STEM and CTEM observations of interference between Laue- and Bragg-diffracted electrons in images of polyhedral crystals

Interference effects have been observed, using both TEM and high resolution STEM, at the edges of images of cubic MgO crystals oriented so that one [100] face is almost parallel to the electron beam. Electron waves entering or leaving this face at a glancing angle will be refracted through a small angle. They thus provide sets of waves, slightly inclined to both the directly transmitted waves and the waves diffracted in the crystal, giving sts of interference fringes in the out-of-focus images, parallel to the crystal edges. In the simplest cases, the fringe pattern is sinusoidal and may be simply related to the relative angle of deviation between the waves, and hence to the crystal inner potenial. In general, the phenomena are very sensitive to the degree of perfection and cleanliness of the crystal surfaces. They are especially so when a wave strikes a crystal face at a glancing angle from within the crystal and undergoes total internal reflection. (orig.)