Question

Explain Point defects in crystals.

Answer 1

In any crystal, at all temperatures above absolute zero, there are always some free
atoms present which lodge themselves anywhere other them their scheduled lattice sites.
This give rise to point defects.
Point defects can be classified into the following categories:

Vacancy defects: This is a defect in which an atom is missing from a regular lattice site, I. Such vacancies arise in close packed structures, i.e., metallic structures. 
Interstitial defects:  An interstitial defect is an imperfection in which an atom that has moved from its
regular lattice dates lodges itself in the interstices i. e., the void space.

Substitutional impurity:  When a regular atom leaves behind a vacancy and goes to the interstices the vacant lattice site can be occupied by a foreign atom.
Interstitial impurity: This is the kind of imperfection when a foreign atom lodges itself in the interstices. Thus can oxide only when the foreign atomism substantially smaller than the host atom.
Schottky defect: In an ionic crystal, a pair of vacancies arise which leas to missing of one Casio and one anion from the regular lattice sites. This is known as Schottky defect in which the charge neutrality is maintained.

Frankel defect: The cations are v smaller than anions. In ionic crystals if a cation leaves its regular lattice site and lodges itself in the interstices it is called Frenkel defect. It is a combination of a cation vacancy and one intersticial defect.