Q.The presence of F-centres in a crystal makes it

An interstitial defect is a stoichiometric point defect found in non-ionic solids where extra particles squeeze into normally-empty lattice voids.

In a crystal lattice, small gaps or voids exist between the regularly arranged constituent particles. In an interstitial defect, some particles occupy these normally-vacant interstitial sites in addition to the regular lattice positions. Since no particles are removed from their normal sites, the total mass increases while the volume stays essentially the same, so the density of the crystal increases. This defect is generally shown by non-ionic solids …

F-centres are anion vacancies occupied by electrons, and these trapped electrons absorb visible light, giving the crystal a characteristic colour.

When a crystal such as NaCl is heated in an atmosphere of sodium vapour, excess Na atoms deposit on the surface, and Cl$^-$ ions diffuse to the surface to combine with them, creating anion (Cl$^-$) vacancies within the crystal. To maintain electrical neutrality, the electrons released by the ionising Na atoms occupy these anion vacancies. These electron-occupied anion vacancies are called F-centres (from German Farbe = …

Colour in alkali-metal-halide crystals comes from F-centres, a type of metal-excess defect.

When a crystal such as NaCl is heated in an atmosphere of sodium vapour, Na atoms deposit on the crystal surface and diffuse into it. Cl⁻ ions diffuse to the surface and combine with the Na atoms, producing NaCl and releasing electrons, which diffuse into the crystal and occupy anion (Cl⁻) vacancies. The anionic sites occupied by unpaired electrons are called F-centres (from German Farbenzentre, colour centres). These trapped electrons absorb energy from visible light and get excited, which imparts colour to t …

ZnS shows a Frenkel (dislocation) defect because the Zn²⁺ cation is much smaller than the S²⁻ anion and can migrate to an interstitial site.

A Frenkel defect arises in ionic solids when a smaller ion (usually the cation) leaves its normal lattice site and occupies an interstitial position, creating a vacancy at its original site while the ion itself sits between the regular lattice points.

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An F-centre is an anion vacancy that traps an electron; the trapped electron absorbs light and imparts colour to the crystal — option (c).

F-centres (from German "Farbenzentre", colour centre) arise in ionic crystals such as NaCl or KCl when they are heated in an atmosphere of the corresponding alkali metal vapour, or exposed to X-rays. Anions leave their normal lattice sites, creating anion vacancies, and to maintain electrical neutrality, an electron takes the place of the missing anion at that vacant site. This trapped electron absorbs specific wavelengths of visible light and gets excited, and the crystal, which was originally colourless, appears coloured as a result …