A TEM study of non-parallel twins inducing thickness growth in silver chloride {111} tabular crystals

Defects in AgCl tabular grains with {111} surfaces are characterized by tra nsmission electron microscopy (TEM) and their influence on the growth proce ss is discussed. In the tabular crystals, twins parallel to the tabular fac e as well as dislocations along different directions are observed. The twin s induce the tabular growth, while the dislocations do not influence the mo rphology. In 10 to 30% of the crystals that have been characterized, thickn ess growth is observed and it is shown that in all cases twins on other pla nes than the tabular ones are present. Two configurations occur more freque ntly and are analyzed in detail. For the first group, twins parallel to the tabular face as well as a microtwin along a non-parallel {111} plane that ends inside the crystal are present. In the crystals of the second group, o nly one extra non-parallel twin occurs giving rise to a bicrystal built up by a tetrahedral shaped part and a flat triangular or trapezoidal part. Mor e complex twin configurations give rise to various, less characteristic mor phologies. From these observations, it appears that the thickness growth is related to the presence of the twins on planes other than on the tabular o nes.