Imperfections in solids
A perfectly ordered crystal can exist only at absolute zero. Above this temperature some departure from ordered arrangement occurs. These are called imperfection or defects.Crystals contain a group of micro crystals with irregularities at crystals boundaries or grain boundaries. These are called lattice defects. The irregularities may also be present at several lattice points. They are called points defects. Point defects These are defects localized at lattice points or at voids. Thus 1 cm3 of a crystal of NaCl with 10^22 Na+ and 10^22Cl- ions has 10^6 vacant Na+ lattice positions at room temperature. Metallic crystals also contain such vacant lattice positions due to missing of metal atom in the lattice. The lattice positions from which lattice atoms or ions are missing are called vacancies. Thus ionic crystals have cation and anion vacancies. Point defects can also arise due to various other factors and it is convenient to classify them as follows. 1. Stoichiometric defects 2. N...