Abstract
The forbidden reflection intensity from a crystal containing the two-fold screw axis or glide plane depends strongly on the incident beam direction, and this influences the contrast of crystal structure image. Taking cementite (Fe3C, orthorhombic, Pbnm) as an example, the calculation of forbidden reflection intensity and the image simulation have been made for 200 kV electrons incident along various directions around [\bar100] of crystal plates with thicknesses of 13.6 and 22.6 nm. When the tilt angle from [\bar100] becomes more than 0.05° (8.7×10−4 rad), the forbidden reflection intensity I(00l) (l: odd) becomes appreciable, and with increasing tilt angle it reaches a value comparable with the normal reflection intensity. The forbidden reflection intensity introduces a contrast modulation with the lattice period to the structure image in the direction of the two-fold axis. These theoretical results explain observed results satisfactorily.
