journal

# Acta Crystallographica. Section A, Foundations and Advances

journal
#1
Gerry Lander, Colin Carlile
May 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
#2
M Dacombe, A T Ashcroft
The report of the Executive Committee for 2016 is presented.
May 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
#3
A quantitative theory of Fraunhofer diffraction by right- and left-handed multiwalled cylindrical scroll nanotubes is developed on the basis of the kinematical approach. The proposed theory is mainly dedicated to structural studies of individual nanotubes by the selected-area electron diffraction technique. Strong and diffuse reflections of the scroll nanotube were studied and explicit formulas that govern relations between the direct and reciprocal lattice of the scroll nanotube are achieved.
May 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
#4
Martin Cramer Pedersen, Olaf Delgado-Friedrichs, Stephen T Hyde
This paper describes an invariant representation for finite graphs embedded on orientable tori of arbitrary genus, with working examples of embeddings of the Möbius-Kantor graph on the torus, the genus-2 bitorus and the genus-3 tritorus, as well as the two-dimensional, 7-valent Klein graph on the tritorus (and its dual: the 3-valent Klein graph). The genus-2 and -3 embeddings describe quotient graphs of 2- and 3-periodic reticulations of hyperbolic surfaces. This invariant is used to identify infinite nets related to the Möbius-Kantor and 7-valent Klein graphs...
May 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
#5
Danut Dragoi
The method described in this paper improves the old methods of crystal orientation, applies new parametric equations for crystallography, and increases the precision and accuracy of measurements. The method applies to inorganic and organic crystals. A breakthrough in crystal orientation happened about 25 years ago when two equations dependent on the Bragg angle and an arbitrary direction in the crystal were developed. Unfortunately, they were analytically insolvable and their unique solution was found numerically...
May 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
#6
Minas K Balyan
For the asymmetrical Laue case the X-ray plane-wave transmission and reflection coefficients and rocking curves are analysed as a function of the deviation angles from the exact Bragg orientation in the diffraction plane and in the direction perpendicular to the diffraction plane. New peculiarities of the rocking curves are obtained. The peculiarities of both the effective absorption coefficient and rocking curves in thick crystals are also investigated.
May 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
#7
David Hicks, Corey Oses, Eric Gossett, Geena Gomez, Richard H Taylor, Cormac Toher, Michael J Mehl, Ohad Levy, Stefano Curtarolo
Determination of the symmetry profile of structures is a persistent challenge in materials science. Results often vary amongst standard packages, hindering autonomous materials development by requiring continuous user attention and educated guesses. This article presents a robust procedure for evaluating the complete suite of symmetry properties, featuring various representations for the point, factor and space groups, site symmetries and Wyckoff positions. The protocol determines a system-specific mapping tolerance that yields symmetry operations entirely commensurate with fundamental crystallographic principles...
May 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
#8
Bertrand Fournier, Benoît Guillot, Claude Lecomte, Eduardo C Escudero-Adán, Christian Jelsch
Estimating uncertainties of property values derived from a charge-density model is not straightforward. A methodology, based on calculation of sample standard deviations (SSD) of properties using randomly deviating charge-density models, is proposed with the MoPro software. The parameter shifts applied in the deviating models are generated in order to respect the variance-covariance matrix issued from the least-squares refinement. This SSD methodology' procedure can be applied to estimate uncertainties of any property related to a charge-density model obtained by least-squares fitting...
May 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
#9
Huibin Chang, Pablo Enfedaque, Yifei Lou, Stefano Marchesini
Coherent ptychographic imaging experiments often discard the majority of the flux from a light source to define the coherence of the illumination. Even when the coherent flux is sufficient, the stability required during an exposure is another important limiting factor. Partial coherence analysis can considerably reduce these limitations. A partially coherent illumination can often be written as the superposition of a single coherent illumination convolved with a separable translational kernel. This article proposes the gradient decomposition of the probe (GDP), a model that exploits translational kernel separability, coupling the variances of the kernel with the transverse coherence...
May 1, 2018: Acta Crystallographica. Section A, Foundations and Advances
#10
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
#11
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
#12
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
#13
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
#14
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
#15
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
#16
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
#17
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
#18
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
#19
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
#20
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
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