A metal excess defect is one of the defects seen in the crystal structures. The other defect is metal deficiency defect. These are the non-stoichiometric inorganic solids that contain constituent elements in non-stoichiometric ration because of the defects in their crystal structures.
Metal excess defect:
This defect is caused due to anionic vacancies and by the presence of extra cations in the interstitial sites. When alkali metal halides are heated in an atmosphere of vapour of the alkali metal, anion vacancies are created. This anion is then diffuse to the surface of the crystal and combine with newly generated metal cations. The electron is lost by the metal atom is then diffuses the crystal and occupy the anionic vacancy site and form F-centres inside the crystal. These F-centres give different colours like NaCl gives a yellow colour. KCl gives a violet colour and HCl gives pink colour.
Types of metal excess defect:
There are two types of metal excess defect. Those two types are listed below
Metal excess defect due to anionic vacancies:
Anionic vacancies are found in alkali halides such as NaCl and KCl. in this defect, the negative ion is missing from the lattice site leaving behind a hole which is then occupied by an electron to maintain electric balance. These electrons are trapped in the anion vacancies.
When NaCl is heated in an atmosphere of Na vapours. The excess of Na atoms deposition the surface of NaCl crystal Cl- ions then diffuse to the surface where the combine with Na+ ions which becomes due to losing electrons.
Metal excess defect due to the presence of extra cations at interstitial sites
In this defect, on heating the compound, it releases extra cations. These cations occupy the interstitial sites in crystals and the same number of electrons goes to neighbouring interstitial sites. Extra cations in the interstitial site can be found in crystals that exhibit Frenkel defect such as ZnO.
Zinc oxide is white at room temperature, on excess heating, it turns yellow as it loses oxygen.