Determination of space-groups using anomalous dispersion

Abstract

In the usual method of determining space-groups, the presence of face or body centring and of screw axes and glide planes is detected by examining the systematic absences of reflexions. However, it is not possible to establish by such a procedure the presence or absence of a centre of inversion, a rotation axis or a mirror plane. This is because the translation-free symmetry elements do not lead to any characteristic absences for the reflexions; also, the symmetry of the X-ray diffraction pattern of a crystal is not exactly the same as that of the point group of the crystal itself, but only the corresponding Laue group which has an additional centre of inversion added to it.