. Imperfection in solids: According to the law of nature nothing is perfect, and so crystals need not be perfect. They always found to have some defects in the arrangement of their constituent particles. These defects affect the physical and chemical properties of the solid and also play an important role in various processes.
For example, a process called doping leads to a crystal r r 2r B C XII U6 Solid State - XII U6 Solid State - - - - - imperfection and it increases the electrical conductivity of a semiconductor material such as silicon. The ability of ferromagnetic material such as iron, nickel etc., to be magnetized and demagnetized depends on the presence of imperfections. Crystal defects are classified as follows ) Point defects ) Line defects ) Interstitial defects ) Volume defects In this portion, we concentrate on point defects, more specifically in ionic solids. Point defects are further classified as follows Point defects stiochiometric defects non- stiochiometric defects impurity defects Schottky defect Frenkel defect metal excess defect metal deficiency defec Stoichiometric defects in ionic solid: This defect is also called intrinsic (or) thermodynamic defect.
In stoichiometric ionic crystals, a vacancy of one ion must always be associated with either by the absence of another oppositely charged ion (or) the presence of same charged ion in the interstitial position so as to maintain the electrical neutrality. . . Schottky defect: Schottky defect arises due to the missing of equal number of cations and anions from the crystal lattice.
This effect does not change the stoichiometry of the crystal. Ionic solids in which the cation and anion are of almost of similar size show Schottky defect. Example: NaCl. Presence of large number of Schottky defects in a crystal, lowers Schottky Defect Na + Cl XII U6 Solid State - XII U6 Solid State - - - - - its density.
For example, the theoretical density of vanadium monoxide (VO) calculated using the edge length of the unit cell is . g cm - , but the actual experimental