratio between right- and left-handed polarized scattering amplitudes has revealed that resonant and non-resonant magnetic scatterings can be observed simultaneously. In this study, synchrotron X-ray intensity experiments were conducted on Gd iron garnet (GdIG) at the pre-edge of the Fe K absorption edge. The asymmetric ratio between circular polarizations exhibited three peaks due to the resonant magnetic responses of Fe(3+) in the Fe2 (24d) site...
May 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28447596/on-the-penrose-and-taylor-socolar-hexagonal-tilings
#19
Jeong Yup Lee, Robert V Moody
The intimate relationship between the Penrose and the Taylor-Socolar tilings is studied, within both the context of double hexagon tiles and the algebraic context of hierarchical inverse sequences of triangular lattices. This unified approach produces both types of tilings together, clarifies their relationship and offers straightforward proofs of their basic properties.
May 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28447595/groupoids-and-labelled-quotient-graphs-a-topological-analysis-of-the-modular-structure-in-pyroxenes
#20
Jean Guillaume Eon
The analysis of the modular structure of pyroxenes, recently discussed in Nespolo & Aroyo [Eur. J. Mineral. (2016), 28, 189-203], has been performed on the respective labelled quotient graphs (LQGs). It is shown that the structure and maximum symmetry of the module, i.e. its layer group, can be determined directly from the LQG. Partial symmetry operations between different modules have been associated with automorphisms of the quotient graph that may not be consistent with net voltages over the respective cycles...
May 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
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