Imperfections in Solids

Real crystals are never perfect. These irregularities, known as imperfections or crystal defects, significantly impact the physical and chemical properties of solids.

Types of Imperfections

Point Defects – At atomic scale
Line Defects – Dislocations in a row

1. Point Defects

a. Stoichiometric Defects

Vacancy: Missing atom
Interstitial: Extra atom in space
Frenkel: Ion leaves lattice and fits in a gap
Schottky: Equal cations and anions missing

b. Non-Stoichiometric Defects

Metal Excess: More cations/electrons than needed
Metal Deficiency: Less cations than expected

c. Impurity Defects

Occurs when foreign ions are added (e.g., NaCl doped with SrCl₂).

2. Line Defects (Dislocations)

Edge Dislocation: Extra half-plane of atoms
Screw Dislocation: Spiral lattice twist

Importance

Affect conductivity and strength
Used in semiconductor doping
Important in metallurgy and materials science

Conclusion: Crystal defects are crucial in determining real-world behavior of solids in chemistry, physics, and engineering.

MCQs: Test Your Understanding

Which defect does not change the density of a crystal?
- A. Frenkel Defect ✅
- B. Schottky Defect
- C. Interstitial Defect
- D. Vacancy Defect
Example of a compound showing Schottky defect is:
- A. ZnS
- B. NaCl ✅
- C. AgCl
- D. FeO

True or False (With Explanation)

Frenkel defect increases the density of a crystal. ❌ False
Explanation: Frenkel defect doesn’t affect density as no ions are lost.
Schottky defect reduces the density of the solid. ✅ True
Explanation: Both cations and anions are missing, lowering mass per unit volume.