Question

Mention one consequence of metal excess defect.

Answer 1

We must remember that the metal excess defect is caused due to the presence of an extra cation in the interstitial spaces present in the crystalline structure. Due to the heating over atmosphere vapour of the alkali metal, anion vacancies are created in the alkali metal halides.

Complete step by step answer:
To start with the discussion let us understand the concept of metal excess defects. This defect is caused due to the presence of an extra cation in the interstitial spaces present in the crystalline structure.
Due to the heating over atmosphere vapour of the alkali metal, anion vacancies are created in the alkali metal halides. This anion in turn combines with the newly generated cation through the crystal diffusion. These defects are caused in 2 cases which are:
Due to the anionic deficiencies- Found in alkali halides like NaCl and KCl
Due to the presence of extra cations at interstitial sites- Found in an element that shows the Frenkel effect like ZnO.
Now various consequences are caused due to the presence of this metal excess defect but one of the most common among them all is the presence of colour in the crystals which have excess metal defects.
This phenomenon is caused when white light falls on the crystal which results in the absorption of energy by the trapped electron and thus exciting them from the ground to a higher energy state and this transition is the reason for the presence of colour in this type of crystal

Note:
Metal defects usually produce coloured crystals and these crystals can be of use. Also due to the metal excess defect the crystal starts conducting electricity which can also be used in many places. The reason behind the crystal becoming conductive in nature is due to the presence of excess cations.