The Solid State - Result Question 37
####37. The appearance of colour in solid alkali metal halides is generally due to
[2006]
(a) Schottky defect
(b) Frenkel defect
(c) Interstitial positions
(d) F-centres
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Solution:
- (d) When the crystal is irradiated with white light, the trapped electrons in a hole absorbs photon for excitation from ground state to excited state. This gives colour to the compund.
Alkali metal halide may have excess metal ion if a negative ion is absent from its lattice site, leaving a hole, which is occupied by electron to maintain electrical neutrality. The ‘holes’ occupied by electrons are called F-centres or colour centres.
