1. Introduction to Solid State Chemistry
Solid state chemistry is the branch of chemistry that deals with the structure, properties, preparation and applications of solid materials. In typical school textbooks, the chapter mainly focuses on crystal structures, unit cells, packing efficiency, defects, and electrical & magnetic properties. But at the postgraduate level, the subject expands into band theory, magnetic ordering, superconductivity, X-ray crystallography, and materials chemistry.
In simple words, solid-state chemistry tries to answer “How particles are arranged in solids, why solids behave differently, and how we can modify solids to create better materials?”
2. Characteristics of the Solid State
Definition: A solid is a state of matter in which particles remain closely packed with very little freedom of movement. Solids have a definite shape, fixed volume and high density because intermolecular forces are strong. The particles only vibrate about their mean positions, which explains why solids are rigid. Depending on the arrangement of particles, solids show different properties like hardness, conductivity, malleability, and brittleness.
Classification of Solids
Solids are broadly classified into crystalline and amorphous solids, and this distinction often becomes an important interview question.
Comparison: Crystalline vs. Amorphous Solids
| Feature | Crystalline Solids | Amorphous Solids |
|---|---|---|
| Arrangement | Long-range order; repeating pattern | Short-range order only |
| Melting Point | Sharp melting point | Gradual softening |
| Isotropy/Anisotropy | Anisotropic (properties vary with direction) | Isotropic (same in all directions) |
| Heat of Fusion | Definite | No definite value |
| X-ray Diffraction Pattern | Sharp, well-defined | Diffuse |
| Examples | NaCl, diamond, quartz | Glass, rubber, plastics |
Advanced interview insight:
Amorphous solids are sometimes called supercooled liquids because their molecular arrangement resembles that of a liquid frozen in time.
3. Crystal Lattices and Unit Cells
Definition: A crystal lattice is a three-dimensional arrangement of points that represent the repeating pattern of a crystal. When we choose the smallest repeating unit which can generate the whole lattice upon translation, we call it a unit cell. Unit cells are characterised by edge lengths (a, b, c) and angles (α, β, γ). The internal symmetry of a solid strongly influences its electrical, optical and mechanical characteristics.
Types of Unit Cells
- Primitive (Simple) Unit Cell – Atoms only at the corners.
- Body-Centered Cubic (BCC) – One atom at centre + corner atoms.
- Face-Centered Cubic (FCC) – One atom at centre of each face + corner atoms.
Bravais Lattices
Bravais recognised that only 14 unique three-dimensional lattices can exist. These belong to seven crystal systems.
Table: Bravais Lattices Summary
| Crystal System | Lattice Parameters | Bravais Lattices | Common Examples |
|---|---|---|---|
| Cubic | a = b = c; α = β = γ = 90° | Simple, BCC, FCC | NaCl (FCC), Fe (BCC) |
| Tetragonal | a = b ≠ c; α = β = γ = 90° | Simple, BCT | Sn |
| Orthorhombic | a ≠ b ≠ c; α = β = γ = 90° | Simple, BCO, FCO, CCO | KNO₃ |
| Hexagonal | a = b ≠ c; γ = 120° | Simple | Mg, Zn |
| Rhombohedral | a = b = c; α = β = γ ≠ 90° | Simple | Calcite |
| Monoclinic | a ≠ b ≠ c; β ≠ 90° | Simple, Base-centered | Gypsum |
| Triclinic | a ≠ b ≠ c; no angles 90° | Simple | K₂Cr₂O₇ |
4. Atomic Packing in Solids
Packing Efficiency
Packing efficiency =
Table: Packing Efficiency in Unit Cells
| Structure | No. of atoms | Coordination Number (CN) | Packing Efficiency |
|---|---|---|---|
| Simple Cubic (SC) | 1 | 6 | 52% |
| BCC | 2 | 8 | 68% |
| FCC | 4 | 12 | 74% |
| HCP | — | 12 | 74% |
FCC and HCP have identical efficiency but differ in stacking sequence:
- FCC → ABCABC
- HCP → ABABAB
5. Voids in Solid Structures
Atoms cannot fill all space; empty spaces called voids appear.
| Void Type | Coordination Number | Size Relationship | Example Occupants |
|---|---|---|---|
| Tetrahedral Void | 4 | r = 0.225R | Zn²⁺ in ZnS |
| Octahedral Void | 6 | r = 0.414R | Na⁺ in NaCl |
| Cubic Void | 8 | r = 0.732R | Cs⁺ in CsCl |
Important concept:
Number of tetrahedral voids = 2 × number of atoms in FCC
Number of octahedral voids = number of atoms in FCC
6. Ionic Solids
Definition: Ionic solids are formed by the strong electrostatic attraction between oppositely charged ions. These solids typically exist as crystalline lattices where each ion occupies a specific site. They are hard, brittle, and have high melting points. They conduct electricity only in molten or aqueous state because ions become mobile.
radius ration rule
Metallic Solids: Metallic solids consist of metal cations surrounded by a “sea of delocalised electrons.” These mobile electrons allow metals to conduct electricity, absorb light, and display malleability and ductility. Metals usually crystallise in BCC, FCC or HCP structures. Their bonding is non-directional and strong, giving rise to high thermal stability.
7. Crystal Defects
Defects are irregularities in the arrangement of atoms or ions in crystals. These defects strongly influence conductivity, colour, stability and mechanical behaviour.
Types of Defects
A. Point Defects
| Type | Description | Example/Application |
|---|---|---|
| Vacancy | Atom missing from lattice site | Increases diffusion |
| Interstitial | Atom occupies a space between regular sites | Carbon in iron (steel) |
i. Stoichiometric Defects
| Type | Description | Example/Application |
|---|---|---|
| Frenkel Defect | Small ion leaves site and occupies interstitial position | AgCl |
| Schottky Defect | Equal number of cations and anions missing | NaCl, KCl |
ii. Non-stoichiometric Defects
| Type | Explanation |
|---|---|
| Metal excess defect | Due to extra cations or electrons in voids (e.g., F-centres) |
| Metal deficiency defect | Cations missing; charge balanced by oxidation state change |
Interview highlight:
F-centres (colour centres) explain why NaCl turns yellow when heated in sodium vapour.
B. Line Defects (Dislocations)
Two types: edge dislocation and screw dislocation.
These are important in materials science because they control mechanical strength.
8. Electrical Properties of Solids
Band Theory: Band theory explains conduction in solids based on allowed and forbidden energy levels.
| Type of Solid | Band Gap | Conductivity Feature | Example |
|---|---|---|---|
| Conductors | Overlapping valence and conduction bands | Free electrons | Metals |
| Semiconductors | Small band gap (1–3 eV) | Conductivity increases with temperature | Si, Ge |
| Insulators | Large band gap (> 5 eV) | Almost no conductivity | Wood, plastic |
Doping: Doping enhances semiconductor conductivity.
| Type of Semiconductor | Dopant | Charge Carriers |
|---|---|---|
| n-type | Group 15 (N, P, As, Sb, Bi) | Electrons |
| p-type | Group 13 (B, Al, Ga, In, Th) | Holes |
9. Magnetic Properties of Solids
Magnetism arises from unpaired electrons.
| Type | Feature | Example |
|---|---|---|
| Diamagnetism | Weakly repelled | Zn |
| Paramagnetism | Weak attraction; unpaired electrons | O₂, Cu²⁺ |
| Ferromagnetism | Strong attraction; domain alignment | Fe, Co, Ni |
| Ferrimagnetism | Opposing but unequal spins | Fe₃O₄ |
| Antiferromagnetism | Opposing equal spins cancel | MnO |
Interview question:
Why does Fe lose ferromagnetism above Curie temperature?
→ Thermal motion destroys domain alignment.
10. X-Ray Diffraction (XRD)
XRD is used to determine crystal structure. When X-rays hit a crystal, they diffract and create patterns that help calculate interplanar distances.
Bragg’s Law: nλ=2dsinθ
Where:
n = order of reflection
λ = wavelength
d = spacing between planes
θ = angle of incidence
Interviewers love questions like:
“How does XRD confirm crystal structure?”
Answer: By comparing measured d-spacings with theoretical lattice parameters.
Frequently Asked PGT Interview Questions
Q1. Difference between crystalline and amorphous solids?
→ Provide definition, melting point, order, anisotropy.
Q2. Why does NaCl show Schottky defect but ZnS shows Frenkel defect?
→ Due to difference in ionic size and ability to move into interstitial sites.
Q3. What is the significance of band gap?
→ Determines conductivity; small band gap → semiconductor.
Q4. Why are metals good conductors?
→ Presence of delocalised electrons.
Q5. Define F-centre.
→ Electron trapped in anion vacancy causing colour.
