Ideally, a perfect crystal has the same unit cell and contains the same lattice points throughout the crystal. Imperfection or defect is generally used to describe any derivation of the ideally perfect crystal crystals from the periodic arrangement of its constitutions. Two types of defects are usually observed in crystals. 1. Point Defects 2. Line Defects
POINT DEFECTS
What are point defects?
If the deviation occurs due to missing atoms, displaced atoms or extra atoms, the imperfection is named a Point defect.
A defect can result from imperfect packing during the original crystalline or from thermal vibrations of atoms at elevated temperatures due to increased thermal energy. Individual atoms are likely to jump out of their positions of lowest energy.
The most common point defects are the Schottky defect and the Frankel defect. Less common point defects are metal excess and metal deficiency defects.
SCHOTTKY DEFECTS
What are Schottky defects?
These defects arise if some of the lattice points in a crystal are unoccupied. The unoccupied points are called lattice vacancies. As shown in the figure, the existence of two vacancies, one due to a missing Na+ ion and the other due to a missing Cl– ion in a crystal of NaCl. The crystal remains neutral due to missing positive and negative ions.
Schottky defects in crystal of NaCl
These defects mostly show in ionic crystals in which the positive and negative ions do not differ much in size. An excellent example of ionic crystals in which Schottky defects occur is Sodium chloride and cesium chloride.
Due to Schottky defects, the crystal is in a position o conduct electricity to a small extent by an ionic mechanism. As an electric field is applied, a nearby ion moves from its lattice site to occupy a vacancy. This creates a new vacancy, and another nearby ion moves into it, and so on.
This process continues resulting in the migration of the vacancy and, thereby, of the ion from one end to the other end of the crystal. In this way, electricity gets conducted across the whole of the crystal.
The existence of vacancies also enables easy movement of atoms or ions in crystal-changing places with one another. It is the phenomenon of diffusion in solids.
