Introduction to Solid State

Jun 22, 2025

Overview

This lecture focused on the main concepts of the Solid State chapter, crystal structure, unit cell, packing efficiency, and important points defining imperfections.

Classification of Solids

  • Solids are mainly divided into crystalline (long range order) and amorphous (short range order).
  • Crystalline solids have a sharp melting point, whereas amorphous solids have a range.
  • Crystalline solids are anisotropic (different properties in different directions), amorphous are isotropic.

Types of Solids

  • Ionic solids: like NaCl, grid of positive and negative ions.
  • Metallic solids: 3D grid of metal atoms, free electrons.
  • Covalent/Network solids: diamond, graphite, quartz (very hard, high MP).
  • Molecular solids: polar (HтВВO), non-polar (COтВВ, HтВВ), hydrogen-bonded.

Unit Cell and Crystal Lattice

  • Unit cell: the smallest, repeatedly repeating unit of a crystal.
  • 7 crystal systemsтАФcubic, tetragonal, orthorhombic, etc.; each with different parameters and angles.
  • In cubic ╬▒ = ╬▓ = ╬│ = 90┬░, a = b = c.
  • Face Centered (FCC), Body Centered (BCC), Simple CubicтАФthree main cubic types.

Packing Efficiency and Void

  • Packing efficiency = (occupied space / total space) ├Ч 100.
  • Highest in FCC (74%), BCC (68%), Simple Cubic (52%).
  • Coordination number 12 in FCC, 8 in BCC, 6 in simple cubic.
  • FCC has tetrahedral (8), octahedral (4) voids (if c=4).
  • Radius ratio tetrahedral: 0.225, octahedral: 0.414, cubic: 0.732.

Imperfections/Defects in Solids

  • Point defects: vacancy, interstitial, Schottky (cation-anion pair missing), Frenkel (cation missing, in interstitial site).
  • Stoichiometric and non-stoichiometric defectsтАФformula/quantity imbalance.
  • Impurity defect: e.g., adding SrClтВВ in NaCl.

Density Formula

  • Density = (Z ├Ч M) / (a┬│ ├Ч NтВР)
  • Z = effective atoms/unit cell, M = molar mass
  • a = edge length, NтВР = Avogadro number

Key Terms & Definitions

  • Crystalline solid тАФ solid with long range order
  • Amorphous solid тАФ irregular, short range order
  • Unit cell тАФ smallest repeatable unit of a crystal
  • Packing efficiency тАФ percentage of filling in solid
  • Coordination number тАФ number of nearest neighbor atoms
  • Void тАФ empty space (tetrahedral, octahedral)
  • Schottky/Frenkel defect тАФ defects in ionic solids

Action Items / Next Steps

  • Revise the table of crystal systems given in NCERT.
  • Solve solid state questions from 2023 advanced.
  • Prepare short notes on density, packing efficiency, and defects.

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