The Plastic Deformation and Crystal Orientation of Thallium Halides*

Abstract

The plastic deformation of cubic thallium halide crystals produced under pressure of a cone perpendicular to the crystal surface, shows two kinds of very sharply limited, raised patterns, namely, the “superficial” and the “transmitted.” In both of these patterns, the shape, symmetry, and position depend on the lattice orientation of the crystal with respect to the observation surface. The first appears around the press point in the form of radial wings. The second develops on the opposite side of the press point and has a square or a rhombic shape. Characteristics of the patterns are explained by the localized distortion of the single crystal around the press point into an aggregate of small crystals. The glide planes (T) of the small crystals lie in the dodecahedron faces (110), and the glide directions (t), in the cubic faces (001). The superficial patterns can be applied to determine the crystal orientation within a few degrees. Inner strains which are not permanent develop symmetrically around the press point. Under pressure of a cone, sodium chloride crystals show splitting along the dodecahedron faces (110), while under the same conditions, silver chloride crystals give only diffuse patterns.