Defects in Ionic Crystals

In a crystalline solid the constituent particles are arranged in a ordered three dimensional network. However, in actual crystals such a perfect order is not there. Every crystal has some deviations from the perfect order. These deviations are called imperfections or defects.

These defects can be broadly grouped into two types: stoichiometric and non-stoichiometric defects, depending on whether or not these disturb the stoichiometry of the crystalline material.

Stoichiometric Defects

In such compounds, the number of positive and negative ions are in stoichiometric proportions. There are two kinds of stoichiometric defects:

1. Schottky Defects
2. Frenkel Defects

These defects cause the crystal to conduct electricity to some extent. The conduction is due to the movement of ions into the holes. When an ion moves into a hole it creates a new hole, which in turn is occupied by another ion, and the process continues.

Schottky Defects

This type of defect is due to the absence of some positive and negative ions from their positions. These unoccupied lattice sites are called holes.

Such defects are found in ionic compounds in which the positive and negative ions are of similar size. for example, NaCl and CsCl. The number of missing positive and negative ions is equal. The presence of Schottky defects decreases the density of the crystal.

Frenkel Defects

This type of defect arises when some ions move from their lattice positions and occupy interstitial sites. The interstitial sites refer to the positions in between the ions. When the ion leaves its lattice site a hole is created there. ZnS and AgBr are examples of ionic compounds showing Frenkel defects.

In these ionic compounds the positive and negative ions are of quite different sizes. Generally, the positive ions leave their lattice positions, as these are smaller and can accommodate themselves in the interstitial sites. The Frenkel defects do not change the density of the solids.