Given below are two statements:
Statement I : Frenkel defects are vacancy as well as interstitial defects.
Statement II : Frenkel defect leads to colour in ionic solids due to presence of F-centres.
Choose the most appropriate answer for the statements from the options given below:

We first recall the standard definitions from solid-state chemistry. A Frenkel defect occurs when an ion, generally the smaller cation, leaves its regular lattice site and occupies an interstitial site in the crystal. Because of this movement:

$$\text{Regular lattice site} \longrightarrow \text{vacancy}$$

and simultaneously

$$\text{Interstitial site} \longleftarrow \text{same ion}$$

Thus, one vacancy defect and one interstitial defect are created at the same time. Hence a Frenkel defect is quite literally the combination of a vacancy defect and an interstitial defect. Therefore Statement I, “Frenkel defects are vacancy as well as interstitial defects,” is true.

Next, we consider the origin of colour in many ionic solids. Colour often arises when an anion vacancy is occupied by an electron; such an electron-filled vacancy is called an F-centre (from the German word “Farbe,” meaning colour). The crystal that contains these F-centres can absorb visible light, which gives it colour. These F-centres are produced in a different kind of imperfection, namely the metal-excess defect (sometimes associated with Schottky-type anion vacancies), not in a Frenkel defect. A Frenkel defect merely displaces ions without introducing extra electrons into the lattice; it does not generate F-centres and therefore does not impart colour for that reason.

Consequently, Statement II, “Frenkel defect leads to colour in ionic solids due to presence of F-centres,” is false.

We have found that Statement I is true while Statement II is false. Examining the options, this correspondence matches Option C.

Hence, the correct answer is Option C.