journal menu
issue contents
March 1998 issue
Cover illustration: Power spectrum of a high-resolution electron-microscopy image of NiAl <110>. The image calculation includes a realistic phonon spectrum by coupling the `molecular dynamics' and `multislice' simulation techniques. The diffuse background, satellite peaks and bands mirror the effect of thermal diffuse scattering and correlated atom vibrations. Courtesy of G. Möbus, T. Gemming and P. Gumbsch.
research papers
Energy-filtered convergent-beam electron diffraction is applied to measure the temperature factors of the intermetallic phase NiAl with high accuracy. A detailed error analysis is given, stochastic and systematic errors are discussed and quantified.
Equations derived from tensors of third and fourth rank describe how f' and f'' depend on polarization, azimuth and Bragg angle for atoms with high symmetry.
By employing high-resolution X-ray diffraction techniques, crystalline perfection has been evaluated and point defect clusters in type Ia natural diamonds have been characterized. Defect parameters have been correlated with nitrogen content.
The Takagi–Taupin equations are written in a representation using angular coordinates. By applying the boundary-value Green-function technique in the solution of the equations, analytical results are obtained for the primary extinction factors.
New analytical results for the absorption factor and the weighted path length are obtained using a surface integration technique.
A quantitative model of both primary extinction and absorption based on the Becker–Coppens procedure for solving the dynamical Takagi–Taupin equations is proposed that results in improved agreement between theory and X-ray diffraction experiments for Si and Ge single spherical crystals.
A rounding operation allows the conversion of point amassments in multilattices into single points of perfect quasicrystalline structures.
The internal and external X-ray standing waves (SW) formed by dynamical diffraction are theoretically discused in the case of weakly distorted crystals. Care is required in the application of SW methods because the location of SW is very sensitive to lattice distortions compared with rocking curves.
Using the statistical approach to dynamical X-ray diffraction, the equations for coherent and diffuse scattered waves in a general case of a deformed crystal are obtained.
A new method to obtain the scale factor to transform powder intensities into electron units is proposed.
In step-scan diffraction measurements, the diffraction angle 2θ is an observation with standard uncertainty u(2θ). Theoretical and experimental investigation of the influence of u(2θ) on the uncertainty of the intensity data shows that u(2θ) usually contributes a multiple of Poisson statistics to the standard uncertainty of the intensity.
There is no fundamental reason why the basic techniques of X-ray crystal structure analysis cannot be applied to unrepeated and very large (i.e. teradalton) structures. The expected properties and current status of the technique involved are described.
General expressions are derived that describe the positions of reflections (the cylindrically projected reciprocal lattice) on a diffraction pattern from an oriented polycrystalline fiber for a triclinic unit cell with any orientation to the fiber axis. Calculations illustrate the characteristics of the projected reciprocal lattice.
short communications
Spatial auto- and cross-correlation functions of quantities distributed radially over spheres of different radii are presented in analytical form. In terms of its application to small-angle (neutron and X-ray) scattering, the distance distribution function is calculated for two-shell ionic micelles and for a spherical Gaussian contrast distribution.
The re-interpretation by Hu (1997) of the 1975 data of Mathieson is shown to be fallacious by reference to the subsequent experimental measurements by Mathieson in 1977.
There is an obvious difference between the behaviour of the dependence of the integrated reflection power ratio on the asymmetric parameter for plane crystals predicted by the H–D equations on mosaic crystals and that predicted by the theory of Hirsch & Ramachandran on perfect crystals. To get rid of the surface layer effect of the crystal sample is most important for the verification of such a difference by experiment.
The Debye–Waller factors of NaBr and NaI have been determined from X-ray intensities.
Reflections with phase angle either 0 or π in 157 noncentrosymmetric space groups are tabulated.
