Defects in Solids covers two main topics:
- Defects in Solids
- Introduction to Semiconductors
Let's start from the very beginning. 📚
📖 PART 1: What is a Crystal?¶
A crystal is a solid whose atoms are arranged in a highly ordered, repeating pattern. These patterns are called crystal systems.
In a crystal, all atoms or ions are arranged in a regular grid pattern. For example, in table salt (NaCl), the crystal is made of cubes of sodium (Na⁺) and chlorine (Cl⁻) ions, where each sodium ion is surrounded by six chlorine ions.
🔷 Four Types of Crystals (By Bonding Type)¶
| Type | How atoms are held | Examples |
|---|---|---|
| Covalent | Strong covalent bonds, forming a giant molecule | Diamond, Quartz (SiO₂) |
| Ionic | Electrostatic forces between ions | NaCl (table salt) |
| Metallic | Metallic bonds with delocalized electrons | Copper, Gold, Steel |
| Molecular | Weak intermolecular forces | Ice, Dry ice |
📖 PART 2: Defects in Solids — Introduction¶
🔷 How Do Defects Form?¶
Solids are formed from many small crystals. During the process of crystallization, defects occur due to the fast or moderate rate of crystal formation.
In the actual formation of a solid, some irregularities or imperfections remain in the crystal lattice. These are called:
Crystal defects or Lattice imperfections
When an atom is missing, misplaced, or substituted in a crystal lattice → it constitutes a defect.
🔷 General Definition of Defects¶
Defects are irregularities in the arrangement of constituent particles.
Based on the type of irregularity, defects are of two basic kinds:
| Type | Description |
|---|---|
| Point Defect | Irregularity around a single point or atom |
| Line Defect | Irregularity along an entire row of lattice points |
🔷 Overall Classification of Defects in Crystalline Solids¶
Defects in Crystal
├── Point Defect → Vacancy, Interstitial, Substitutional, Schottky, Frenkel
├── Line Defect → Edge Dislocation, Screw Dislocation
├── Surface Defect → Grain Boundary, Twin Boundary
└── Volume Defect → Void, Crack, Inclusion, Precipitate
📖 PART 3: Point Defects — In Detail¶
🔷 What is a Point Defect?¶
When there are any irregularities of arrangement around any point or any atom in an ideal crystal, it is called a point defect.
Point defects are localized disruptions in the regular arrangement of atoms caused by:
- Missing atoms
- Extra atoms
- Atoms of a different type
🔷 5 Types of Point Defects¶
1️⃣ Vacancy Defect¶
- An atom is missing from its normal lattice site, creating a vacant space
- Affects: density, electrical conductivity, optical properties
2️⃣ Interstitial Defect¶
- An extra atom occupies a space between normal lattice sites
- The extra atom can be of the same type or different type
3️⃣ Substitutional Impurity¶
- An atom of a different type occupies a normal lattice site
- The original atom is replaced by a foreign atom
4️⃣ Frenkel Defect¶
- An ion is displaced from its regular lattice site
- It moves to an interstitial site
- This creates both a vacancy AND an interstitial at the same time
5️⃣ Schottky Defect¶
- A pair of oppositely charged ions (both cation AND anion) are missing from the lattice
- Creates two vacancies while maintaining electrical neutrality
🔷 Impact of Point Defects on Material Properties¶
| Property | Effect |
|---|---|
| Mechanical | Affects strength, ductility, and toughness |
| Electrical | Alters electrical conductivity |
| Optical | Influences how materials interact with light |
| Diffusion | Facilitates movement of atoms within material |
| Chemical | Acts as sites for chemical reactions |
📖 PART 4: Types of Defects in Solids¶
🔷 Important Note First!¶
According to the Third Law of Thermodynamics:
At absolute zero temperature (0K), any substance will be perfectly crystalline — NO defects!
But as temperature increases, defects are created.
🔷 3 Main Types of Defects¶
Defects in Solids
├── 1. Stoichiometric Defects
├── 2. Non-Stoichiometric Defects
└── 3. Impurity Defects
1️⃣ STOICHIOMETRIC DEFECTS¶
🔷 What is it?¶
- The defect in which the stoichiometry (ratio of atoms) remains the same as shown by the chemical formula
- Also called Intrinsic or Thermodynamic defect
🔷 Two Subtypes:¶
👉 A) Vacancy Defect¶
- An atom is missing from its normal lattice site
- Creates a vacant space
- Causes decrease in density
- Only for non-ionic solids
👉 B) Interstitial Defect¶
- An atom occupies an interstitial site (between regular lattice sites)
- The atom can be of same or different type
- Only for non-ionic solids
⚠️ Important: Both vacancy and interstitial defects are ONLY for non-ionic solids. Ionic solids show Frenkel and Schottky defects instead!
🔷 Schottky Defect (For Ionic Solids)¶
Key Points:¶
- Occurs due to missing of equal number of cations AND anions from lattice site
- Shown by substances with high coordination number
- The radius ratio of cation and anion [r⁺/r⁻] is not far below unity
- It is a type of vacancy defect
- Crystal neutrality is maintained
- Density decreases
- Crystal can conduct electricity to a small extent
Examples:¶
NaCl, CsCl, KCl, AgBr
🔷 Frenkel Defect (For Ionic Solids)¶
Key Points:¶
- Some cations are missing from their lattice site
- They move to occupy an interstitial site
- Shown by substances with low coordination number
- The radius ratio [r⁺/r⁻] is LOW
- It is a type of dislocation/interstitial defect
- The smaller ion (usually cation) gets displaced
- Density remains the SAME (no ions leave the crystal)
- Increases the dielectric constant of the crystal
Examples:¶
ZnS, AgBr, AgI, AgCl
⭐ Special Note: AgBr shows BOTH Frenkel AND Schottky defects!
🔷 Quick Comparison: Schottky vs Frenkel¶
| Feature | Schottky | Frenkel |
|---|---|---|
| What's missing | Both cation & anion | Only cation displaced |
| Where does ion go | Leaves crystal | Goes to interstitial site |
| Coordination number | High | Low |
| Radius ratio | Close to unity | Low |
| Density | Decreases | Stays same |
| Examples | NaCl, KCl, CsCl | ZnS, AgI, AgCl |
2️⃣ IMPURITY DEFECTS¶
🔷 What is it?¶
- Occurs when a foreign ion replaces the original ion in the lattice
- Example: NaCl containing SrCl₂ as impurity
How it works:¶
- In molten NaCl, a small amount of SrCl₂ is present as impurity
- Sr²⁺ occupies the site of Na⁺
- To maintain neutrality → one Sr²⁺ replaces TWO Na⁺ ions
- The remaining Na⁺ site becomes vacant
Examples:¶
CdCl₂ and AgCl
3️⃣ NON-STOICHIOMETRIC DEFECTS¶
🔷 What is it?¶
- Shown by compounds of d-block elements
- The ratio of atoms is NOT the same as the chemical formula suggests
🔷 Two Types:¶
👉 A) Metal Excess Defect — Due to Anionic Vacancy¶
- When a compound has excess metal ions
- An anion is absent from its lattice site
- The vacant site is occupied by an electron (called F-centre or colour centre)
- Shown by alkali metal halides like NaCl, KCl, LiCl
🌈 Colours produced by F-centres:¶
| Compound | Colour |
|---|---|
| NaCl | Yellow |
| KCl | Violet |
| HCl | Pink |
👉 B) Metal Excess Defect — Due to Interstitial Cation¶
- An excess positive ion is located in the interstitial site
- Example: ZnO
- When ZnO is heated → loses oxygen reversibly
- Excess Zn²⁺ ions occupy interstitial sites
- Electrons enclosed in neighboring interstitial sites to maintain neutrality
- ZnO turns yellow on heating
👉 C) Metal Deficiency Defect¶
- Some cations are missing from the lattice site
- To maintain electrical neutrality → another cation increases its valency
- Example: FeO (found as Fe₀.₉₅O, ranging from Fe₀.₉₃O to Fe₀.₉₆O)
- Some Fe²⁺ are missing → compensated by presence of Fe³⁺ ions
🔷 Importance of Crystal Defects (Summary)¶
| # | Importance |
|---|---|
| 1 | Point defects in metals reduce electricity and semi-conductivity |
| 2 | Substitution defects govern electronic conductivity in semiconductors |
| 3 | Vacancies control ionic conductivity and diffusion |
| 4 | Dislocations govern plastic deformation |
| 5 | Defects influence the color of crystals |
| 6 | Defects strongly influence mechanical strength |