ABSTRACT
An ideal crystal in which every lattice site is occupied by an atom, a molecule, or a group of atoms is a geometric concept. In real crystals, a variety of deviations from the ideal lattice structure occur. Any deviation from the perfect atomic arrangement in a crystal is considered an imperfection or a defect. It has now become clear that many important properties of solids are controlled as much by imperfections as by the nature of the host crystal, which may act only as a solvent or matrix for the imperfections. As we know, the electrical conductivity of doped semiconductors is entirely due to a trace amount of intentionally added impurities. Considerable deviations from the theoretical values of the measured intensities of x-rays diffracted from crystals occur due to the presence of defects. The colors of many crystals are due to imperfections. Luminescence in crystals is nearly always connected to the presence of impurities. Diffusion of atoms through solids may be accelerated enormously by impurities and imperfections, and nally the mechanical properties of solids are usually controlled by imperfections.
