What are the various types of point defects? How are they caused?

(i) Vacancies: This refers to a missing atom or a vacant atomic site because of absence of a matrix atom.

Missing  of  one  cation  and  one  anion  ion  in  an  ionic  crystal  is known as Schottky imperfection. Electrical neutrality is sustained in this type of imperfection which is observed in alkali halides.

(ii) Interstitialcies: An extra atom (substantially smaller than parent atoms) enters interstitial void or space between regularly positioned atoms. Vacancy and interestitialcy are, hence inverse phenomena.

Displacement  of  cation  ions  from  a  lattice  site  into  the  void  space  is  known as Frankel imperfection and this results in creation of vacancy. An imperfection don't affect the overall electrical neutrality of the crystal.

Compositional defect:

(iii) Substitutional impurity: Presence of foreign atom in place of a matric atom is known as substitutional impurity.

If  a  foreign  atom  occupies  a  vacant  position  within  crystal  lattice,  this  defect  is known as interstitial impurity.

(iv)  Electronic  defects:  Errors  in  charge  distribution  in  solids  are  termed  as  electronic defects.  There is departure from the normal regularity of charge distribution. This effect is responsible for the operation of p-n junction and transistors.

Point defects formed by

(i) Thermal fluctuations during preparation of crystals.

(ii) Quenching (quick cooling) from a higher temperature

(iii) Severe deformation such as hammering or rolling.

(iv) External  bombardment  by  atoms  or  high  energy  particles( for example Cyclotron of nuclear reactor).