Defects in Solids: Stoichiometric & Non-Stoichiometric
Perfect crystals exist only at absolute zero ($0\text{ K}$). At higher temperatures, deviations from the perfectly ordered arrangement occur, known as crystal defects, which profoundly impact electrical and magnetic properties.
1. Stoichiometric Defects (Intrinsic)
These are point defects that do not disturb the stoichiometry of the solid. They are also called intrinsic or thermodynamic defects.
Schottky Defect
- Type: Vacancy defect in ionic solids.
- Condition: High Coordination Number; Cation and Anion sizes are similar (e.g., $\text{NaCl, KCl, CsCl, AgBr}$).
- Effect: Density decreases because ions are missing.
Frenkel Defect
- Type: Dislocation defect. Smaller ion (usually cation) moves to an interstitial site.
- Condition: Low Coordination Number; Large difference in ion sizes (e.g., $\text{ZnS, AgCl, AgBr, AgI}$).
- Effect: Density remains constant.
Note: $\text{AgBr}$ shows both Schottky and Frenkel defects.
Figure: Classification of Point Defects in Solids
2. Non-Stoichiometric Defects
These defects disturb the stoichiometry of the compound. They arise from metal excess or metal deficiency.
Metal Excess & F-Centers
- Anionic Vacancies: A negative ion may be missing from its lattice site, leaving a hole occupied by an electron to maintain electrical neutrality.
- F-Center (Farbenzenter): This unpaired electron absorbs energy in the visible region, imparting color to the crystal.
- Examples: $\text{NaCl}$ turns Yellow; $\text{LiCl}$ turns Pink; $\text{KCl}$ turns Violet.
3. Impurity Defects
These defects arise when foreign atoms are introduced into the host crystal lattice. This process is crucial for creating semiconductors.
- Ionic Solids: If molten $\text{NaCl}$ containing a little $\text{SrCl}_2$ is crystallized, some $\text{Na}^+$ sites are occupied by $\text{Sr}^{2+}$. Each $\text{Sr}^{2+}$ replaces two $\text{Na}^+$ ions to maintain charge, creating one cation vacancy.
- Covalent Solids (Doping): Group 14 elements (Si, Ge) doped with Group 15 (P, As) create n-type semiconductors. Doping with Group 13 (B, Al) creates p-type semiconductors.
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