21 results match your criteria Acta crystallographica. Section A Foundations of crystallography[Journal]

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A new vision for Acta Crystallographica Section A.

Acta Crystallogr A 2013 Nov 17;69(Pt 6):533-4. Epub 2013 Oct 17.

Department of Applied Physics and Applied Mathematics, Columbia University, 200 Mudd, 500 W 120th Street, New York, NY 10027, USA.

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http://dx.doi.org/10.1107/S0108767313023702DOI Listing
November 2013
3 Reads

Keyhole electron diffractive imaging (KEDI).

Acta Crystallogr A 2012 Nov 7;68(Pt 6):687-702. Epub 2012 Sep 7.

Istituto di Cristallografia-Consiglio Nazionale delle Ricerche (IC-CNR), Bari, Italy.

Electron diffractive imaging (EDI) relies on combining information from the high-resolution transmission electron microscopy image of an isolated kinematically diffracting nano-particle with the corresponding nano-electron diffraction pattern. Phase-retrieval algorithms allow one to derive the phase, lost in the acquisition of the diffraction pattern, to visualize the actual atomic projected potential within the specimen at sub-ångström resolution, overcoming limitations due to the electron lens aberrations. Here the approach is generalized to study extended crystalline specimens. Read More

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http://scripts.iucr.org/cgi-bin/paper?S0108767312031832
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http://dx.doi.org/10.1107/S0108767312031832DOI Listing
November 2012
5 Reads

Geometric parameters of isotropic ensembles of right cylinders from the small-angle-scattering correlation function.

Acta Crystallogr A 2010 Sep 5;66(Pt 5):597-601. Epub 2010 Aug 5.

Martin-Luther-University Halle-Wittenberg, Institute of Physics, Hoher Weg 8, D-06120 Halle, Germany.

The scattered intensity of ensembles of right homogeneous quasi-diluted cylinders with constant oval right section (RS) and volume fraction phi are analyzed using the small-angle-scattering (SAS) correlation function (CF) gamma(r) = gamma(r, phi) in the isotropic two-phase approximation. A relation between the CF of the cylinder RS, beta(0)(r), and the CF of the single cylinder of height H, gamma(0)(r, H), allows the calculation of the explicit cylinder parameters of height, surface area, RS surface area, RS perimeter and volume. This is accomplished by evaluating the first two derivatives of gamma(0)(r) at r = 0. Read More

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http://dx.doi.org/10.1107/S0108767310022166DOI Listing
September 2010

Time-resolved synchrotron diffraction and theoretical studies of very short-lived photo-induced molecular species.

Acta Crystallogr A 2010 Mar 18;66(Pt 2):179-88. Epub 2010 Feb 18.

Chemistry Department, University at Buffalo, State University of New York, Buffalo, NY 14260-3000, USA.

Definitive experimental results on the geometry of fleeting species are at the time of writing still limited to monochromatic data collection, but methods for modifications of the polychromatic Laue data to increase their accuracy and their suitability for pump-probe experiments have been implemented and are reviewed. In the monochromatic experiments summarized, excited-state conversion percentages are small when neat crystals are used, but are higher when photoactive species are embedded in an inert framework in supramolecular crystals. With polychromatic techniques and increasing source brightness, smaller samples down to tenths of a micrometre or less can be used, increasing homogeneity of exposure and the fractional population of the excited species. Read More

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http://scripts.iucr.org/cgi-bin/paper?S0108767309055342
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http://dx.doi.org/10.1107/S0108767309055342DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2824528PMC
March 2010
2 Reads

Dynamical calculation for X-ray 24-beam diffraction in a two-plate crystal cavity of silicon.

Acta Crystallogr A 2008 May 18;64(Pt 3):394-403. Epub 2008 Apr 18.

Department of Physics, National Tsing Hua University, Taiwan.

The X-ray back diffraction of (1240) in a monolithic two-plate silicon cavity occurs at photon energy 14.4388 keV, at which 24 beams are simultaneously excited. Based on the dynamical theory of X-ray diffraction, a theoretical approach has been developed for solving the fundamental equation of dynamical theory to investigate this back diffraction and the interference patterns generated by the Fabry-Perot-type resonance that produces intensity undulation in both transmitted and back-reflected beams. Read More

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http://dx.doi.org/10.1107/S0108767308006545DOI Listing

The electron-spin--nuclear-spin interaction studied by polarized neutron scattering.

Acta Crystallogr A 2007 Nov 17;63(Pt 6):455-64. Epub 2007 Oct 17.

GKSS Forschungszentrum, Geesthacht, Germany.

Dynamic nuclear spin polarization (DNP) is mediated by the dipolar interaction of paramagnetic centres with nuclear spins. This process is most likely to occur near paramagnetic centres at an angle close to 45 degrees with respect to the direction of the external magnetic field. The resulting distribution of polarized nuclear spins leads to an anisotropy of the polarized neutron scattering pattern, even with randomly oriented radical molecules. Read More

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http://dx.doi.org/10.1107/S0108767307043759DOI Listing
November 2007
1 Read

Correlated grain-boundary distributions in two-dimensional networks.

Acta Crystallogr A 2007 Jul 1;63(Pt 4):315-28. Epub 2007 Jul 1.

Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.

In polycrystals, there are spatial correlations in grain-boundary species, even in the absence of correlations in the grain orientations, due to the need for crystallographic consistency among misorientations. Although this consistency requirement substantially influences the connectivity of grain-boundary networks, the nature of the resulting correlations are generally only appreciated in an empirical sense. Here a rigorous treatment of this problem is presented for a model two-dimensional polycrystal with uncorrelated grain orientations or, equivalently, a cross section through a three-dimensional polycrystal in which each grain shares a common crystallographic direction normal to the plane of the network. Read More

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http://dx.doi.org/10.1107/S0108767307021782DOI Listing
July 2007
3 Reads

Double-helix structure in multiwall boron nitride nanotubes.

Acta Crystallogr A 2005 Nov 22;61(Pt 6):533-41. Epub 2005 Oct 22.

Department of Nuclear, Plasma, and Radiological Engineering, University of Illinois at Urbana-Champaign, IL 61801, USA.

A new nanotube structural form is reported that resembles a double helix in multiwall boron nitride nanotubes (MW-BNNT) grown by a carbon-free chemical-vapor-deposition process as documented by evidence obtained by transmission electron diffraction and microscopy. The double-helix structure is found in MW-BNNTs exhibiting the same chirality in its different walls. The MW-BNNTs deviate from the structure of ideal nested coaxial cylindrical tubes. Read More

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http://dx.doi.org/10.1107/S0108767305026723DOI Listing
November 2005
2 Reads

Intensity of N-beam X-ray diffraction: kinematical theory for a small crystal.

Authors:
Shijo Nagao

Acta Crystallogr A 2005 Mar 22;61(Pt 2):246-51. Epub 2005 Feb 22.

Department of Physics, Kyushu University, Fukuoka 812-8581, Japan.

For a small and ideally imperfect crystal, a method for calculating n-beam X-ray diffraction intensities has been developed on the basis of macroscopic intensity exchanges among the beams. This kinematical formulation results in a set of simultaneous equations that can be solved by numerical calculation. To validate the macroscopic formulae, the Darwin intensity transfer equations, which describe microscopic interactions by both diffraction and absorption, are integrated on a spherical crystal. Read More

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http://dx.doi.org/10.1107/S0108767305003508DOI Listing

Generalized quasiperiodic patterns and superstructures in quasicrystals.

Authors:
Akiji Yamamoto

Acta Crystallogr A 2005 Mar 22;61(Pt 2):185-93. Epub 2005 Feb 22.

Advanced Materials Laboratory, NIMS, Tsukuba 305-0044, Japan.

Patterns with octagonal and icosahedral symmetries obtained from periodic two-dimensional (2D) 8-grids and three-dimensional (3D) 12-grids by the dual method are shown to be superstructures of the Beenker pattern and the 3D Penrose pattern with the same edge length. The superstructures are described in the same spaces as those of the latter (4D for the Beenker and 6D for the 3D Penrose patterns) in the section method but their lattice constants are doubled. The occupation domains and the diffraction patterns for these cases are given. Read More

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http://dx.doi.org/10.1107/S0108767304027333DOI Listing

Precise analysis of the components of diffraction by cylindrical crystals.

Acta Crystallogr A 2004 Nov 26;60(Pt 6):562-4. Epub 2004 Oct 26.

China Institute of Atomic Energy, PO Box 275 (18), Beijing 102413, People's Republic of China.

A detailed quantitative analysis of the various Bragg and Laue components of the integrated reflection power ratio for cylindrical crystals, and the dependence of these components on the Bragg angle, thetaB, the reduced radius, tau0 = sigma0rho, and the ratio of absorption coefficient to diffraction cross section, mu/sigma0 = xi0, is presented. The result indicates that the percentage of Laue and Bragg components of the integrated reflection power ratio is larger than 50% when thetaB < or = 20 degrees and murho < or = 1, and when thetaB > or = 10 degrees and murho > or = 5. The reflection power ratio profile for cylindrical crystals with large murho is also discussed. Read More

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http://dx.doi.org/10.1107/S0108767304017404DOI Listing
November 2004
1 Read

A five-dimensional model of dodecagonal Ta-Te quasicrystals with fractal occupation domains.

Authors:
Akiji Yamamoto

Acta Crystallogr A 2004 Mar 17;60(Pt 2):142-5. Epub 2004 Feb 17.

Advanced Materials Laboratory, NIMS, Tsukuba, 305-0044 Japan.

The consideration of two approximants of the first stable dodecagonal Ta-Te quasicrystal shows that the five-dimensional (5D) space group of the dodecagonal phase is P12m2(12(5)mm) and the structure consists of five layers within a 10 A period. A 5D quasicrystal model leading to the two approximate structures by the introduction of appropriate linear phason strains is derived by the section method. The model shows 20 A dodecagonal clusters (DDC) arranged at each vertex of the square-triangle dodecagonal pattern and has fractal occupation domains, suggesting that the quasicrystal has either a deterministic or a random square-triangle packing of the DDC. Read More

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http://dx.doi.org/10.1107/S0108767303029003DOI Listing

On the autocorrelation functions of right cylindrical particles.

Authors:
S Ciccariello

Acta Crystallogr A 2003 Sep 29;59(Pt 5):506-12. Epub 2003 Aug 29.

Dipartimento di Fisica 'G.Galilei' e unità INFM, via Marzolo 8, I-35131 Padova, Italy.

The autocorrelation function of a finite particle with the shape of a right cylinder is determined by the autocorrelation function of the particle right section, whatever the latter's shape, and by the cylinder height. In fact, the first function is an integral transform of the second with a simple kernel depending on the cylinder height. This integral relation is solved to determine the autocorrelation function of the particle section in terms of the autocorrelation function of the full particle. Read More

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http://dx.doi.org/10.1107/S0108767303016957DOI Listing
September 2003

'Six-beam X-ray section topograph' images and computer-simulated images using the new n-beam dynamical theory based on the Takagi-Taupin equations.

Acta Crystallogr A 2003 Jul 26;59(Pt 4):311-6. Epub 2003 Jun 26.

Engineering Research Institute, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan.

A 'six-beam X-ray section topograph' experiment was performed whose images are in excellent agreement with the images computer-simulated using the theory derived by one of the authors [Okitsu (2003). Acta Cryst. A59, 235-244] based on the Takagi-Taupin dynamical theory. Read More

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July 2003
4 Reads

Secondary extinction and diffraction behaviors in cylindrical crystals.

Authors:
Hua-Chen Hu

Acta Crystallogr A 2003 Jul 26;59(Pt 4):297-310. Epub 2003 Jun 26.

China Institute of Atomic Energy, PO Box 275 (18), Beijing 102413, People's Republic of China.

The X-ray and neutron diffraction properties in absorbing cylindrical crystals are systematically explored within the framework of transfer equations and the kinematic diffraction approximation. The calculated power ratio distribution, the integrated reflection power ratio and the secondary-extinction factor y( micro ) are expressed as functions of the Bragg angle theta(B), the reduced radius sigma(0)rho = tau(0) and the ratio of absorption coefficient to diffraction cross section micro /sigma(0) = xi(0). Numerical solutions were obtained for all theta(B) (0-90 degrees ) and samples with tau(0) from 0 to 30, and xi(0) from 0 to 25. Read More

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July 2003
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X-ray Takagi-Taupin dynamical theory generalized to n-beam diffraction cases.

Authors:
Kouhei Okitsu

Acta Crystallogr A 2003 May 25;59(Pt 3):235-44. Epub 2003 Apr 25.

Engineering Research Institute, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan.

A new X-ray dynamical diffraction theory that can deal with n-beam cases comprehensively (n in [3, 4, 6, 8, 12]) has been derived based on the Takagi-Taupin dynamical theory. The new theory takes into account correctly the effects of arbitrarily polarized incident X-rays and the polarization states of X-ray wavefields in a crystal. Furthermore, an arbitrary lattice displacement in the crystal can be dealt with. Read More

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May 2003
3 Reads

Quasiperiodic packings of fibres with icosahedral symmetry.

Authors:
Duneau Audier

Acta Crystallogr A 1999 Jul;55(Pt 4):746-754

Centre de Physique Théorique, Ecole Polytechnique, CNRS UMR 7644, 91128 Palaiseau, France.

Different icosahedral packings of fibres have been experimentally realized. A packing construction with straight fibres of the same circular cross section, only parallel to fivefold icosahedral axes and respecting the closest packing condition, is reported. Its characteristics of point-group symmetry and related two-dimensional tilings are analysed. Read More

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International Union of Crystallography Acta Crystallographica Section A Notes for Authors.

Authors:

Acta Crystallogr A 1999 Jan;55(Pt 1):96-101

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January 1999
2 Reads

Solving crystal structures without Fourier mapping. I. Centrosymmetric case. Erratum.

Authors:
Pilz Fischer

Acta Crystallogr A 1999 May;55(Pt 3):564

Institute of Crystallography, University of Saarbrücken, D-66041 Saarbrücken, Germany, and Laboratory of Crystallography, University of Bayreuth, D-95440 Bayreuth, Germany.

Unfortunately, some printing errors escaped our attention in the proof-reading of our paper [Pilz & Fischer (1998). Acta Cryst. A54, 273-282]. Read More

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May 1999
1 Read

The use of multiple-scattering data to enhance small-angle neutron scattering experiments.

Authors:
Sabine Bertram

Acta Crystallogr A 1999 May;55(Pt 3):500-507

Materials Division, ANSTO, Menai, NSW 2234, Australia.

Multiple scattering of neutrons by the inhomogeneities responsible for small-angle neutron scattering (SANS) during the passage of the beam through the specimen can be used to provide valuable information about the shape of the objects and the absolute value of the contrast between the scattering particles and the matrix. The neutrons emerging from the specimen are classified into those that have been scattered n times. The index n ranges from zero to infinity. Read More

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May 1999
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Solution of the phase problem in crystallography and application to dynamical electron diffraction.

Authors:
W F Tivol

Acta Crystallogr A 1995 Sep;51 ( Pt 5):708-16

Wadsworth Center for Laboratories and Research, School of Public Health, Albany, NY 12201-0509, USA.

Unitarity, a fundamental principle of scattering theory, leads to the prediction of an essentially unique set of phases for the scattering amplitude from a complete knowledge of the differential cross section or, in the case of a crystal, from the diffracted intensities. The Sayre equation and all the direct methods of phasing following there from are derived as a special case of unitarity for zero excitation error. Dynamical and kinematical scattering are considered, and the relationship between them, S = exp(i tau nzK), is obtained. Read More

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September 1995
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