Defects in crystals refer to imperfections or irregularities in the orderly arrangement of atoms in
a crystal lattice. These defects can have significant effects on the physical, chemical, and
electrical properties of materials. There are several types of crystal defects, categorized based
on their dimensional characteristics:
1. Point Defects
Vacancies: Missing atoms from their lattice sites, which can result from thermal
vibrations or other causes.
Interstitials: Extra atoms positioned in spaces (interstices) between the regular lattice
sites.
Substitutional Impurities: Foreign atoms replacing host atoms in the lattice.
Frenkel Defect: A combination of a vacancy and an interstitial defect. An atom leaves
its lattice site and occupies an interstitial site.
Schottky Defect: A type of vacancy defect where equal numbers of cations and anions
are missing from their lattice sites, maintaining charge neutrality.
2. Line Defects (Dislocations)
Edge Dislocation: A defect where an extra half-plane of atoms is inserted in the lattice.
The distortion extends along a line, hence the name.
Screw Dislocation: The crystal layers are shifted relative to each other, causing a
helical structure along the defect line.
3. Surface Defects (Planar Defects)
Grain Boundaries: The regions between different crystal grains in a polycrystalline
material. These boundaries disrupt the orderly arrangement of atoms.
Twin Boundaries: Mirror-image defects where two parts of the crystal lattice are
symmetrically arranged.
Stacking Faults: A disturbance in the stacking sequence of atomic planes, common in
metals like face-centered cubic structures.
4. Volume Defects
Pores or Voids: Empty spaces within the crystal, which can occur due to insufficient
material during the crystallization process.
Inclusions: Foreign materials trapped inside the crystal, such as another phase or an
impurity.
These defects play a crucial role in influencing properties such as mechanical strength,
electrical conductivity, and optical behavior. For example, defects can strengthen materials
(via mechanisms like dislocation hardening) or degrade them (such as by increasing
brittleness).
