CBSE Class 12-science Answered

Metal excess defect is one of the reason for Non-stoichiometric defect.

Metal excess defects occurs due to:

1.Anion vacancies

2.Presence of extra cations at interstitial sites

1.Anion vacancies: In this case, negative ions may be missing from their lattice sites leaving holes in which the electrons remain entrapped to maintain the electrical neutrality. The holes that are occupied by electrons are called ‘F’ centres (or colour centres) and are responsible for the colour of the compound and many interesting properties.

2.Presence of extra cations at interstitial sites: In this case, there are extra positive ions occupying interstitial sites and the electrons in another interstitial sites to maintain electrical neutrality. The defect may be visualised as the loss of non-metal atoms which leave their electrons behind. The excess metal ions occupy interstitial positions. This defect is similar to Frenkel defect and is found in crystals having Frenkel defects.