EFFECT OF SPECIMEN THICKNESS ON SYMMETRY DETERMINATIONS BY CONVERGENT-BEAM ELECTRON-DIFFRACTION

The application of convergent-beam electron diffraction to the study of local symmetry is becoming increasingly popular in the materials science community. One of the most important tests for determining the point and/or space group of a material is the plus or minus g experiment which is used to test for a center of symmetry. This study shows that thin specimens may appear to lack centrosymmetry due only to their limited thickness along the beam direction, rather than to the actual space group of the material. Conversely, zone axis convergent-beam patterns from thin specimens may display a symmetry which is higher than the acutal symmetry of the material due to a loss of high-order Laue zone lines in the patterns under weak dynamical scattering conditions.