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Structure of the Crystals

All the crystals are not perfect in structure. We will present 4 types of defects in the crystals' structure.

1. Point defects- zero-dimensional imperfections

Point Defects - zero-dimensional imperfections

Point defects have the volume of atomic dimensions: Examples are: foreign atoms, vacant lattice sites, extra or missing electrons.  The most common are:

a) Substitutional impurity atoms (usually large)

b) Interstitial impurity atoms (usually small)

c) Vacancies (missing atoms)

Interstitial DefectsInterstitial DefectsVacancies Vacancies Substitutional impurity atoms Substitutional impurity atoms

2. Line defects-one dimensional imperfections

Line Defects - one-dimensional imperfections

These type of defects are also called dislocations or line defects (1-D).   They are caused when a portion of the crystal slides or slips relative to another.  the plane in which the dislocation moves through the lattice is called the slip plane.

 The result is a distorted region along a line AB also called the dislocation line. There are three possible cases:

1. atoms are displaced to AB ® edge dislocation

2. atoms are displaced  || to AB  ® screw dislocation

3. atoms are displaced at other angles ® mixed dislocation:

Edge dislocation 1D Edge dislocation 1D

3. Surface defects- two-dimensional or planar defects

Surface Defects - two-dimensional imperfections

Surface defects (2-D) are the boundary between two orderly regions of a crystal.  In other words, they are separate regions having different crystallographic orientations.

  • the simplest form composed of parallel array of edge dislocations , tilt boundary
  • twin boundaries are a particular type of boundary where one portion of the lattice is a mirror image of the other; the mirror is the twinning plane growth

Twin Boundary 2DTwin Boundary 2D

4. Bulk defects- three dimensional imperfections

Bulk Defects - three-dimensional imperfections
In some cases a second phase (second compound or element)  is present in the crystal in the form of inclusions (formed during growth) or precipitates (formed during the cooldown of the crystal)  They range from 0.1 to 50 micron in size and have various shapes.  Their presence can lower the optical transparency of the crystal or impede the electrical carrier transport through the crystal.   In metals they can improve the mechanical properties of the metal.