Question

The appearance of colours in solid metal halides is generally due to which of the following?
(a) f-centres
(b) Frenkel defect
(c) Schottky defect
(d) interstitial defect

Answer 1

Colour shown by alkali metal halide is due to metal excess defects which occur when there is a greater number of cations (positively charged ions) than anions (negatively charged ions). To balance the stoichiometry, an electron goes to the lattice site which impart colour to the crystal.

Complete step by step answer:
When ionic or solid metal halide is heated, it undergoes metal excess defects. During heating anions from the ionic metal halide crystal goes out and leaves a vacant space in its interstitial site. Electron then occupies that vacancy and during this it absorbs light in the visible region and emits colour.
In option (a) f-centres is a non-stoichiometric metal excess defect in which electrons are trapped in anion vacancies are called f-centres as they show colours upon excitation. So, f-centre is responsible for colour appearance in solid metal halides.
In option (b) Frenkel defect is joint defect in crystal solids where a cation smaller in size dislocates from its positive to create a vacancy in the interstitial site of the ionic crystal. It does not bring any colour to the compounds.
In option (c), Schottky defect occurs due to the dislocation of oppositely charged ions from their lattice sites and occupies apposition in the interstitial site. This creates oppositely charged vacancies.
In option (d) interstitial defect is a crystallographic defect in which atoms occupy a site of the crystal lattice of site at which there is already an atom. This leads to more atoms than the vacancies.

So, the correct answer is Option A .

Note:
${\text{NaCl}}$ is a solid metal halide and it has f-centres due to which ${\text{NaCl}}$ shows yellow colour upon heating in the atmosphere of sodium. The Cl- anions leave the lattice site which is filled by the electron released by the Sodium atom in the atmosphere. Another example is ${\text{KCl}}$ which shows violet colour.