Which of the following compounds is likely to show both Frenkel and Schottky defects in its crystalline form?

First, let us recall what the two kinds of point defects are. A Schottky defect is produced when an equal number of cations and anions leave the bulk crystal lattice, creating corresponding vacant lattice sites. This maintains overall electrical neutrality. Such a defect is favoured in ionic compounds in which the ions are of comparable size, so that the simultaneous absence of both kinds of ions from their lattice points does not create excessive strain.

On the other hand, a Frenkel defect arises when an ion (usually the smaller cation) leaves its normal lattice site and occupies an interstitial position in the crystal. The vacancy created at the original site together with the interstitial ion still keeps charge balance intact. This type of defect is favoured when there is a large difference in size between cation and anion, making it easy for the smaller ion to get into an interstitial void.

We now have to decide which given compound can simultaneously satisfy both conditions: similar ionic sizes (for Schottky) and a much smaller cation capable of occupying an interstitial site (for Frenkel).

Let us examine each option.

For $$\text{AgBr}$$, the radius of $$\text{Ag}^+$$ is not very different from that of $$\text{Br}^-$$, so the creation of equal numbers of cation and anion vacancies (Schottky defect) is possible without large lattice distortion. At the same time, $$\text{Ag}^+$$ is still small enough compared with $$\text{Br}^-$$ to slip into an interstitial site, thus also permitting the Frenkel defect. Therefore, $$\text{AgBr}$$ is able to exhibit both defects in the same crystal.

For $$\text{CsCl}$$, the $$\text{Cs}^+$$ ion is quite large, almost as large as $$\text{Cl}^-$$, so while Schottky defects are possible, the large cation cannot easily occupy an interstitial site; hence a Frenkel defect is unlikely.

For $$\text{KBr}$$, the argument is similar: $$\text{K}^+$$ is also relatively large and cannot readily fit into an interstitial void, so Frenkel defects are not favoured.

For $$\text{ZnS}$$ (zinc blende structure), the sizes of $$\text{Zn}^{2+}$$ and $$\text{S}^{2-}$$ differ significantly, but $$\text{Zn}^{2+}$$ being dipositive must move two negative charges whenever it relocates, making Schottky pair creation (requiring equal numbers of cation and anion vacancies) energetically expensive. Thus simultaneous occurrence of both defects is not common.

Among the four, only $$\text{AgBr}$$ meets both criteria and is experimentally known to display a mixture of Frenkel as well as Schottky defects.

Hence, the correct answer is Option A.