Q.Explain the following terms with suitable examples:

• Schottky defect: a vacancy defect of ionic solids where an equal number of cations and anions are missing, so electrical neutrality is preserved. Density falls. Shown by solids with high coordination number and nearly equal ionic sizes, e.g. $NaCl$, $KCl$, $CsCl$, $AgBr$.
• Frenkel defect: a dislocation defect where the smaller ion (usually the cation) leaves its lattice site and lodges in an interstitial site. No ion is lost, so density is unchanged. Shown by solids with a large size difference and low coordination number, e.g. $ZnS$, $AgCl$, $AgBr$, $AgI$.
• Interstitial defect: constituent particles occupy an otherwise-vacant interstitial site, increasing the density of the solid. …

Step 1 – Schottky defect.

This is a vacancy defect in which an equal number of cations and anions are missing from their lattice sites. Removing equal charges keeps the crystal electrically neutral.

Consequence: the density decreases. It appears in ionic solids with a high coordination number and cations/anions of similar size.

Examples: $NaCl,\;KCl,\;CsCl,\;AgBr$ (AgBr shows both).

Step 2 – Frenkel defect.

A dislocation defect: the smaller ion (generally the cation) is displaced from its normal site to an interstitial site. Since nothing leaves the crystal, mass and volume are essentially unchanged, so density stays the same. It occurs in solids with a large difference in ionic radii and low coordination number.

Examples: $ZnS,\;AgCl,\;AgBr,\;AgI$.

Step 3 – Interstitial defect.

When some constituent particles (atoms, molecules or ions) occupy an interstitial site that is normally empty, the defect is an interstitial defect. Extra particles packed into the same volume increase the density of the substance.

Step 4 – F-centres. …

Classify each as the appropriate point defect. Distinguish Schottky (paired vacancies, density down) from Frenkel (displacement to interstitial, density unchanged) by what is lost vs displaced; note the interstitial defect raises density; and treat the F-centre as a metal- …

• Confusing Schottky and Frenkel — forgetting that Schottky removes an ion pair (density falls) while Frenkel only relocates an ion to a void (density unchanged).
• Saying an F-centre is a cation vacancy; it is an anion vacancy occupied by an electron. …