Which of the following defects are shown by \(KCl\) Crystal?

(A) Schottky defect

(B) Frenkel defect

(C) Metal excess defect

(D) Metal deficiency defect

Choose the correct answer from the options given below:

(A) and (C) only

The correct answer is option 2. (A) and (C) only.

(A) Schottky Defect: \(KCl\) is likely to show Schottky defects. Schottky defects occur in ionic crystals where both a cation and an anion leave their lattice sites, creating vacancies. This type of defect is favored when the sizes of the cation and anion are similar, as is the case with \(K^+\) and \(Cl^-\) in \(KCl\).

Here's why size similarity matters:

If the cation is much smaller than the anion, the cation can more easily squeeze into an interstitial space (a space between lattice sites) rather than leaving the crystal altogether. This is called a Frenkel defect.

Conversely, if the anion is much smaller than the cation, it's energetically unfavorable for a large anion to leave a vacancy.

Therefore, \(KCl\) with its comparably sized ions shows Schottky defects.

(C) Metal Excess Defect: Alkali metals solid like \(KCl\) show non-stoichiometric defect. When crystals of \(KCl\) are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The \(Cl^-\) ions get diffused to the surface of the crystal and combine with \(K\) atoms to give \(KCl\). This happens by loss of electron by \(K\) atoms to form \(K^+\) ions. The released electron diffused into the crystal and occupy anionic sites. Hence, the crystal now has anionic sites. Therefore, the crystal now has an excess of potassium. These anionic sites occupied by unpaired electrons are called, F-centres (derived from the German word Farbenzenter for colour centre).

Let us analyze why other two defects are not shown by \(KCl\):

(B) Frenkel Defect: \(KCl\) does not typically exhibit Frenkel defects. Frenkel defects involve the displacement of an ion from its regular lattice site to an interstitial position within the crystal structure.

In the case of \(KCl\), the relatively large size of both the potassium ion \((K^+)\) and the chloride ion \((Cl^-)\) compared to the interstitial spaces in the crystal lattice makes Frenkel defects less likely to occur. The size mismatch between the ions and the interstitial sites means that it would require a significant amount of energy to displace an ion from its lattice site to an interstitial position without disrupting the crystal structure significantly.

(D) Metal deficiency defect: \(KCl\) does not exhibit metal deficiency defects. Metal deficiency defects occur in compounds where the cations are missing from their regular lattice positions, leading to an excess of anions to maintain charge neutrality.

In \(KCl\), the crystal lattice consists of alternating potassium \((K^+)\) and chloride \((Cl^-)\) ions in a one-to-one ratio. There are no metal cations missing from the lattice sites, so there is no excess of anions to compensate for. Therefore, metal deficiency defects do not apply to \(KCl\).