Measurement of diffuse X-ray scattering between reciprocal-lattice points as a new experimental method in determining interstitial structures

For interstitials the scattering cross section between the Bragg reflections is very sensitive to the position of the defect atom and the displacements of the neighbouring lattice atoms. In contrast to this the scattering near the Bragg reflections (Huang scattering) is mainly governed by the displacement field at large distances from the defect and gives information about defect symmetry and strength [Trinkaus, H. (1972). Phys. Stat. Sol. (b) 51, 307-319]. For determining structures of point defects a measurement of scattering far from reciprocal-lattice points is therefore the best suited and most direct method. X-ray measurements of point-defect scattering in this region far from reciprocal-lattice points are difficult. With conventional X-ray set-ups it is not possible to separate the defect scattering from the thermal diffuse and Compton background scattering which is up to two orders of magnitude larger in the case of typical defect concentrations of some 10^-4. Such measurements, however, become possible if large X-ray sources of high luminous density are used in connexion with multidetector arrangements. As an example an experimental arrangement [Haubold, H.-G. & Schilling, W. (1975). To be published; Haubold, H.-G. (1974). Rep. Kernforschungsanlage Jülich, JUL-1090-FF] is reviewed, in which the scattering cross-section of self-interstitials in aluminium was measured in samples containing an atomic density of interstitials up to 5 × 10^-4.