what metal excess defects? explain with example


Asked by Avadhut Katkar | 23rd Aug, 2018, 06:48: PM

Expert Answer:

1)    Metal Excess:

This occurs in two ways,

A. By anion vacancies:

  In this defect, a negative ion is missing from its lattice site, leaving behind a hole which is occupied by an electron and maintain the electrical balance.

  The sites which contain the electrons are trapped in the anion vacancies.
  They are known as F-centres because they are responsible for giving colour to the crystal.
B. By the presence of extra cations in the interstitial sites:
 Metal excess defect may also cause by extra cations which occupy the interstitial sites.
 Electrical neutrality is maintained by an electron present in another interstitial site.
This defect is found in the crystals which show Frenkel defect.
When ZnO is heated, it loses oxygen and turns yellow.
The excess of Zn2+ ions are trapped into the vacant interstitial sites and electrons are trapped into the neighbouring interstitial sites.

The crystals with this type of effect act as a semiconductor as they contain some free electrons.

Answered by Varsha | 24th Aug, 2018, 01:55: PM