issue contents

ISSN: 2053-2733

January 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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Nonsymmetrical diffraction in a perfect rectangular t × l crystal is analysed using the Takagi–Taupin equations. Analytical and numerical results are presented for the extinction and absorption factors..

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Darwin theory is used for computer simulations of diffraction from perfect Si and Ge crystals with thermal vibrations. Dynamical theory with a Debye–Waller factor fits most results, but deviations at high temperature call into question the usual quasi-ergodic hypothesis.

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It is shown that the additional ATS main reflections and ATS satellites can appear in the diffraction pattern of an incommensurately modulated crystal when the wavelength of the incident X-radiation is close to the absorption edge.

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Model X-ray data are computed from a superposition of ab initio molecular electron densities in the crystal, as well as from periodic crystal Hartree–Fock electron densities, for several hydrogen-bonded systems and the weakly bound acetylene. Interaction effects on the electron density are illustrated and retrieved quantitatively by suitable multipole models. Multipole refinement reveals enhancement of the molecular dipole moment for hydrogen-bonded crystals but negligible change in molecular second moments. Electric field gradients at H nuclei are also discussed.

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Particular quasiperiodic structures with non-crystallographic symmetry exhibit discrete periodic average structures. Examples for one-, two- and three-dimensional quasiperiodic structures are discussed.

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A new procedure for the derivation of minimal surfaces with self-intersections along straight lines is introduced. The deduction of several properties of each such surface from its generating polygon is described.

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From the analysis of the elastic neutron scattering data by Dolino et al. [J. Phys. (Paris) (1984), 45, 361–371]>, it is shown that in the incommensurate (IC) phase of quartz there exists a strong component of the acoustic displacements u_z of the modulation. The existence of a large u_z is not consistent with the currently accepted model and a new model is needed in order to understand the origin of the IC modulation in quartz.

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Using Monte Carlo simulations, the equilibrium and growth behaviour of crystal faces having more than one connected net are studied. The crystal habit is in such cases dependent on the supersaturation. Flat, rough and disordered flat phases are found.

books received

international union of crystallography

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