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ISSN: 2053-2733

May 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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A model for the surface free energy is proposed for incommensurate intergrowth compounds. Facetting of this type of aperiodic crystals is thus explained, and it is predicted that main facets common to the subsystems will have the highest morphological importance.

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The properties of moiré fringes on X-ray diffraction topographs of bicrystals are derived from the dynamical theory of X-ray diffraction and discussed for experimentally relevant cases.

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A new method to analyse moiré fringes allows the relative displacement field between the two parts of bicrystals to be reconstructed with a strain sensitivity of 10−6 to 10−8.

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It is shown that the small-angle scattering of X-rays or neutrons by dislocations within a deformed metal, which are partially ordered into wall-like structures, is characterized by several factors. Principally, these are associated with: (i) a single dislocation or dipole; (ii) the dislocation configuration in the plane of the wall; and (iii) the distribution of dislocations across the wall thickness.

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A direct analyis of the hydrated catalase crystal structure at room temperature is consistent with earlier studies of frozen hydrated samples by electron microscopy.

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Numerical calculation of the angular and spectral distributions of the intensities of the specularly diffracted waves in the case of three-wave grazing-incidence X-ray diffraction is carried out using the dynamical theory. A method of determining the value of the triplet phase invariant for thin crystal surface layers is also developed.

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A theoretical analysis of the parametric X-radiation (PXR) produced by the nonrelativistic electrons in an X-ray tube with a monocrystalline anode is presented.

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The quantitative zone-axis CBED technique ZAPMATCH is applied to low-order structure-factor measurements in nickel and copper in an attempt to resolve the debate regarding the accuracy of various experimental methods and theory calculations that have previously been applied to these materials. The extension of ZAPMATCH to the study of thermal diffuse scattering effects is also considered.

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The possibility of using the ZAPMATCH quantitative CBED pattern-matching technique to measure Debye–Waller factors accurately is investigated. The application of this approach to nickel and copper 〈110〉 zone-axis patterns is discussed.

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The minimum value of the electron density in the unit cell is proposed as an objective function for phase determination. An algorithm for maximizing this objective function, based on linear programming, finds the correct phases in numerical experiments with simulated crystalline and quasicrystalline structures.

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It is shown that SANS patterns from specimens of different thicknesses but otherwise identical not only provide information on the shape and size of the scattering entity but also give absolute values of the scattering contrast between particles responsible for SANS and the matrix. No separate calibration of the incident beam is required.

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A mixing of lattices derived from a dodecahedral star and an icosahedral star is considered giving a 16 × 16 matrix for the cut-and-project method of generating 3D quasiperiodic patterns.

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Joint probability distribution functions of structure factors with rational indices are derived in P1. Phase relationships are obtained.

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Probabilistic formulae for estimating structure factors of half-integral index reflections are given and experimental tests are performed.

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The sharply peaked maxima of electron intensity formed at the exit face of a thin atomic focuser crystal may be used to derive the structure of a thin specimen crystal with a resolution of better than 0.04 nm.

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A method is proposed to calibrate all the relevant instrumental parameters of a multicircle diffractometer equipped with an area detector.

short communications


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The positive-definite conditions of the elastic constants of two-dimensional quasicrystals with non-crystallographic symmetries have been derived.

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The well known spherical-wave results are obtained by a simple and direct calculation.

Acta Cryst. (1999). A55, 564
doi: 10.1107/S0108767399003232

international union of crystallography


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