Electron Diffraction from Small Crystals

Abstract

Small crystals of silver and silver bromide were studied by electron diffraction. Small crystals of these materials are of interest because published experimental and theoretical considerations indicate changes in their structure which may occur. Since the diffraction theory which is applicable to large crystals cannot always be used in interpreting the photographs from materials of small size, computations were made for crystal structures and shapes which might be expected for silver and silver bromide. As a result of the computations, it was found that positions of diffraction peaks of substances having the sodium chloride structure depend on the crystal shape. It was also shown that the determination of particle size by the usual method of the Scherrer equation may give values which are much too small. Comparison of experimental and theoretical diffraction effects indicates that small silver particles have the same structure as the large particles, but that there is a real contraction of the lattice in the smaller particles which amounts to 2.7 percent for particles with a diameter of about 31A. The small silver bromide particles have the same structure as the large particles, and an apparent expansion of the silver bromide lattice of about 1.0 percent appears to be best explained not as an expansion but as a particle-shape effect which changes the position of the diffraction peak. The crystals appear to be plates bounded by (111) faces, just as in many photographic emulsions, although this is not the usual form of crystals having this structure.