Role of upper-layer interactions in electron diffraction symmetries (reply)

DR STEEDS REPLIES: Dr Goodman1 casts doubt on the bright field rotation method2 of identifying the presence of faults parallel to the surface of a specimen. The examples quoted in support of his point of view are, however, inadequate. Whereas our experiments are often performed over rotations of 20° or more, in the convergent beam work in gold, cited by Dr. Goodman, the angular range in view is 1.29° at 4.21° from [111], which is much too limited for a test. During large rotations the systematics-only pattern is interrupted by various non-systematic reflections, but there are several ranges of orientation in which non-systematics are not important and the asymmetric pattern returns to essentially the same form. The graphite example is irrelevant because the field of view is restricted to a narrow angular range close to the [0001] zone axis where the pattern is dominated by rapidly varying cross-grating contributions; our ‘rule’ refers to contours which result essentially from one-dimensional diffraction. Clearly, the misunderstanding arises from the very great difference of angular view in convergent beam and bend contour patterns, despite their formal equivalence (to a first approximation). Effects of inclination, although important for really large tilts, such as the 54° from the normal of the foil used by Dr Goodman, are not in our experience significant for smaller rotations of the order which we propose in our experimental test, at least not for bend contour patterns, where a certain amount of distortion is almost inevitable.