In an ideal crystal atoms or molecules are arranged in a regular and periodic manner. Any deviation in a crystal from a crystal from a perfect periodic structure is called IMPERFECTION. Real crystals are always imperfect in some respect as ideal crystal do not exist in nature and cannot even be produced artificially. Imperfections have some advantages as the resistivity, conductivity, colour, luminescence , mechanical and plastics properties of matter can be controlled by regulating imperfection. Point defects are localized defects which means that in the same crystal different types of points are observed at its different parts.

The crystal defects are classified into the following types:

• POINT DEFECTS which are zero dimensional defects.
• LINE DEFECTS which are one dimensional.
• SURFACE DEFECTS which are two dimensional defects and
• VOLUME DEFECTS which are two dimensional defects.