Question

Which type of ‘defect’ has the presence of cations in the interstitial sites?
A.Frenkel defect
B.Metal deficiency defect
C.Schottky defect
D.Vacancy defect

Answer 1

Hint: Any deviation from the perfectly ordered arrangement of atoms or ions constitutes a defect or imperfection in a solid. There are two types of imperfections, namely atomic imperfection/ point defects and electronic imperfection. The type of defect mentioned here is a point defect.

Complete step by step solution:
Atomic imperfection or point defects occur when there are deviations from the regular or periodic arrangement around an atom or a group of atoms in a crystalline substance.
Frenkel defect – This type of defect arises when an ion is missing from its lattice site and occupies an interstitial position in the crystal. Electrical neutrality is maintained even in the presence of this defect because the number of cations and anions remain the same. As cations are smaller than anions, they tend to occupy the interstitial sites.
Metal deficiency defect – This is a type of defect caused when a metal ion or cation is missing from its lattice site. Whenever such a type of defect occurs, the electrical neutrality is maintained by one of the nearest cations which acquires an extra positive charge. This is possible in crystals which are made of metals with variable valency.
Schottky defect – This type of defect occurs when the same or equal number of cations and anions are missing from their lattice sites. Electrical neutrality is maintained in case of this defect also. This type of defect is mostly exhibited by highly ionic compounds which have high coordination number and cations and anions are of similar sizes.
Vacancy defect – This is a type of defect in a crystal where an atom is missing from one of its lattice sites in the crystal.

Hence, option (A) is the correct answer.

Note:
A clear understanding of the various types of defects in crystalline substances is mandatory to answer such types of questions. Remember that a large number of Schottky defects in a crystal lowers the density of a crystal, whereas Frenkel defects do not cause any decrease in the density of the crystal.