Lets DEFECTS in CRYSTALS

 Hello readers,

In order to examine a crystal, the crystal lattice must be taken into consideration. It is the systematic 3-dimensional arrangement of atoms in space such that the crystal attains perfect stability. But these crystal lattice contain some irregularities in the rearrangement of atoms known as defects. Defects are of two types, point defects and line defects.

Point defects:

The deviations of the arrangement of an atom from its lattice site results in Point defects. They are of three types: Stoichiometric defects, Impurity defects and Non-stoichiometric defects.

1. Stoichiometric defects:

Stoichiometric defects do not disturb the stoichiometry of the compound. They are also known as intrinsic or thermodynamic defects. Ionic and non-Ionic solids show different kind of Stoichiometric defects.

Defects in non-ionic solids:

(i) Vacancy defects:

As the name suggests this defect is due to some vacant lattice sites. It results in decrease of density of substance due to decrease in mass of substance. It is created when the substance is heated.

(ii) Interstitial defects:

When constituent particles occupy interstitial sites of the crystal then Interstitial defect is created. It does not affect the density of substance.

Defects in Ionic solids:

(iii) Frenkel defects:

When the smaller ions (usually cation) occupy interstitial site, they result in formation of vacancy at its original site. This type of defects does not change the density of substance. Examples, AgBr, ZnS and AgCl.

(iv) Schottky defects:

Here, ionic neutrality is maintained as equal number of cations and anions are missing from lattice site. It results in decrease in density of system due to loss of atoms from crystal site. Example, AgBr, CsCl and NaCl.

2. Non-stoichiometric defects:

Non-stoichiometric defects disturb the stoichiometry of the crystalline substance. They are classified into two types, metal excess defects and metal deficiency defects.

(i) Metal excess defects:

These defects are created due to the anion vacancies or due to the presence of extra cations in the interstitial sites. Alkyl halide like NaCl or KCl shows metal excess defect due to the anion vacancies.

(ii) Metal deficiency defect:

Crystals which show this type of defect contain less amount of metal as compared to stoichiometry proportions. For example, FeO which is found as Fe0.95O.

3. Impurity defects:

An example of Impurity defect is: NaCl crystal which contain SrCl2 as impurity.

Since, some sites of Na+ is occupied by Sr2+ ions. Since, each strontium ions replaces two sodium ions form its lattice site. Therefore, equal number of cationic vacancies are created as the number of strontium ions are present in lattice.

Explanation:

Why the violet color?

When KCl is heated in the atmosphere of potassium vapors, the potassium atoms get attached to the surface of the crystal system. Chloride ions diffuse to the surface from their lattice sites creating anion vacancies in the lattice and combine with potassium atoms to give KCl.

This occurs due to the loss of electrons from potassium atoms to form K+ ions. These electrons diffuse into the lattice and occupy anionic vacancies. These anionic sites or vacancies now contain unpaired electrons and are known as F-centers. These unpaired electrons absorb energy, get excited and when they come back to the ground state, they release energy in the form of light making KCl crystal violet.