Question

Frenkel defect is noticed in (a) \(\mathrm{ZnS}\) (b) AgI (c) \(\mathrm{AgBr}\) (d) all of these

Short answer

The Frenkel defect is noticed in all of these: ZnS, AgI, and AgBr.

Step-by-step solution

Understanding Frenkel Defect

A Frenkel defect occurs when an ion is displaced from its lattice site to an interstitial site. This typically occurs in ionic crystals where the difference in size between the cation and anion is large and the cation is much smaller in size, allowing it to fit into the spaces between lattice sites.

Analyzing Each Option

We have four options: ZnS, AgI, AgBr, and 'all of these.' We need to identify if Frenkel defects are commonly observed in one or more of these materials.

ZnS Analysis

ZnS (zinc sulfide) can exhibit Frenkel defects due to the small size of the Zn²⁺ ion compared to the lattice spaces available. However, Frenkel defects are more common in materials with large differences between the sizes of cations and anions.

AgI Analysis

AgI (silver iodide) is known for exhibiting Frenkel defects because the Ag⁺ ion is significantly smaller than the I⁻ ion, making it more likely to occupy interstitial positions.

AgBr Analysis

AgBr (silver bromide) also displays Frenkel defects, similarly to AgI. The silver ions are smaller and can relocate to interstitial sites within the lattice.

Conclusion

All these compounds—ZnS, AgI, and AgBr—can exhibit Frenkel defects due to the size differences in their respective cations and anions. Hence, Frenkel defects are noticed in all of these materials.

Key concepts

Ionic Crystals

Ionic crystals form part of the large family of crystalline solids. In these structures, ions are held together by electrostatic forces, creating a neat, repeating pattern. This organization is known as a crystal lattice. The fundamental building blocks in ionic crystals are positively charged cations and negatively charged anions.

These ions line up with each other to maximize attraction and minimize repulsion. This arrangement can result in remarkable properties such as high melting points and brittleness. Ionic crystals can conduct electricity when melted or dissolved in water as the ions are free to move and carry an electric current.

• They have a precise, geometric arrangement known as a lattice structure.
• The strength of the ionic bonds contributes to their high melting and boiling points.
• Common examples include sodium chloride (table salt) and calcium fluoride.

Understanding the behavior of ions within these crystals is crucial for grasping concepts like Frenkel defects. It’s the peculiar positioning and potential movement of ions within this arrangement that lead to defects.

Lattice Sites

A lattice site in a crystalline solid is a distinct position where ions reside. Picture it like a tidy, invisible grid within the material. Every ion has a specific spot, and all spots are regularly spaced. Lattice sites help maintain the structural integrity of the crystal.

In ionic crystals, these lattice sites are occupied by cations and anions, holding each other in place through electrostatic attraction. If an ion leaves its lattice spot and relocates to another part of the structure, this forms a defect.

• Each cation and anion has a specific site it should occupy.
• The uniformity of lattice sites allows for the unique properties of the crystal.
• Defects occur when ions move from these regular positions.

The Frenkel defect specifically happens when a smaller ion, usually a cation, leaves its designated site, occupying an interstitial space instead. This movement doesn't alter the overall charge balance but can affect the crystal's properties.

Cation and Anion Size Difference

The size difference between cations and anions plays a significant role in the potential for defect formation within a crystal. Anions are generally larger because they have additional electrons that increase the electron cloud size, leading to a greater radius.

Cations, on the other hand, lose electrons and therefore have smaller radii. This difference is fundamental to the occurrence of Frenkel defects, as it allows the smaller cations to move easily between lattice sites without disrupting the overall lattice structure.

• Cations are smaller because they lose electrons to gain a positive charge.
• Anions acquire electrons, making them larger.
• This size variance allows smaller cations to fit into interstitial spaces, setting up the conditions for Frenkel defects.

In materials like AgI and AgBr, we notice how the small size of silver (Ag⁺) ions compared to iodine (I⁻) or bromine (Br⁻) ions facilitates the occurrence of Frenkel defects. The smaller size facilitates their movement into spaces between larger ions, creating distinctive lattice imperfections.