Question

When an extra cation and electron occupies the interstitital sites in its crystal lattice, it is called:

• A. Schottky defect
• B. Metal excess defect
• C. Stoichiometric defect
• D. Frenkel defect

Answer 1

The correct option is B Metal excess defect

Presence of excess metallic cations in the crystal is called metal excess defect.
Metal excess defect can occur in two ways.
1. A negative ion may be absent from its lattice site, leaving a hole which is occupied by an electron, thereby maintaining the electrical neutrality.
2. When an extra positive ion occupies an interstitial position in the lattice followed by the inclusion of an interstitial electron to maintain the electrical neutrality.

Metal excess defect is a non-stoichiometric defect. Schottky and Frenkel defects are stoichiometric defects and have no free electrons.
Thus, correct option is (c).