Imperfections or defects: Any deviation from the perfect atomic arrangement in a crystal is said to contain imperfections or defects. In fact, using the term “defect” is sort of a misnomer since these features are commonly intentionally used to manipulate the mechanical properties of a material. Adding alloying elements to a metal is one way of introducing a crystal defect. Crystal imperfections have strong influence upon many properties of crystals, such as strength, electrical conductivity and hysteresis loss of ferromagnets. Thus some important properties of crystals are controlled by as much as by imperfections and by the nature of the host crystals.

1. The conductivity of some semiconductors is due entirely to trace amount of chemical impurities.
2. Color, luminescence of many crystals arise from impurities and imperfections
3. Atomic diffusion may be accelerated enormously by impurities or imperfections
4. Mechanical and plastic properties are usually controlled by imperfections

Imperfections in crystalline solids are normally classified according to their dimension as follows

1. Point imperfections (Zero dimensional defects)
2. Line imperfections (one dimensional defects)
3. Plane or surface imperfections (Two dimensional defects)
4. Volume imperfections (three dimensional defects)

Point Defects:- Point defects are where an atom is missing or is in an irregular place in the lattice structure. Point defects include self interstitial atoms, interstitial impurity atoms, substitutional atoms and vacancies.

Line Imperfections:

In linear defects groups of atoms are in irregular positions. Linear defects are commonly called dislocations. Any deviation from perfectly periodic arrangement of atoms along a line is called the line imperfection.

Planar defects: which are interfaces between homogeneous regions of the material. Planar defects include grain boundaries, stacking faults and external surfaces.

Volume Defects: Voids are regions where there are a large number of atoms missing from the lattice. The image to the right is a void in a piece of metal. The image was acquired using a Scanning Electron Microscope (SEM).