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ISSN: 2053-2733

July 1999 issue

Highlighted illustration

Cover illustration: One quadrant of each of four fibre diffraction patterns from disordered polynucleotide fibres. The patterns were calculated for fibres with disorder as follows. Upper left: crystalline (no disorder); upper right: noncrystalline (completely disordered); lower left: discrete rotational disorder and correlated lattice disorder; lower right: random screw disorder. See Stroud & Millane [Acta Cryst. (1996), A52, 812-829].

research papers


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The crystallographic phase problem is recast in terms of feasible sets, facilitating new technique development and improved mathematical understanding.

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Different formulae of the Darwin theory in the two-beam case are obtained by a uniform view of basic properties of tridiagonal matrices, their determinants (continuants) and their close relationship to continued fractions and difference equations.

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P1 and space-group-specific methods of ab initio crystal structure prediction are compared, especially with consideration of restrictions introduced by the number of molecules per cell.

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This study has shown that the polysomatic model can be efficiently applied to the structural analysis of complex cuprates. The structure of ladder-type cuprates was analysed in more detail.

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The spectral decomposition of the compliance fourth-rank tensor related to crystalline or other anisotropic media belonging to the monoclinic crystal system is described.

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Modified form factors, to be used in the calculation of the structure factors F(hkl), are calculated from multiconfiguration relativistic Dirac–Fock wave functions. Tables are presented for neutral elements in the range Z = 70–100.

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Off-axis electron holography has been used to determine that the mean inner potential of germanium is 14.3 (2) V using cleaved 90° wedge samples.

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A revised procedure to compute (a program is available) the periodic intensity distribution (PID) function of mica polytypes is given and results are presented in the most suitable axial settings. Symbolism to describe mica polytypes is unified.

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A dynamical calculation scheme that employs Cartesian coordinates with a z axis normal to the crystal surface to define polarization unit vectors and wavefields is applied to interpret the intensity distribution of crystal truncation rods for surfaces and interfaces.

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A model for analysing the phase sensitivity of the reflection coefficients of a diffracted wave in the case of three-wave X-ray diffraction is proposed. This model considers three-wave diffraction as the interference of the directly excited and the Umweg-excited diffracted waves.

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Equations derived from a statistical model for large structures give matrix elements as functions of the lengths and directions of Patterson vectors and the distribution of weights without summation over the individual reflections.

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A method is described whereby the normal matrix of least-squares refinement can be calculated using the fast Fourier transformation to replace all loops over the structure factors.

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Three Nd1−xCaxMnO3 (0.3 ≤ x ≤ 0.5) compounds have been studied at room temperature as well as below the charge ordering temperature by electron microscopy and neutron diffraction.

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In an X-ray anomalous-scattering experiment using powder samples, the phases of structure factors cannot be uniquely determined when the crystal is noncentrosymmetric. A new constraint type for this case has been built into the maximum-entropy program MEED, so that now all types of information obtained from such an experiment can be used simultaneously in a MEM calculation of the electron density.

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The experimental electrostatic potential is used as an observable property to derive molecular fragment electric moments. The method based on analytical transformations of the real spherical harmonics is presented and applied to a sample of six water molecules.

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A generalized likelihood-based search can be used for determining the position of a macromolecule in the cell. In some cases, spurious maxima are eliminated and the signal-to-noise ratio is enhanced in comparison with the straightforward R-factor-based search.

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Quasiperiodic packing of fibres with global icosahedral symmetry are studied and described using a cut and project method.

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A least-squares procedure is described that can in principle resolve the SAS phase ambiguity provided that unbiased ω(h, k) estimates are available. SAS tangent formula methods may be used to either validate or reject the correctness of the anomalously scattering heavy-atom sites.

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Crystallographic symbols for use in arbitrary dimensions are considered and recommendations presented together with symbols for orthogonal transformations, geometrical crystal classes and standard settings of lattices. Standard metric tensors recommended for four-, five- and six-dimensional crystal families are given in full.
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