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ISSN: 2053-2733

May 1996 issue

Highlighted illustration

Cover illustration: Indicatrix of the inverse of Young's modulus for ADP, a tetragonal crystal. Courtesy of A. Authier, Laboratoire de Minéralogie-Cristallographie, Université Pierre et Marie Curie, Paris, France.

obituaries


topical reviews


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The various problems that have to be faced when direct phasing procedures are applied to powder data are described.

research papers


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For a powder sample, the overlap function is a cubic function when the interface consists of planar facets. The explicit expressions of the Scherrer constants are reported.

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Molecular geometry calculations in the superspace framework of incommensurate structure analysis are presented in detail.

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The comparison of ab initio generated phase sets requires preliminary map alignment. A space-group-symmetry-specific algorithm for optimal origin and enantiomorph choices is proposed.

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Anharmonic motions and effects of chemical bonding of heavy atoms result in nearly identical features of difference electron-density maps, which vary similarly with temperature except at very low temperatures. Examples are K2PtCl6 and Si.

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From the Dirac equation with periodic scalar potential, an n-beam dynamical formula of high-energy diffraction by a crystal is developed by the matrix representation.

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Traditional direct methods are combined with anomalous scattering techniques for their potential applications to mediun/large molecules.

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A new classification of coincidence lattices is derived from the factorization of transition matrices into a product of shear transformations.

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As the effects of chemical bonding are small compared to the dynamic range of the total density, the two-channel maximum-entropy method is applied to the deformation density of α-glycine. The molecular dipole moment is reasonably well preserved in the reconstruction provided a non-uniform informative prior density is used.

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The Schwinger scattering of neutrons has been used to determine the sense of rotation of the helices of atoms in a crystal of tellurium. Its optical activity shows that the rotation of the plane of polarization of infrared radiation traversing the crystal parallel to the triad axis is in the same sense as the rotation of the helices.

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Theoretical structure factors Fq of rhombohedral magnesite (MgCO3) have been computed by quantum-mechanical periodic Hartree-Fock methods and compared with observed |Fo| (literature data) and conventional Fc (independent-spherical-atom model) values. Difference electron-density maps have been calculated on the most important sections.

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The low-resolution shape restoration from small-angle scattering data is shown to be unique if the number of shape parameters is 1 to 1.5 times the number of Shannon channels covered by the data set.

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A new set-up for magnetic X-ray diffraction is described and the form of the non-resonant magnetic X-ray scattering cross section is investigated for the model system MnF2.

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A new method for the microscopic investigation of magnetic materials, namely the non-resonant diffraction of high-energy X-rays (E > 80 keV), is discussed and experimental results for MnF2 are reported.

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The potential used to calculate electron diffraction intensities depends on the amplitudes of atomic vibrations. This paper provides a method for calculating this potential.

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Debye-Waller factors are given for 66 elemental crystals and absorptive scattering factors are given for 44 elemental crystals over the temperature range 1 to 1000 K.

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A slab method is developed for dynamical RHEED calculations from the surface of a semi-infinite crystal.

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A Patterson correlation function has been used to define a locked translation function and to refine rotational parameters of a search atomic model.

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The Sayre equation is evaluated as a technique for phase extension or ab initio phase determination, using electron crystallographic data from a bacterial porin.

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When single-wavelength anomalous acattering (SAS) diffraction intensities are available, the phase problem may be formulated as one in global optimization. This formulation permits a substantial strengthening of the traditional techniques of direct methods, particularly in the case that data are available at low resolution only.

short communications


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Contrary to statements in the recent literature, there is no evidence for strong anisotropy in the Debye–Waller factor for silicon at room temperature.

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White and monochromatic X-ray topographic methods are used to image defects in protein crystals.

international union of crystallography



books received


Acta Cryst. (1996). A52, 508
doi: 10.1107/S0108767396099941
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