Question

Mark the correct statement (a) CsCl crystal show Schottky defect (b) Crystals having F-centres are coloured and paramagnetic (c) Photo sensitivity of AgBr is due to the presence of Frenkel defect in its crystals (d) all of these are correct

Short answer

(b) Crystals having F-centres are coloured and paramagnetic.

Step-by-step solution

Define Schottky Defect

A Schottky defect occurs in ionic crystals where equal numbers of cations and anions are missing from their lattice sites. This defect is typical in crystals with similar-sized ions like NaCl but is not commonly observed in CsCl due to the significant size difference between Cs and Cl atoms.

Discuss F-Centres

F-centres occur when an anion vacancy in a crystal is filled by an electron, leading to color and magnetic properties. The presence of F-centres can indeed cause crystals to become colored and paramagnetic.

Analyze AgBr and Frenkel Defects

Frenkel defects involve a cation leaving its lattice site and occupying an interstitial site. However, the photosensitivity of AgBr is attributed to the release of electrons when exposed to light, rather than the presence of Frenkel defects.

Evaluate the Statements

(a) is incorrect because Schottky defects are not typical in CsCl crystals. (b) is correct because F-centres cause coloration and paramagnetism. (c) is incorrect because AgBr's photosensitivity is due to light-induced reactions, not Frenkel defects. (d) is incorrect since not all statements are correct.

Key concepts

Schottky Defect

A Schottky defect is a type of point defect in a crystal lattice where both a cation and an anion are missing from their respective positions. This defect maintains electrical neutrality as the numbers of missing positive and negative ions are equal.
In ionic compounds like NaCl, where ions are similar in size, Schottky defects frequently occur. In such crystals, the lattice can adjust to the absence of ions without sacrificing stability due to their balanced proportions.
However, in crystals like CsCl, the ionic sizes are quite different. The large size difference between cesium (Cs) and chlorine (Cl) ions makes it difficult for the crystal to accommodate such vacancies without distorting the structure. Thus, Schottky defects are rarely seen in CsCl.

F-centres

F-centres, also known as color centers, are vacancies in a crystal lattice occupied by electrons. When an anion vacancy exists and becomes occupied by an electron, this electron can absorb part of the visible light, giving the crystal its color.
The absorption of light shifts the electron’s energy level and releases energy when the electron returns to its ground state. This mechanism imparts the crystal with both its characteristic color and paramagnetic properties.

• F-centres are responsible for bright colors seen in some alkali halides.
• Their presence indicates the ability of a crystal to conduct electricity and interact magnetically.

Through these properties, F-centres are quite significant in solid-state physics and materials science.

Photo Sensitivity of Crystals

Photo sensitivity in crystals, such as silver bromide (AgBr), involves the interaction of light with the crystal's electrons. When light hits AgBr, it excites electrons, enabling them to move into conduction bands or leave their original atoms to create electron-hole pairs.
This process results in a temporary change to the crystal's electrical conductivity and is crucial in photographic film development, where light exposure creates an enduring imprint.

• Photosensitivity is not a result of Frenkel defects. It is tied to more transient light-based reactions.
• The process is reversible; shielding from light ceases the photosensitive change.

Understanding photo sensitivity is key for applications like sensors and optical devices.

Frenkel Defect

A Frenkel defect creates a disorder within a crystal lattice by the displacement of a cation from its normal position to an interstitial site, leaving its original site vacant. This type of defect does not change the overall charge of the crystal, but it does create physical overcrowding in areas with surplus ions.
Unlike Schottky defects, which involve both cation and anion vacancies, Frenkel defects involve only one type of ion, typically the smaller and more mobile cations.

• Common in crystals such as silver halides, Frenkel defects are also evident in zinc sulfide and silver chloride.
• They can affect the crystal's ionic conductivity and mechanical properties.

These defects are significant for materials used in solid-state ionic devices and contribute to the understanding of ionic mobility in solids.