Crystal, defect, point etching

Optical microscopic examinations of the reactant at room temperature can provide information on the shapes and sizes of the crystallites and structural information from features of observed symmetry. The degree of perfection of the crystallites can be assessed and damage, major defects and inclusions may be identified. Surface defects, such as the points of emergence of dislocations, may be revealed by etching the sample surface with a suitable solvent. Cleaving of a crystal gives two closely related faces and one section may be etched to reveal the location of defects while the other section is partially decomposed and then re-examined [30,31 ] to enable the distributions of the different surface features (usually dislocations and nuclei) to be compared. Such techniques have been used to investigate the role of defects in the initiation of decomposition [32,33]. 

The grov h and dissolution of crystals are opposing processes. It follows that an inhibitor for grov h of a particular crystal plane can also influence the dissolution rate of the same plane. This increases the contrast between the protected planes and the exit points of defect lines, since there only a protected crystal plane is involved. In this way, etch craters are formed with whose aid one can count the defect lines (see also Fig. 4.8). 

In Sec. 2 the overall surface dissolution was considered. Except in the case of surface diffusion, we assumed that a dissolving surface is free from defects. Since real crystals usually contain dislocations and other defects, it Is necessary to know their effect on dissolution rates and the mechanism of formation of etch pits at their emergence points. 

---