Question

When electrons are trapped into the crystal in anion vacancy, the defect is known as
A) Schottky defect
B) Frenkel defect
C) Stoichiometric defect
D) F-centres

Answer 1

Hint: There are various types of defects that arise in a crystal. In some defects the cation is removed, in some defects both the cation and anion are removed and in a special type of defect an anion is removed from the crystal.

Complete Step by Step Answer:
In a crystal sometimes some anion is removed to form vacant sites in the crystal. To make the complex neutral an electron occupies the vacant position of the crystal and gives rise to this defect. This vacant centre is called F-centre.

When a cation or anion leaves its site and occupies the interstitial position of a crystal lattice then the defect arises is called Frenkel defect.

In Schottky defect the equal number of anions and cations are removed from the crystal lattice to produce vacancy in the crystal which in turns causes defect in the crystal.

In this defect at first one cation is removed from the crystal solid. Now to maintain the neutrality of the crystal lattice an anion is also removed from the crystal lattice.
Thus the correct option is D.

Note: Due to this defect the salt or crystal gives colour. In this defect the electron goes to the higher energy level and when it comes back to the ground state level it gives colour in the visible range. Like sodium chloride gives yellow colour.