Various point defects in crystals:

Point Defects are imperfect point- like regions, one or two atomic diameters in size and hence called as zero dimensional defects or imperfections.

There are different kinds of point defects like Vacancies, Schottky defect, Substitutional Impurity, Interstitial impurity and Frenkel defects.

Vacancies:

• If an atom is missing from its normal site in the matrix, the defect is called a vacancy defect. It could be a single vacancy, divacancy or a trivacancy. In metals vacancies and created by thermal excitation. When the temperature is sufficiently high, as the atoms vibrate around their regular positions, some acquire enough energy to leave the site completely. A vacancy is created when the regular atom leaves,.

Schottky imperfection:

• A pair of one cation and one anion can be missed from an ionic crystal. Such a pair of vacant ion sites is called Schottky imperfection. This type of defect is dominant in alkali halides.

   

Substitutional Impurity:

• It refers to a foreign atom that substitutes for or replaces a parent atom in the crystal. Pentavalent or trivalent impurity atoms doped in Silicon or Germanium are also substitution impurities in the crystal.

   

Interstitial impurity.:

• An interstitial defect arises when an atom occupies a definite position in the lattice that is not normally occupied in the perfect crystal. In crystals, packing density is always less than 1. If a small sized atom occupies the void space in the parent crystal without disturbing the parent atoms from their regular sites, then it is called as ‘interstitial impurity’.

   

Frenkel  defects:

• In ionic crystals, an ion displaced from a regular site to an interstitial site is called ‘Frenkel imperfection’.
• As cations are generally the smaller ones, it is possible for them to get displaced into the void space. Anions do not get displaced as the void space is too small compared to the size of the anions. A Frenkel defect does not change the overall electrical neutrality of the crystal. This type of defect occurs in silver halides and$CaF_2$.