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Acta Crystallographica. Section A, Foundations and Advances

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https://www.readbyqxmd.com/read/29493543/monoclinic-sphere-packings-ii-trivariant-lattice-complexes-with-mirror-symmetry
#1
Heidrun Sowa
All homogeneous sphere packings were derived that refer to the three trivariant monoclinic lattice complexes with mirror symmetry. In total, 29 types of sphere packings have been found. Only for three types is the maximal inherent symmetry of their sphere packings monoclinic whereas the inherent symmetry is orthorhombic for 13 types, tetragonal for eight types, hexagonal for four types and cubic for one type.
March 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29493542/development-of-a-joint-refinement-model-for-the-spin-resolved-one-electron-reduced-density-matrix-using-different-data-sets
#2
Saber Gueddida, Zeyin Yan, Jean Michel Gillet
The paper describes a joint refinement model of the spin-resolved one-electron reduced density matrix using simultaneously magnetic structure factors and magnetic directional Compton profiles. The model is guided by two strategies: (i) variation of basis functions and (ii) variation of the spin population matrix. The implementation for a finite system is based on an expansion of the natural orbitals on basis sets. To show the potential benefits brought by the joint refinement model, the paper also presents the refinement results using magnetic structure factors only...
March 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29493541/phasing-via-pure-crystallographic-least-squares-an-unexpected-feature
#3
Maria Cristina Burla, Benedetta Carrozzini, Giovanni Luca Cascarano, Carmelo Giacovazzo, Giampiero Polidori
Crystallographic least-squares techniques, the main tool for crystal structure refinement of small and medium-size molecules, are for the first time used for ab initio phasing. It is shown that the chief obstacle to such use, the least-squares severe convergence limits, may be overcome by a multi-solution procedure able to progressively recognize and discard model atoms in false positions and to include in the current model new atoms sufficiently close to correct positions. The applications show that the least-squares procedure is able to solve many small structures without the use of important ancillary tools: e...
March 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29493540/coordination-numbers-of-the-vertex-graph-of-a-penrose-tiling
#4
Anton Shutov, Andrey Maleev
A new approach to study coordination shells and coordination sequences of quasiperiodic graphs is suggested. The structure of the coordination shells in the vertex graph of a Penrose tiling is described. An asymptotic formula for its coordination numbers is obtained. An essentially different behaviour of the coordination numbers for even and odd shells is revealed.
March 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29493539/electron-density-critical-points-analysis-and-catastrophe-theory-to-forecast-structure-instability-in-periodic-solids
#5
Marcello Merli, Alessandro Pavese
The critical points analysis of electron density, i.e. ρ(x), from ab initio calculations is used in combination with the catastrophe theory to show a correlation between ρ(x) topology and the appearance of instability that may lead to transformations of crystal structures, as a function of pressure/temperature. In particular, this study focuses on the evolution of coalescing non-degenerate critical points, i.e. such that ∇ρ(xc ) = 0 and λ1 , λ2 , λ3 ≠ 0 [λ being the eigenvalues of the Hessian of ρ(x) at xc ], towards degenerate critical points, i...
March 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29493538/ab-initio-structure-determination-and-quantitative-disorder-analysis-on-nanoparticles-by-electron-diffraction-tomography
#6
Yaşar Krysiak, Bastian Barton, Bernd Marler, Reinhard B Neder, Ute Kolb
Nanoscaled porous materials such as zeolites have attracted substantial attention in industry due to their catalytic activity, and their performance in sorption and separation processes. In order to understand the properties of such materials, current research focuses increasingly on the determination of structural features beyond the averaged crystal structure. Small particle sizes, various types of disorder and intergrown structures render the description of structures at atomic level by standard crystallographic methods difficult...
March 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29493537/the-development-of-powder-profile-refinement-at-the-reactor-centre-netherlands-at-petten
#7
Bob van Laar, Henk Schenk
With thousands of references to `Rietveld refinement' it is forgotten that the method did not suddenly appear in a flash of inspiration of a single person, but was the result of the work of three individuals working in the 1960s at the Reactor Centre Netherlands at Petten, Loopstra, van Laar and Rietveld. This paper outlines the origins of `profile refinement', as it was called at Petten, and also looks at why it took so long for the scientific community to recognize its importance. With the recent passing of Hugo Rietveld, the death of Bert Loopstra in 1998 and before other pioneers also disappear, it is important to set down a first-hand account...
March 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29493536/spatial-displacement-of-forward-diffracted-x-ray-beams-by-perfect-crystals
#8
A Rodriguez-Fernandez, V Esposito, D F Sanchez, K D Finkelstein, P Juranic, U Staub, D Grolimund, S Reiche, B Pedrini
Time-delayed, narrow-band echoes generated by forward Bragg diffraction of an X-ray pulse by a perfect thin crystal are exploited for self-seeding at hard X-ray free-electron lasers. Theoretical predictions indicate that the retardation is strictly correlated to a transverse displacement of the echo pulses. This article reports the first experimental observation of the displaced echoes. The displacements are in good agreement with simulations relying on the dynamical diffraction theory. The echo signals are characteristic for a given Bragg reflection, the structure factor and the probed interplane distance...
March 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29269599/multislice-imaging-of-integrated-circuits-by-precession-x-ray-ptychography
#9
Kei Shimomura, Makoto Hirose, Yukio Takahashi
A method for nondestructively visualizing multisection nanostructures of integrated circuits by X-ray ptychography with a multislice approach is proposed. In this study, tilt-series ptychographic diffraction data sets of a two-layered circuit with a ∼1.4 µm gap at nine incident angles are collected in a wide Q range and then artifact-reduced phase images of each layer are successfully reconstructed at ∼10 nm resolution. The present method has great potential for the three-dimensional observation of flat specimens with thickness on the order of 100 µm, such as three-dimensional stacked integrated circuits based on through-silicon vias, without laborious sample preparation...
January 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29269598/the-limit-of-application-of-the-scherrer-equation
#10
M A R Miranda, J M Sasaki
The Scherrer equation is a widely used tool to obtain crystallite size from polycrystalline samples. Its limit of applicability has been determined recently, using computer simulations, for a few structures and it was proposed that it is directly dependent on the linear absorption coefficient (μ0 ) and Bragg angle (θB ). In this work, a systematic study of the Scherrer limit is presented, where it is shown that it is equal to approximately 11.9% of the extinction length. It is also shown that absorption imposes a maximum value on it and that this maximum is directly proportional to sin θB /μ0 ...
January 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29269597/a-one-step-mechanism-for-new-twinning-modes-in-magnesium-and-titanium-alloys-modelled-by-the-obliquity-correction-of-a-58%C3%A2-a-2b-prototype-stretch-twin
#11
Cyril Cayron
The \{ 11{\overline 2}2\} and \{ 11{\overline 2}6\} twinning modes were recently discovered by Ostapovets et al. [Philos. Mag. (2017), 97, 1088-1101] and interpreted as \{ {10{\overline 1}2} \}-\{ {10{\overline 1}2} \} double twins formed by the simultaneous action of two twinning shears. Another interpretation is proposed here in which the two conjugate twinning modes result from a one-step mechanism based on a (58°, a + 2b) prototype stretch twin and differ from each other only by their obliquity correction...
January 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29269596/improving-the-convergence-rate-of-a-hybrid-input-output-phasing-algorithm-by-varying-the-reflection-data-weight
#12
Hongxing He, Wu Pei Su
In an iterative projection algorithm proposed for ab initio phasing, the error metrics typically exhibit little improvement until a sharp decrease takes place as the iteration converges to the correct high-resolution structure. Related to that is the small convergence probability for certain structures. As a remedy, a variable weighting scheme on the diffraction data is proposed. It focuses on phasing low- and medium-resolution data first. The weighting shifts to incorporate more high-resolution reflections when the iteration proceeds...
January 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29269595/construction-of-weavings-in-the-plane
#13
Eden Delight Miro, Aliw Iw Zambrano, Agnes Garciano
This work develops, in graph-theoretic terms, a methodology for systematically constructing weavings of overlapping nets derived from 2-colorings of the plane. From a 2-coloring, two disjoint simple, connected graphs called nets are constructed. The union of these nets forms an overlapping net, and a weaving map is defined on the intersection points of the overlapping net to form a weaving. Furthermore, a procedure is given for the construction of mixed overlapping nets and for deriving weavings from them.
January 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29269594/small-angle-x-ray-scattering-tensor-tomography-model-of-the-three-dimensional-reciprocal-space-map-reconstruction-algorithm-and-angular-sampling-requirements
#14
Marianne Liebi, Marios Georgiadis, Joachim Kohlbrecher, Mirko Holler, Jörg Raabe, Ivan Usov, Andreas Menzel, Philipp Schneider, Oliver Bunk, Manuel Guizar-Sicairos
Small-angle X-ray scattering tensor tomography, which allows reconstruction of the local three-dimensional reciprocal-space map within a three-dimensional sample as introduced by Liebi et al. [Nature (2015), 527, 349-352], is described in more detail with regard to the mathematical framework and the optimization algorithm. For the case of trabecular bone samples from vertebrae it is shown that the model of the three-dimensional reciprocal-space map using spherical harmonics can adequately describe the measured data...
January 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29269593/quasicrystals-what-do-we-know-what-do-we-want-to-know-what-can-we-know
#15
Walter Steurer
More than 35 years and 11 000 publications after the discovery of quasicrystals by Dan Shechtman, quite a bit is known about their occurrence, formation, stability, structures and physical properties. It has also been discovered that quasiperiodic self-assembly is not restricted to intermetallics, but can take place in systems on the meso- and macroscales. However, there are some blank areas, even in the centre of the big picture. For instance, it has still not been fully clarified whether quasicrystals are just entropy-stabilized high-temperature phases or whether they can be thermodynamically stable at 0 K as well...
January 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29072201/the-rome-de-lisle-problem
#16
Yury L Voytekhovsky, Dmitry G Stepenshchikov
The `Rome de Lisle problem' on the vertex and edge truncations has been formulated and solved for all crystal closed simple forms (two, eight, five and 15 for orthorhombic, trigonal + hexagonal, tetragonal and cubic syngonies, respectively). The collections of simple forms obtained are enumerated and considered as special combinations of simple forms in symmetry classes.
November 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29072200/a-note-on-x-ray-spherical-wavefields-in-the-laue-case-for-perfect-crystals
#17
Takashi Saka
Spherical wavefields in the Laue case are obtained when the real part of the crystal structure factor is zero or small compared with the imaginary part. The results are the same as in the conventional case where the real part is large. Through this work, it is shown that virtual wavefields on the vacuum side should be taken into account to explain the obtained results. It is also shown that, to obtain the virtual wavefields, a modification of the conventional spherical wave theory is necessary.
November 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29072199/about-difference-electron-densities-and-their-properties
#18
Maria Cristina Burla, Benedetta Carrozzini, Giovanni Luca Cascarano, Carmelo Giacovazzo, Giampiero Polidori
Difference electron densities do not play a central role in modern phase refinement approaches, essentially because of the explosive success of the EDM (electron-density modification) techniques, mainly based on observed electron-density syntheses. Difference densities however have been recently rediscovered in connection with the VLD (Vive la Difference) approach, because they are a strong support for strengthening EDM approaches and for ab initio crystal structure solution. In this paper the properties of the most documented difference electron densities, here denoted as F - Fp , mF - Fp and mF - DFp syntheses, are studied...
November 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29072198/extrinsic-faulting-in-3c-close-packed-crystal-structures-computational-mechanics-analysis
#19
Ernesto Estevez-Rams, Raimundo Lora-Serrano, Arbelio Penton-Madrigal, Massimo Nespolo
Extrinsic faulting has been discussed previously within the so-called difference method and random walk calculation. In this contribution it is revisited under the framework of computational mechanics, which allows expressions to be derived for the statistical complexity, entropy density and excess entropy as a function of faulting probability. The approach allows one to compare the disordering process of an extrinsic fault with other faulting types. The ℇ-machine description of the faulting mechanics is presented...
November 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/29072197/the-phase-problem-for-two-dimensional-crystals-i-theory
#20
Romain D Arnal, Rick P Millane
Properties of the phase problem for two-dimensional crystals are examined. This problem is relevant to protein structure determination using diffraction from two-dimensional crystals that has been proposed using new X-ray free-electron laser sources. The problem is shown to be better determined than for conventional three-dimensional crystallography, but there are still a large number of solutions in the absence of additional a priori information. Molecular envelope information reduces the size of the solution set, and for an envelope that deviates sufficiently from the unit cell a unique solution is possible...
November 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
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