Top-bottom effects in double diffraction

A top-bottom effect in double diffraction has been observed in selected-area diffraction (SAD) patterns recorded from epitactic overgrowths. In SAD patterns recorded from epitactic α-Fe2O3 islands grown on an (0001) Al2O3 substrate, the double-diffraction reflections were found always to surround the primary reflections associated with the bottom crystal. This observation is explained using simple geometric constructions and the concepts of kinematical diffraction in a deformed crystal. The effect may be useful as a simple method for determining on which side of a TEM specimen a particular epitactic feature lies.