Question

Select the correct statements regarding F-centres
A. Electrons are held in the voids of crystals
B. F-centre impart colours to the crystals
C. Conductivity of the crystal increases due to F-centre
D. All of the above

Answer 1

Hint: F center is described as a type of crystallographic defect. There are many types of defects like Schottky, Frenkel defects that can be shown in crystal structures. F center is one of them which showed defects due to vacant orbitals.

Complete answer:
When Potassium chloride is heated in sodium vapour then we observe that it acquires a yellow colour and if we heat it in excess then sodium creates new lattice sites for sodium. Hence to keep the structure stable we have to add an equal number of chloride sites that are 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 move freely into the crystal by absorbing visible light and hence it imparts the colour. A trapped electron at an anion vacancy is an electronic defect and it is known as F-centre.

From the above discussion we can say that option D is the correct answer.

Note:
Generally, F-center defect occurs due to an anionic vacancy present in a crystal which is filled by one or more electrons generally based on charge on the missing ion in the crystal. Electrons which are present in this vacancy tries to absorb light in a visible spectrum in such a way that the transparent material seems to be coloured.