defects in crystals Defects in solids are of two categories.

1. Point  defects,  which  are  localized  within  a very small region consisting of few atomic sites.

2. Line or surface defects, which comprise of many atoms in one or two dimensions.

1. Point defects : Simplest possible defect in a crystal is a `vacancy',   the  absence  of  an  atom  from  a  lattice  point. `Interstitial'  is  an  atom  that  occupies a void space. In a crystal  an  impurity atom with a size such that it fits into the interstitial  void  occurs.  These  are  called  interstitial  - impurity.

In   substitution  impurity  an  impurity  atom  occupies  a position of the parent atom of the crystal. It happens when the size of the impurity atom is of the order of parent atom.

In ionic crystals, some positive and negative ions are found absent from their respective positions in proportion to the ratio of the ions in the compound. In Nacl crystal for example Na⁺ and Cl^-  ions  will  be  missing  in ratio 1:1, while in a crystal of Cu₂O,  copper  vacancies  will  be  twice  the  number  of oxygen vacancies.  These  missing  ions are found on the surface of the crystal.  This type of  defects is called Schottky defect (see fig. d1)

It is also possible that an ion is absent from a site, while an extra ion of same kind is found to be in interstitial position elsewhere  in the crystal, keeping the overall charge neutrality. This  type  of  defect  is  known  as  Frankel defect. These are created when a crystal is exposed to radiation (fig.d2).

2.  Line or surface defects : also called dislocations. Such defects generally  run  through curved routes in the crystals, but can be understood by two simple types, in edge and screw dislocations.

In  edge  dislocation  a  plane of atoms abruptly terminates which resembles a knife edge, as illustrated in the fig.d3.

In  screw dislocation, normal crystallographic planes appear to spiral about a row of atoms (fig.d4).

Dislocations  are  present  in  a crystal that has suffered accidents  during its growth. They can be removed by heating the crystal to a high temperature, and then cooling slowly. The process is called annealing.