Question

Which of the following is a non-stoichiometric defect?

• A. Frenkel defect
• B. Schottky defect
• C. Interstitial defect
• D. Metal deficiency defect

Answer 1

The correct option is D

Metal deficiency defect

Explanation for correct option:

D. Metal deficiency defect

Non-stoichiometric defect:

• “A large number of nonstoichiometric inorganic solids are known which contain the constituent elements in non-stoichiometric ratio due to defects in their crystal structures.”
• These defects are of two types:

1. Metal excess defect
2. Metal deficiency defect

Metal-deficiency defect:

• It is a type of crystal defect in which, either a cation or an anion vacancy causes the defects
• An example of metal-deficiency defect is $\mathrm{FeO}$ which is mostly found with a composition of ${\mathrm{Fe}}_{0.95}\mathrm{O}$.

Stoichiometric defect:

“These are the point defects that do not disturb the stoichiometry of the solid. They are also called intrinsic defects.”

Explanation for incorrect options:

A. Frenkel defect:

• “This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site.”

B. Schottky defect:

• “It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal.”

C. Interstitial defect:

• “When some constituent particles (atoms or molecules) occupy an interstitial site, the crystal is said to have the interstitial defect.”

Hence, option D is the correct option i.e. Metal deficiency defect.