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

We begin by recalling the standard point defects possible in an ionic (or even non-ionic) crystalline solid. In the syllabus, four named defects are important: vacancy defect, metal (cation or anion) deficiency defect, Schottky defect and Frenkel defect. Each defect is identified by what actually happens to the ions in the lattice.

For the present question, we are asked: which defect involves "the presence of cations in the interstitial sites"? An interstitial site is a normally empty space between the regular lattice points. Therefore the defect must be one in which at least one of the ions leaves its normal lattice position and occupies such a gap, without maintaining its original site.

We now briefly review each listed defect in this context.

For a vacancy defect, an atom or ion simply leaves its lattice point, creating a vacant site, but it does not relocate to an interstitial space; it may completely leave the crystal or remain on the surface. Thus, interstitial occupation is not involved.

For a Schottky defect, equal numbers of cations and anions leave their lattice points, again creating vacancies but not occupying interstitials. The crystal maintains electrical neutrality because the missing positive and negative ions are equal in number, yet no interstitial occupancy is introduced.

For a metal deficiency defect (also called cation deficiency defect), certain cations are missing from their lattice points, and to maintain charge balance, some of the remaining cations attain a higher positive oxidation state. Here too, no ion sits in an interstitial position; instead, oxidation state adjustment compensates for the missing cations.

Finally, consider the Frenkel defect. In this defect, an ion—usually the smaller ion, most often a cation—leaves its normal lattice site and migrates to an interstitial site within the same crystal. Because the ion remains inside the crystal but in a non-lattice position, the overall stoichiometry and charge balance are preserved. The presence of the same number of vacancies (where the ion left) and interstitials (where it now sits) characterises the Frenkel defect.

Hence, the only defect among the four that definitely "has the presence of cations in the interstitial sites" is the Frenkel defect.

Therefore, the correct choice is Option 4 (Frenkel defect).

Hence, the correct answer is Option 4.