Question
Compare Schottky and Frenkel defects in ionic crystals. Give one example of each and explain how they affect density.
Solution — Step by Step
In a perfect crystal, every atom/ion is at its designated lattice position. Real crystals always have imperfections called defects. Point defects involve individual ions or vacancies.
There are two main types of stoichiometric point defects (where the ratio of cations to anions remains unchanged):
- Schottky defect (vacancy defect)
- Frenkel defect (dislocation defect)
In a Schottky defect, pairs of ions (one cation and one equal-charge anion) are missing from their lattice positions. The ions leave the crystal to maintain electrical neutrality — you cannot remove just one ion type without charging the crystal.
Key characteristics:
- Equal number of cation and anion vacancies (for 1:1 salts)
- No ions move to interstitial positions — ions are completely absent
- Density decreases (vacancies reduce mass without changing volume significantly)
- Common in compounds with ions of similar size
Examples: NaCl, KCl, KBr, CsCl, AgBr (Schottky AND Frenkel)
In NaCl: occasionally, one Na⁺ and one Cl⁻ are missing from their sites, creating paired vacancies. At room temperature, NaCl has roughly 1 Schottky defect per 10¹⁵ ions.
In a Frenkel defect, an ion (usually the smaller cation) leaves its normal lattice site and moves to an interstitial position (a gap between regular positions). The original site becomes vacant, but the ion is still present in the crystal.
Key characteristics:
- The displaced ion occupies an interstitial space
- Electrical neutrality is maintained (same number and types of ions — just misplaced)
- Density does NOT change (same mass, same volume)
- Common when there is a large size difference between cation and anion (small cation can fit interstitially)
Examples: AgCl, AgBr, AgI (all silver halides — Ag⁺ is small and can move to interstitial sites), ZnS (zinc blende)
In AgCl: occasionally, Ag⁺ (small cation) leaves its lattice position and occupies a gap in the crystal lattice, leaving a vacancy at its original position.
| Feature | Schottky Defect | Frenkel Defect |
|---|---|---|
| Nature | Ions absent (vacancies) | Ions displaced to interstitial sites |
| Ions present? | No (gone from crystal) | Yes (in interstitial position) |
| Effect on density | Decreases | No change |
| Electrical neutrality | Maintained (equal vacancies) | Maintained (same ions, different position) |
| Favoured by | Similar sized ions | Large size difference (small cation) |
| Examples | NaCl, KCl, KBr | AgCl, AgBr, ZnS |
Why This Works
Both defects maintain stoichiometry and electrical neutrality — that’s what makes them “stoichiometric defects.” The key difference is whether the displaced ions stay in the crystal (Frenkel — interstitial) or leave entirely (Schottky — vacancies).
The density effect follows directly: Schottky removes ions (less mass → lower density), while Frenkel just relocates them (mass unchanged → same density).
Alternative Method — Visual Analogy
Schottky: Imagine a hostel where some rooms are completely empty — the students left the building. Fewer students, same building size → lower “population density.”
Frenkel: Students moved from their assigned rooms to sleep in the corridors. Same number of students, same building → same “population density.”
Common Mistake
The most common confusion: saying “Frenkel defect decreases density.” It does NOT — the ions are still inside the crystal (in interstitial positions), so the total mass doesn’t change. Only Schottky defect decreases density because ions actually leave the crystal. Also: AgBr shows BOTH Schottky and Frenkel defects — this is specifically asked in JEE and CBSE. Don’t classify AgBr as only one type.
Frenkel = Floating ion (moves to interstitial), density Fixed (unchanged). Schottky = Stolen ion (leaves crystal), density Shrinks (decreases). This alliteration helps during exams.