An optical method of studying the diffraction from imperfect crystals I. Modulated structures

Abstract

The correspondence between the X-ray diffraction pattern of a crystal and the optical diffraction pattern of a two-dimensional grating has been used to determine the nature of the diffraction from imperfect crystals. A two-dimensional grating representing the structure of the imperfect crystal is prepared on a very fine-grained photographic plate by a technique which gives an error of less than 1 $\mu $ in the positions of the elements of the grating. The grating is placed in a bath of cedar-wood oil between optically flat glass plates, and its Fraunhofer diffraction pattern is observed in a modified Lipson diffractometer. Illustrations are given of the application of the optical method to the study of the diffraction from modulated structures, such as the alloy Cu$_{4}$FeNi$_{3}$ with a periodic variation of lattice parameter, and the age-hardening aluminium-copper alloy with a variation of both lattice parameter and structure amplitude.