Consequences of Metal Excess Defects
The crystals with metal excess defects are generally coloured due to the presence of free electrons in them.
The crystals with metal excess defects conduct electricity due to the presence of free electrons and are semiconductors. As the electric transport is mainly by “excess” electrons, these are called n-type (n for negative) semiconductor.
The crystals with metal excess defects are generally paramagnetic due to the presence of unpaired electrons at lattice sites.
When the crystal is irradiated with white light, the trapped electron absorbs some component of white light for excitation from ground state to the excited state. This gives rise to colour. Such points are called F-centres. (German word Farbe which means colour) such excess ions are accompanied by positive ion vacancies. These vacancies serve to trap holes in the same way as the anion vacancies trapped electrons. The colour centres thus produced are called V-centres.