Frenkel Defects and Schottky defect certain defects can be introduced in a crystal by adding an impurity to the material. Introduction of defects by incorporating small amount of an impurity in the host crystal is termed doping. This defect arises when a action from its regular site in ionic crystal lattice is replaced by different actions. If the impurity cation is substituted in the place of regular cation, then it is called as frenkel defect.The impurity containing crystal is called doped crystal. The impurity atom or ion can occupy an interstitial or a lattice point in the crystal. The doped crystal may be regarded as a solid solution: Substitutional solid solution, or interstitial solid solution.
1} for substitution type solid solutions, the electronic structure of the impurity is very important parameter.
2} for interstitial solid solutions, the size of the impurity atom is the most important parameter.
The introduction of the defects by doping process is of great interest to the electronic industry. Transistors are made of doped Si. Alloys are formed by substitution defects. E.g. Brass, in which Cu metal is substituted by zinc metal in the ratio of 3:1 at regular Cu sites. Stainless steel involves an interstitial impurity defect by carbon atoms.
The interstitial impurity defect changes the properties of the original crystalline solid. If the impurity of cation is present in the interstitial positions then it is called interstitial impurity defect. If the impurity cation is substituted in the place of regular cation, then it is called stoichiometry impurity defect.
Non – Stoichiometric Defects:
The compound which contains the combining elements in a ratio different from that required by their stoichiometric formulae, are called non – stoichiometric compounds. Generally non – stoichiometric defects are of two types. The first type of this non – stoichiometric defect is Metal – excess defect and another type is Metal – deficiency defect.