Extension of the Howie‐Whelan Equations for Electron Diffraction to Non‐Centro Symmetrical Crystals

The Howie‐Whelan system of equations [2] is extended to non‐centro symmetrical crystals. This system of equations, which is valid for the Laue case, describes the scattering of electrons by a perfect crystal in the two‐beam approximation and takes anomalous absorption into account. The theory predicts that Friedel's law no longer holds for the dark field image but is still valid in the bright field. Observations on 180° walls in barium titanate support the theory. In addition the theory shows that 180° walls in inclined planes should produce α‐type fringes. This phenomenon is also observed experimentally.