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Acta Crystallographica. Section A, Foundations and Advances

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https://www.readbyqxmd.com/read/28042809/x-ray-crystallography-second-edition-by-william-clegg-oxford-university-press-2015-pp-128-price-gbp-14-99-paperback-isbn-9780198700975
#1
John R Helliwell
No abstract text is available yet for this article.
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042808/basic-elements-of-crystallography-2nd-edition-by-nevill-gonzalez-szwacki-and-teresa-szwacka-pan-stanford-publishing-2016-pp-x-324-price-usd-79-95-paperback-isbn-9789814613576
#2
Massimo Nespolo
No abstract text is available yet for this article.
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042807/ordering-of-convex-polyhedra-and-the-fedorov-algorithm
#3
Yury L Voytekhovsky
A method of naming any convex polyhedron by a numerical code arising from the adjacency matrix of its edge graph has been previously suggested. A polyhedron can be built using its name. Classes of convex n-acra (i.e. n-vertex polyhedra) are strictly (without overlapping) ordered by their names. In this paper the relationship between the Fedorov algorithm to generate the whole combinatorial variety of convex polyhedra and the above ordering is described. The convex n-acra are weakly ordered by the maximum extra valencies of their vertices...
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042806/mpf-a-multipurpose-figure-of-merit-for-phasing-procedures
#4
Maria Cristina Burla, Benedetta Carrozzini, Giovanni Luca Cascarano, Carmelo Giacovazzo, Giampiero Polidori
The efficient multipurpose figure of merit MPF has been defined and characterized. It may be very helpful in phasing procedures. Indeed, it might be used for establishing the centric or acentric nature of an unknown structure, for identifying the presence of some pseudotranslational symmetry, for recognizing the correct solution in multisolution approaches and for estimating the quality of structure models as they become available during the phasing process. Thus, phase improvement or deterioration may be monitored and useless models may be discarded to save computing time...
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042805/lattice-reduction-using-a-euclidean-algorithm
#5
A Mujica
The need to reduce a periodic structure given in terms of a large supercell and associated lattice generators arises frequently in different fields of application of crystallography, in particular in the ab initio theoretical modelling of materials at the atomic scale. This paper considers the reduction of crystals and addresses the reduction associated with the existence of a commensurate translation that leaves the crystal invariant, providing a practical scheme for it. The reduction procedure hinges on a convenient integer factorization of the full period of the cycle (or grid) generated by the repeated applications of the invariant translation, and its iterative reduction into sub-cycles, each of which corresponds to a factor in the decomposition of the period...
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042804/constraint-induced-direct-phasing-method
#6
Hongliang Xu
The best and most detailed structural information is obtained when the diffraction pattern of a single crystal a few tenths of a millimetre in each dimension is analyzed, but growing high-quality crystals of this size is often difficult, sometimes impossible. However, many crystallization experiments that do not yield single crystals do yield showers of randomly oriented microcrystals that can be exposed to X-rays simultaneously to produce a powder diffraction pattern. Although single-crystal diffraction data consist of discrete spots or X-ray reflections, the diffraction of microcrystals in a powder forms rings so that the reflections overlap...
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042803/revisiting-la0-5sr1-5mno4-lattice-distortion-and-charge-ordering-with-multi-beam-resonant-diffraction
#7
Wen Chung Liu, Yi Hua Chiu, Ying Yu Kung, Po Yu Liao, Chih Hao Cheng, Yu Chieh Chih, Yi Wei Tsai, Chia Hung Chu, Chia Hung Lai, Di Jing Huang, Yun Liang Soo, Shih Lin Chang
Sinusoidal wave type distortions of La0.5Sr1.5MnO4 in the low-temperature orthorhombic phase were observed using multi-beam resonant X-ray diffraction (MRXD) with (7/4 7/4 0) fractional primary diffraction. Two four-beam diffractions with opposite asymmetry were measured at 6.5545 keV and compared with the curves simulated by the dynamical X-ray diffraction theory. This approach provides the possibility of resolving the distortion modes which are perpendicular to the momentum transfer by a single azimuthal scan...
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042802/sphere-packings-as-a-tool-for-the-description-of-martensitic-phase-transformations
#8
Heidrun Sowa
Martensitic transformations which play an important role in metallurgical processes are analysed using group-subgroup relations and sphere-packing considerations. This approach is applied to the transformations between body-centred cubic (b.c.c.) and face-centred cubic (f.c.c.) phases and yields the orientation relations according to the Nishiyama-Wassermann, the Kurdjumov-Sachs and the Pitsch mechanisms. The models proposed by Pitsch and Schrader and by Burgers for the transition between b.c.c. and hexagonally closest-packed (h...
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042801/computer-simulations-of-x-ray-six-beam-diffraction-in-a-perfect-silicon-crystal-ii
#9
V G Kohn
This paper reports computer simulations of X-ray six-beam (000, 220, 242, 044, -224, -202) diffraction in a perfect silicon crystal of large thickness where the super-transmission effect prevails, i.e. about 2 cm or more for an X-ray photon energy of 8 keV. Both the plane-wave angular dependence and the six-beam section topographs, which are obtained in experiments with a two-dimensional slit, are calculated. The angular dependence is computed by means of an eigenvalue problem in accordance with Ewald's theory...
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042800/how-many-photons-are-needed-to-reconstruct-random-objects-in-coherent-x-ray-diffractive-imaging
#10
T Jahn, R N Wilke, Y Chushkin, T Salditt
This paper presents an investigation of the reconstructibility of coherent X-ray diffractive imaging diffraction patterns for a class of binary random `bitmap' objects. Combining analytical results and numerical simulations, the critical fluence per bitmap pixel is determined, for arbitrary contrast values (absorption level and phase shift), both for the optical near- and far-field. This work extends previous investigations based on information theory, enabling a comparison of the amount of information carried by single photons in different diffraction regimes...
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042799/2-periodic-self-dual-tilings
#11
Olaf Delgado-Friedrichs, Michael O'Keeffe
2-Periodic self-dual tilings are important in fields ranging from crystal chemistry to mathematical physics. They have been systematically enumerated using combinatorial tiling theory, and 1, 5 and 62 uninodal, binodal and trinodal self-dual tilings have been found. This paper illustrates all uninodal and binodal self-dual tilings and selected trinodal self-dual tilings. Most of these structures are described for the first time.
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042798/a-general-algorithm-for-generating-isotropy-subgroups-in-superspace
#12
Harold T Stokes, Branton J Campbell
This paper presents a general algorithm for generating the isotropy subgroups of superspace extensions of crystallographic space groups involving arbitrary superpositions of multi-k order parameters from incommensurate and commensurate k vectors. Several examples are presented in detail in order to illuminate each step of the algorithm. The practical outcome is that one can now start with any commensurate parent crystal structure and generate a structure model for any conceivable incommensurate modulation of that parent, fully parameterized in terms of order parameters of irreducible representations at the relevant wavevectors...
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/28042797/to-superspace-and-beyond
#13
Mark S Senn
No abstract text is available yet for this article.
January 1, 2017: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/27809210/prices-of-iucr-journals
#14
M H Dacombe
No abstract text is available yet for this article.
November 1, 2016: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/27809209/an-alternative-to-the-goodness-of-fit
#15
Julian Henn
An alternative measure to the goodness of fit (GoF) is developed and applied to experimental data. The alternative goodness of fit squared (aGoFs) demonstrates that the GoF regularly fails to provide evidence for the presence of systematic errors, because certain requirements are not met. These requirements are briefly discussed. It is shown that in many experimental data sets a correlation between the squared residuals and the variance of observed intensities exists. These correlations corrupt the GoF and lead to artificially reduced values in the GoF and in the numerical value of the wR(F(2))...
November 1, 2016: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/27809208/quantitative-theory-of-diffraction-by-ordered-coaxial-nanotubes-reciprocal-lattice-and-diffraction-pattern-indexing
#16
Zufar Khalitov, Azat Khadiev, Diana Valeeva, Dmitry Pashin
A quantitative theory of diffraction by right- and left-handed coaxial nanotubes with an ordered structure is developed. Their reciprocal lattices, including pseudo-orthogonal nodes, are studied. The explicit formulas that govern relations between direct and reciprocal lattices of a nanotube are achieved and a simple descriptive tool for diffraction pattern indexing is proposed.
November 1, 2016: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/27809207/the-complete-classification-of-five-dimensional-dirichlet-voronoi-polyhedra-of-translational-lattices
#17
Mathieu Dutour Sikirić, Alexey Garber, Achill Schürmann, Clara Waldmann
This paper reports on the full classification of Dirichlet-Voronoi polyhedra and Delaunay subdivisions of five-dimensional translational lattices. A complete list is obtained of 110 244 affine types (L-types) of Delaunay subdivisions and it turns out that they are all combinatorially inequivalent, giving the same number of combinatorial types of Dirichlet-Voronoi polyhedra. Using a refinement of corresponding secondary cones, 181 394 contraction types are obtained. The paper gives details of the computer-assisted enumeration, which was verified by three independent implementations and a topological mass formula check...
November 1, 2016: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/27809206/residue-based-scattering-factors
#18
Hongliang Xu
A glob is defined as a group of atoms in the crystal which can be chosen in various ways. Globs themselves can be used as scattering elements in the theory of structure determination, just as atoms are used at present. In this paper, amino-acid residues are chosen to form globs and empirical formulas for residue-based scattering factors have been developed.
November 1, 2016: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/27809205/direct-observation-of-incommensurate-structure-in-mo3si
#19
Ahmet Gulec, Xiaoxiang Yu, Matthew Taylor, John H Perepezko, Laurence Marks
Z-contrast imaging, electron diffraction, atom-probe tomography (APT) and density functional theory calculations were used to study the crystal structure of the Mo3Si phase which was previously reported to have an A15 crystal structure. The results showed that Mo3Si has an incommensurate crystal structure with a non-cubic unit cell. The small off-stoichiometry in composition of the sample which was revealed by APT and atomic resolution Z-contrast imaging suggested that site substitution caused the development of split atomic positions, disorder and vacancies...
November 1, 2016: Acta Crystallographica. Section A, Foundations and Advances
https://www.readbyqxmd.com/read/27809204/thermal-evolution-of-mg-al-and-ni-al-layered-double-hydroxides-the-structure-of-the-dehydrated-phase
#20
Svetlana Cherepanova, Natalya Leont'eva, Vladimir Drozdov, Vladimir Doronin
Simulation of X-ray diffraction patterns on the basis of the models of one-dimensional disordered crystals was used to investigate the structure of the dehydrated phase produced by dehydration of Mg-Al and Ni-Al layered double hydroxides at a temperature of ∼473-498 K. It was found that the removal of water molecules transforms the initial structure, which is a mixture of 3R1 and 2H1 polytypes, into a structure that comprises preferentially fragments of 3R2 and 1H polytypes and has some turbostratic disorder...
November 1, 2016: Acta Crystallographica. Section A, Foundations and Advances
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