Random elastic deformation (RED) - an alternative model for extinction treatment in real crystals

A modification of the Hamilton-Zachariasen theory of extinction in imperfect crystals is reported. Unlike the generally adopted Darwin mosaic model the crystal is supposed to consist of elastically deformed domains so that individual reflection events can be treated within the quasiclassical approach developed by the author in an earlier study [Acta Cryst. (1984), A40, 120-126]. In this way a modified expression for the scattering cross section, taking into account multiple wave interference, is introduced into the energy transport equations. Under the assumption of a constant magnitude of the strain gradient and of spherical isotropy of its orientation distribution, generally valid results formally compatible with the standard solution of Becker & Coppens [Acta Cryst. (1974), A30, 129-147] are obtained. A significant achievement is the physically correct description of the primary extinction effect.