What metal excess defects? explain with example

CBSE Class 12
1

what metal excess defects? explain with example

2

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.
    image

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.
    image