Crystal Imperfections

Although we speak of an ideal crystal as consisting of an extended array of atoms with perfectly regular periodicity in all directions, such perfection is rarely found in the internal structure of real crystals. This is not surprising considering the enormous magnitude of activity that occurs on an atomic scale on the surface of a crystal during growth. Thus, even for a very small growth rate, such as 1 mm per day, about one hundred layers of atoms are deposited on the surface per second. On an atomic scale, the presence of imperfections or defects is thus very common. Moreover, it may be shown by thermodynamic reasoning that crystal defects are to be expected at all temperatures above absolute zero, although the fraction of defects at ordinary temperatures may be negligibly small. Defects in crystals may be of several forms—point defects, line defects and planar defects. Discussion of these defects will occupy most of this chapter.