There are mainly two types of defects:-

Point defect:-When the deviation or irregularities exists from ideal arrangement around a point or an atom in a crystalline substance the defect is called the point defect.

Line defect:-When the deviation from the ideal arrangement exists in the entire row of lattice points the defect is called as line defect.

Types of the point defects:-

1. Stoichiometric defects

2. Non stoichiometric defects

3. Impurity defects

Stoichiometric defect:-

If imperfection in the crystal are such that the ratio between cation and onions remains same. Stoichiometry of substance do not disturbed defect is called stoichiometric defect these defects are of the following types.

1. Vacancy defect:-When is in a crystalline substance, some of the lattice sites are vacant the crystal is said to have vacancy defect it results in decrease of density of substance.

2. Interstitial defect:-

When some extra constituent particles are present in the interstitial site the crystal is said to be have interstitial defect.

This defect increases density of the crystal

These above types of defects are shown only by non – ionic solids.

.Schottky defect:-

If in an ionic crystal of the type A+B- equal number of cations and anions are missing from the lattice site. So that electrical neutrality is remained is called schottky defect.Compounds exhibiting schottky defect are NaCl, KCl

Which compounds have small difference in size of cation and anions show defect.

Frenkel defect:-If an ion is missing, from its lattice site and is occupies the interstitial site, electrical neutrality as well as Stoichiometry of the compound are maintained this type of defect is called frenkel defect. It is also called dislocation defect.

Example:-(AgCl, AgBr, AgI, ZnS) shows this defect which have a large difference in size of cations and anions.

Non stoichiometric defects:-

If a result of the imperfactions in the crystal the ratio, of the cations and anions becomes difference from that indicated by ideal chemical formula. The defects are called non-stoichiometric defects.

They are of two types:-

i. Metal excess defects:-

A negative ion may be missing from its lattice site, leaving a hole which is occupied by an electron, there by maintain the electrical neutrality.

The interstitial sites containing the electron thus trapped in the anion vacancies are called the F – centers. They are responsible for imparting colour to the crystals.Example:-When NaCl is heated in an atmosphere of Na vapours. The excess of Na atoms deposition the surface of NaCl crystal Cl- ions then diffuse to the surface where the combine with Na+ ions which becomes due to losing electrons.

These electrons loses by Na atom are diffuse back into the crystal and occupy the vacant site created by Cl- ions and imparts Yellow colour to NaCl crystal

ii. By presence of extra cation in interstitial sites:-

• Metal excess may also be caused by an extra cation occupying the interstial site. For example when ZnO is heated it loses oxygen and turns yellow due to followingZnO → Zn+2 + (1/2)O2 + 2e-The excess interstitial sites and the electrons in neighbouring interstitial sites.

Metal deficiency defect:-This defect occurs when the metal shows the variable valancy. Due to metal deficiency the compounds obtained are non stoichiometric. For example it is difficult to prepare ferrous oxide with ideal composition because ferum exists as both Fe+2 and Fe+3 ions thus we obtain Fe0.95O or FexO withx = 0.93 to 0.96

Impurity defects:-

These defects are arises when foreign atoms are present at the lattice site in place of the host atom. Or it is present at the vacant interstitial site example n – type semi conductor, p – type semi conductor.