An ideal crystal is one which has the same unit cell containing the same lattice points throughout the whole of the crystal. At absolute zero (0 K) most of the ionic crystals are ideal but with rise in temperature, there is a chance of distortion. The defect may appear at a point, along a line or over a surface.
Generally imperfections are of two types :
1. Electronic Imperfections : In a pure covalent crystals (e.g., Si) or ionic crystals (e.g., NaCl) at absolute zero (0 K) electrons are present in lowest energy states. But, at higher temperatures, of the electrons may occupy higher energy states depending upon the temperature. For example, in crystals of pure silicon, some electrons are released from covalent bonds at temperature above 0 K. The bonds from which electrons have been removed become electron deficient and these are called holes. These electrons become mobile and are responsible for electrical conductivity. This is known as intrinsic condition.
The holes in an electric field move in opposite direction to that in which electron move. The electrons and holes in solids give rise to electronic imperfections.
2. Atomic or Point Imperfections -The defects or imperfections arise due to the irregularity in the arrangement of atoms or ions are known as point defects or atomic imperfections. These are produced when an equal number of cations and anions are missing from the regular lattice positions and thus an equal number of cations and anionic vacancies are produced or when an ion of an ionic crystal is missing and is shifted to a vacant interstitial site existing in the structure of the crystal.
Atomic or point defects may occur in stoichiometric as well as in non-stoichiometric crystals. The point defects found in stoichiometric crystals are called stoichiometric defects while the point defects found in non-stoichiometric crystals are known as non-stoichiometric defects.