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)
2. 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:
3. Surface defects- two-dimensional or planar defects
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