Question

Explain with diagram, Frenkel defect. What are the conditions for its formation? What is its effect on density and electrical neutrality of the crystal?

Answer 1

• Frenkel defect: 

a. Frenkel defect arises when an ion of an ionic compound is missing from its regular lattice site and occupies an interstitial position between lattice points. The cations are usually smaller than anions. Therefore, the cations occupy interstitial sites.

b. The smaller cation is displaced from its normal site to interstitial space. It, therefore, creates a vacancy defect at its original position and an interstitial defect at its new location in the same crystal. Frenkel defect can be regarded as the combination of vacancy defect and interstitial defect.

c. This defect is found in ionic crystals like ZnS, AgCl, AgBr, AgI and CaF².

Frenkel defect

• Conditions for the formation of Frenkel defect:

a. Frenkel defect occurs in ionic compounds with large difference between sizes of cation and anion.

b. The ions of ionic compounds must be having low coordination number.

• Consequences of Frenkel defect:

a. As no ions are missing from the crystal lattice as a whole, the density of solid and its chemical properties remain unchanged.

b. The crystal as a whole remains electrically neutral because the equal numbers of cations and anions are present.