Twinning and the Tabular Growth of Silver Bromide Crystals

Abstract

SILVER bromide has the sodium chloride-type structure, so that a perfect crystal, growing in a uniform environment, would not be expected to show any tendency to unsymmetrical growth. The fact that dispersions of silver bromide in gelatin, for example in photographic emulsions, can be obtained which do consist of regular polyhedra (Fig. 1) shows that a uniform environment for growth can be obtained. In the same environment during growth, but with different conditions of nucleation, however, dispersions can be obtained in which the crystals are tabular, the tabular faces having triangular or hexagonal shapes (Fig. 2). It can be shown that under identical environmental conditions, the uniform growth in three dimensions of the regular polyhedral crystals proceeds much more slowly than the ‘outward’ growth of the tabular crystals, that is, the tabular growth is due to enhanced growth parallel to the tabular faces rather than to repression of growth on these faces.