Interbranch transient beating of X-ray intensities in deformed crystals.
X-ray dynamical diffraction in a deformed crystal is studied using the interbranch resonance concept. It is shown that appreciable beating of the X-ray intensities may be induced by a lattice distortion that produces interbranch transformations of the local dispersion surface. In X-ray plane-wave topography, this effect may be observed as interference fringes arising around the kinematical image of a defect. It is predicted that such interbranch fringes can be induced by edge dislocations.
Structure of X‐ray wave packets in perfect and highly distorted crystals
It is shown that, in order to treat the problem of the propagation of an X-ray wave in a distorted crystal, the plane-wave assumption, which is one of the fundamental ingredients in the usual dynamical theory, should be removed. Both the incident and the crystal waves should be built as wave packets, i.e. continuous distributions of k vectors, characterized by their extensions in both reciprocal and direct spaces. The characteristic structure of the crystal wave packet for a perfect crystal, and the changes undergone by this structure as a result of the crystal distortions, are examined; the criterion for the validity of geometrical optics is thus reformulated.
Study of the New Wave Fields Appearing in Crystals Deformed by Large Thermal Gradients
The criterion for the validity of geometrical optics is quantitatively examined through a 'computer experiment' in the specific case of a crystal containing substantial deformations induced by a large thermal gradient. It is shown that the above-mentioned criterion is numerically correct. For larger deformations than those corresponding to the criterion the computed intensity shows that two wave fields appear where only one would normally have been expected, in good agreement with theoretical predictions. The intensity of the new wave field has been plotted as a function of deformation. I. Introduction The Ewald-Laue dynamical treatment of X-ray propagation in a triply periodic medium provides a complete understanding of how an X-ray wave propagates in a perfect crystal. The main concern of people working in the field at present is to give a correct account of the propagation of X-ray waves in crystals exhibiting defects of periodicity. For small deformations (slightly curved reflecting planes, for instance) different authors have shown that the Ewald-Laue theory can be extended (Penning & Polder, 1961; Kato, 1963); it is still possible to define ray trajectories for the energy propagation, the only difference from the perfect crystal case being that these trajectories are curved - they 'adjust' themselves to the curvature of the reflecting planes just as ordinary light rays 'adjust' themselves when they encounter a variation of index of refraction in the so-called geometrical optics treatment. There is experimental evidence that this extension of the theory for small deformations no longer holds in the case of large deformations.