Question

Which of the following statement(s) for crystals having Frenkel defect is/are correct?

• A. Frenkel defects are observed where the difference in sizes of cations and anions is large.
• B. The density of crystals having Frenkel defect is lesser than that of a pure perfect crystal.
• C. In an ionic crystal having Frenkel defect may also contain Schottky defect.
• D. Usually alkali halides do not have Frenkel defect.

Answer 1

Answer: (A), (C), (D)

The correct options are
A Frenkel defects are observed where the difference in sizes of cations and anions is large.
C In an ionic crystal having Frenkel defect may also contain Schottky defect.
D Usually alkali halides do not have Frenkel defect.

Schottky Defect:

This defect is caused when some of the lattice points are unoccupied called vacancies or holes as shown in figure (a). The number of missing positive and negative ions is the same so that the crystal remains neutral in all. Schottky defects are more common in ionic compounds with high co-ordination number and where the sizes of positive and negative ions are almost equal. For example, NaCI, KCI, CsCI and KBr.

The number of defects increases with increase in temperature. The number of defects increases to one in $106$ sites at $775$ K and one in $104$ sites at $1075$ K. The presence of large number of Schottky defects in crystal results in significant decrease in its density.

Frenkel Defect:

This defect is caused when some of the ions leave their lattice sites to occupy an interstitial site as shown in figure (b). Frenkel defects are more common in ionic compounds with low co-ordination number and where there is large difference in size between positive and negative ions. For example, ZnS, AgCI, AgBr and Agl.

In pure alkali metal halides, these defects are not very common because the ions cannot get into interstitial positions due to their large sizes.