Question

Which type of ‘defect’ has the presence of cations in the interstitial sites?

• A. Vacancy Defect
• B. Frenkel Defect
• C. Metal Deficiency Defect
• D. Schottky Defect

Answer 1

The correct option is B

Frenkel Defect

Explanation of correct option

B. Frenkel Defect

1. Frenkel defects arise when an atom or a cation leaves its lattice site within the crystal lattice and occupies a position in between lattice sites called an interstitial position.
2. Frenkel defects arise in crystals where the anion is much larger than the cation such as in Silver bromide ($\mathrm{AgBr}$) and Zinc sulphide ($\mathrm{ZnS}$) crystals.
3. Since cations do not leave the crystal lattice altogether, the overall density of the crystal and its electrical neutrality is preserved.
4. Frenkel defects are responsible for the conduction of electricity in crystals due to vacant lattice sites.

Explanations of incorrect options

A. Vacancy defect

1. A vacancy defect is a type of defect in a crystal where an atom or ion is missing from its lattice site.
2. These may arise due to imperfect packing during crystallization or due to thermal vibrations of the atoms/ions at higher temperatures.

C. Metal deficiency defect

1. In this case, one of the positive ions is missing from its lattice site and the extra negative charge in the lattice is balanced by some nearby positive ion acquiring an extra positive charge. The ion missing from its lattice site is not found in the interstitial position.
2. This type of defect is generally found in transition metal compounds that can exhibit variable valency such as the crystal of Ferrous oxide ($\mathrm{FeO}$) also known as wustite. These crystals are iron deficient with compositions ranging from ${\mathrm{Fe}}_{0.93}\mathrm{O}$ to ${\mathrm{Fe}}_{0.96}\mathrm{O}$.

D. Schottky defect

1. Schottky defects arise when the same number of anions and cations are missing from the crystal lattice.
2. Schottky defects occur in crystals where the sizes of anions and cations are comparable. Some good examples of crystals where Schottky defects appear are Sodium chloride ($\mathrm{NaCl}$), Potassium chloride ($\mathrm{KCl}$) and Caesium chloride ($\mathrm{CsCl}$).
3. Schottky defects do not affect the electrical neutrality of the crystal.
4. Since anions and cations are completely missing from the crystal lattice, Schottky defects reduce the density of the crystal.

Final Statement: The correct option is B. Frenkel defects have the presence of cations in the interstitial sites.