The space-group determination of GaS and Cu₃As₂S₃I by convergent-beam electron diffraction

Space-group determinations were carried out on GaS and Cu₃As₂S₃I single crystals using convergent-beam electron diffraction data. The space groups found were P6₃/mmc and Cmcm, respectively. The effectiveness of the specific test for centrosymmetry was examined. In both structures the test was made clearer by use of the inclined-axis technique, in which the incident beam is at an appreciable angle to the principal zone axis. It was concluded that the test was superior to optical methods in structures such as GaS which have a high dislocation density.