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Question

Select the correct statement regarding F-centre.

A
Electrons are held in the voids of crystals.
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B
F-centre imparts colour to the crystal.
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C
Conductivity of the crystal increases due to F-centre.
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D
All the above.
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Solution

The correct option is D All the above.
When Potassium chloride is heated in sodium vapour, we see that is acquires a yellow colour. The heating in excess sodium creates new lattice sites for sodium. To maintain the structure, an equal number of chloride sites are also created but the latter are vacant. The electron of the potassium atom that is ionized in the KCl matrix is trapped by the chloride ion vacancy. This trapped electron can be freed into the crystal by absorbing visible light and hence, the colour. A trapped electron at an anion vacancy is an electronic defect and it is known as F-centre.

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