Question

Which of the following compounds is likely to show both Frenkel and Schottky defects in its crystalline form?

• A. ZnS
• B. CsCl
• C. KBr
• D. AgBr

Answer 1

The Correct Option is A

Solution:

In solid-state chemistry, defects in crystalline structures are categorized into different types. Two common types of point defects in crystals are Frenkel defects and Schottky defects.

Frenkel Defect: This defect occurs when an ion or atom is displaced from its regular lattice site to an interstitial site, creating a vacancy at the original position and an interstitial defect. Frenkel defects typically occur in crystals with ions of different sizes, where the smaller ion can fit into the interstitial site without disrupting the lattice structure.

Schottky Defect: This defect involves the simultaneous vacancy of an equal number of cations and anions in the crystal lattice, maintaining the overall neutrality of the crystal. Schottky defects typically occur in ionic compounds where both cations and anions are of similar size, making it easier for both to leave their lattice sites.

Now, let's evaluate the given compounds:

Option 1: ZnS (Zinc Sulfide)

Zinc sulfide (ZnS) is a compound where Zn2+ ions and S2- ions have a noticeable size difference. This compound is capable of exhibiting both Frenkel and Schottky defects:

Schottky Defect: ZnS can have vacancies for both Zn2+ and S2- ions due to their similar sizes.

Frenkel Defect: Zn2+ ions, being small, can also move to interstitial sites, creating Frenkel defects.

Option 2: CsCl (Cesium Chloride)

CsCl has a structure where Cs+ ions and Cl- ions are of similar size, and this allows CsCl to form Schottky defects easily. However, due to the large size of Cs+ ions, this compound is less likely to exhibit Frenkel defects.

Option 3: KBr (Potassium Bromide)

Potassium bromide (KBr) typically exhibits Schottky defects as both the K+ and Br- ions have similar ionic sizes, allowing for the simultaneous formation of vacancies for both ions. However, KBr does not easily show Frenkel defects as the ions are not significantly different in size.

Option 4: AgBr (Silver Bromide)

Silver bromide (AgBr) is an ionic compound where Ag+ ions and Br- ions are quite different in size. This compound typically shows Schottky defects due to the similar ionic sizes of Ag+ and Br-. However, it is unlikely to show Frenkel defects as the Ag+ ions are large and don't easily fit into interstitial sites.

Conclusion:

The compound that is most likely to show both Frenkel and Schottky defects is ZnS (Zinc Sulfide). Therefore, the correct answer is:

Option 1: ZnS