Animated Solution for Chemistry - Solid State: 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.

Frenkel vs. F-Centres

Crystal defects might sound like flaws, but they are actually the secret ingredients that give solid materials their unique and fascinating properties. In this problem, we are presented with two statements regarding point defects in ionic crystals. Let's break them down conceptually to uncover the truth.

The Frenkel Defect

Statement I claims that a Frenkel defect is both a vacancy and an interstitial defect. To understand this, imagine a perfectly ordered crystal lattice. In ionic solids, cations are generally smaller than anions. Occasionally, a small cation decides to abandon its designated lattice site.

When the cation leaves its original position, it leaves behind an empty space. This empty space is precisely what we call a vacancy defect. But the cation doesn't leave the crystal entirely; it simply squeezes into the empty voids between the other ions. These voids are known as interstitial sites. By occupying this new position, the cation creates an interstitial defect.

Because a single ion's movement creates a vacancy at its old site and an interstitial defect at its new site simultaneously, a Frenkel defect is indeed a combination of both. Therefore, Statement I is absolutely true.

The Mystery of F-Centres

Statement II links Frenkel defects to the presence of F-centres and the resulting colour in ionic solids. Let's investigate what an F-centre actually is.

An F-centre (from the German word Farbenzenter, meaning colour centre) is formed when an anion is missing from its lattice site, and to maintain electrical neutrality, an unpaired electron occupies that vacant site. This is a classic example of a Metal Excess Defect, which is a non-stoichiometric defect.

The trapped electron in the F-centre is not tightly bound. When visible light falls on the crystal, this electron absorbs energy and gets excited to a higher energy level. When it falls back, it emits light of a specific wavelength, imparting a characteristic colour to the crystal. A famous example is when white $NaCl$ crystals are heated in sodium vapor, they turn a brilliant yellow due to the formation of F-centres.

However, notice the crucial difference: F-centres are caused by missing anions and trapped electrons (Metal Excess Defect). Frenkel defects merely involve the internal rearrangement of cations without any trapped electrons. Therefore, Frenkel defects do not create F-centres and do not typically impart colour to the crystal. Thus, Statement II is false.

By carefully distinguishing between stoichiometric defects (like Frenkel) and non-stoichiometric defects (like Metal Excess), we can confidently conclude that Statement I is true while Statement II is false. This makes option (c) the correct choice.