Question
Explain Schottky and Frenkel defects in ionic crystals. How does each defect affect the density of the crystal? Give examples of crystals showing each type.
(JEE Main 2022, similar pattern — theory-based question worth 4 marks)
Solution — Step by Step
In a Schottky defect, equal numbers of cations and anions are missing from their lattice sites, creating vacancies. The missing ions migrate to the surface or leave the crystal entirely.
Conditions: Occurs in ionic crystals where cation and anion are of similar size (high coordination number). Example: NaCl, KCl, CsCl, AgBr.
Effect on density: Since ions are missing but the crystal volume remains nearly the same, the density decreases.
In a Frenkel defect, an ion (usually the smaller cation) leaves its normal site and occupies an interstitial site (a gap between regular positions). This creates a vacancy at the original site and an interstitial defect.
Conditions: Occurs when there is a large difference in size between cation and anion. The smaller ion can fit into interstitial sites. Example: AgCl, AgBr, ZnS.
Effect on density: No ions leave the crystal — they just move to different positions. So the density does not change.
| Feature | Schottky | Frenkel |
|---|---|---|
| What happens | Ion pairs leave lattice | Small ion moves to interstitial site |
| Vacancies | Both cation and anion vacancies | One vacancy + one interstitial |
| Density | Decreases | No change |
| Size condition | Similar-sized ions | Large size difference |
| Examples | NaCl, KCl, CsCl | AgCl, AgBr, ZnS |
| Also called | Vacancy defect | Dislocation defect |
Why This Works
Both defects are thermodynamic inevitabilities — at any temperature above 0 K, a perfect crystal is less stable than one with a few defects. The entropy gain from creating vacancies outweighs the energy cost, up to an equilibrium defect concentration that increases with temperature.
The density argument is straightforward: Schottky defects remove mass from the crystal (ions leave), so density drops. Frenkel defects simply rearrange mass within the crystal (no ions leave), so density stays the same. This density test is the simplest way to distinguish between the two defects experimentally.
Alternative Method
You can also identify defect types from the stoichiometry. Schottky defects maintain stoichiometry (equal cations and anions missing). Frenkel defects also maintain stoichiometry (no ions lost). Non-stoichiometric defects (like metal excess or metal deficiency) are a separate category — often caused by variable oxidation states (e.g., ).
AgBr is special — it shows BOTH Schottky and Frenkel defects. This is a favourite trick question in JEE. Also, remember that Frenkel defects are more common in crystals with low coordination numbers, where there is more empty space for displaced ions.
Common Mistake
Students sometimes say “Frenkel defect decreases density because an ion is missing from its site.” The ion is not missing from the crystal — it has moved to an interstitial position. The total number of ions in the crystal is unchanged, so mass is unchanged, and density does not change. Only in Schottky defects do ions actually leave the crystal.