Stoichiometric Defects

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If imperfections in the crystal are such that the ratio between the anions and cations remain same as represented by the molecular formula, the defect is called a stoichiometric defect.

There are three types of stoichiometric defects:

• Schottky defect

• Frenkel defect

• Interstitial defect.

Schottky Defect

In in ionic crystal of the type A⁺ B^-, equal number of anions and cations are missing from the lattice sites so that electrical neutrality is maintained it is called schottky defect.

fig 2.29 - Schottky defect

Types of compounds exhibiting Schottky defect

This type of defect is shown by highly ionic compounds which have

• High coordination number

• Cations and anions of similar sizes

Examples: KCl, NaCl, KBr and CsCl.

Consequences of Schottky Defect

• As the number of ions decrease, volume is the same, so density decreases.

• Crystals with Schottky defect conduct electricity to a small extent.

• Due to the presence of holes the stability of the crystal decreases.

Number of Schottky defects at room temperature

In NaCl there are approximately (10)⁶ Schottky pairs per cm³ at room temperature. In one cm³ there are 10²² ions, so there will be one Schottky defect per 10¹⁶ ions.

Frenkel Defect

If an ion is missing from its lattice site (causing a vacancy or a hole there) and it occupies the interstitial site so that electrical neutrality as well as stoichiometry of the compound are maintained. This type of defect is called Frenkel defect.

Since the size of cations are generally smaller it is more likely that cations occypy interstitial sites.

fig 2.30 - Frenkel defect

Types of compounds exhibiting Frenkel defect

This type of defect is present in those compounds which have

• Low co-ordination number.

• Large difference in size of anion and cation.

Examples: AgCl, AgBr, AgI and ZnS.

Consequences

• Solids with this defect conduct electricity to a small extent

• The dielectric constant of the crystal increases

• The density of the solid is unchanged

• Due to the presence of holes, the stability of the crystal decreases.

Interstitials

fig 2.31 - Interstitia ls

Atoms or ions, which occupy normally vacant interstitial positions in a crystal, are called interstitials. The important factor determining the formation of interstitials is the size of the atom/ion, because they are accomodated in the voids.