A systematic method to identify the space group from PED and CBED patterns part I--theory.

This systematic method allows the unambiguous identification of the extinction and diffraction symbols of a crystal by comparison of a few experimental Precession Electron Diffraction (PED) patterns with theoretical patterns drawn for all the extinction and diffraction symbols. The method requires the detection of the Laue class, of the kinematically forbidden reflections and of the shift and periodicity differences between the reflections located in the First-Order Laue Zone (FOLZ) with respect to the ones located in the Zero-Order Laue Zone (ZOLZ). The actual space group can be selected, among the possible space groups connected with each extinction symbol or diffraction symbol, from the identification of the point group. This point group is available from observation of the 2D symmetry of the ZOLZ on Convergent-Beam Electron Diffraction (CBED) patterns.