Which of the following shows both, Frenkel and Schottky defect?
AgBr
Understanding Crystal Defects: Frenkel vs. Schottky
In crystalline solids, atoms or ions are arranged in a highly ordered, repeating pattern. However, this perfect arrangement can sometimes be disturbed, leading to what are called crystal defects. These defects can significantly affect the physical and chemical properties of the crystal. Two common types of point defects (defects affecting a single point or a few points) in ionic crystals are the Frenkel defect and the Schottky defect.
What are Frenkel and Schottky Defects?
Let's define these two important crystal defects:
- Frenkel Defect: This defect occurs when an ion leaves its regular lattice site and occupies an interstitial site (a space between the regular lattice sites). This creates a vacancy at the original site and an interstitial defect at the new location. Cations are usually smaller than anions, so they are more likely to move into interstitial sites. Frenkel defects maintain the overall electrical neutrality and the stoichiometry (ratio of cations to anions) of the crystal. This type of defect is favored in ionic compounds where there is a large difference in the size of cations and anions, and the coordination number is low.
- Schottky Defect: This defect involves a pair of vacancies – one cation vacancy and one anion vacancy. A cation leaves its lattice site, and an anion also leaves its lattice site. This results in two vacancies. The number of missing cations and anions is equal, so the crystal remains electrically neutral and stoichiometric. Schottky defects lead to a decrease in the density of the crystal. This defect is favored in ionic compounds where the sizes of cations and anions are roughly similar, and the coordination number is high.
Analyzing the Options for Crystal Defects
Now let's examine the given options to see which compounds exhibit these defects:
| Compound | Typical Defect(s) Observed | Reasoning |
|---|---|---|
| $\text{KCl}$ | Schottky defect | $\text{KCl}$ is an alkali halide. Alkali halides typically show Schottky defects due to the similar sizes of alkali metal cations ($\text{K}^+$) and halide anions ($\text{Cl}^-$) and high coordination number. |
| $\text{AgCl}$ | Frenkel defect (primarily) | $\text{AgCl}$ shows primarily Frenkel defects. The $\text{Ag}^+$ ion is relatively small compared to $\text{Cl}^-$ and can easily move into interstitial sites. While Schottky defects can theoretically exist in any ionic crystal, they are less dominant in $\text{AgCl}$ compared to Frenkel defects. |
| $\text{AgBr}$ | Both Frenkel and Schottky defects | $\text{AgBr}$ is unique among the silver halides and alkali halides discussed here because it exhibits both types of defects significantly. The $\text{Ag}^+$ ion is small enough to create Frenkel defects by moving to interstitial positions. At the same time, due to factors related to lattice energy and ion sizes, $\text{AgBr}$ also readily forms Schottky defects. |
| $\text{NaCl}$ | Schottky defect | $\text{NaCl}$ is also an alkali halide, similar to $\text{KCl}$. It primarily shows Schottky defects due to the similar sizes of $\text{Na}^+$ and $\text{Cl}^-$ ions and high coordination number. |
Based on this analysis, $\text{AgBr}$ is the compound that shows both Frenkel and Schottky defects simultaneously to a significant extent.
Summary of Defects in $\text{AgBr}$
$\text{AgBr}$ stands out because:
- The $\text{Ag}^+$ ion is relatively small, making it possible for it to move from its lattice site into an interstitial position, leading to a Frenkel defect (vacancy at lattice site + interstitial cation).
- The lattice structure and ion sizes in $\text{AgBr}$ also facilitate the formation of cation-anion vacancy pairs, characteristic of a Schottky defect.
Therefore, $\text{AgBr}$ provides a good example where both Frenkel and Schottky defects coexist.
| Compound | Predominant Defect | Can Show Both? |
|---|---|---|
| $\text{NaCl}$ | Schottky | Rarely (compared to Schottky) |
| $\text{KCl}$ | Schottky | Rarely (compared to Schottky) |
| $\text{AgCl}$ | Frenkel | Rarely (compared to Frenkel) |
| $\text{AgBr}$ | Both | Yes (significant presence of both) |
| $\text{ZnS}$ | Frenkel | Rarely (compared to Frenkel) |
| $\text{CsCl}$ | Schottky | Rarely (compared to Schottky) |
The question asks which compound shows *both*. While other compounds might theoretically show traces of the less favored defect type, $\text{AgBr}$ is commonly cited as a material where both Frenkel and Schottky defects are significantly present and contribute to its properties, such as its unique behavior in photography.
Revision Table: Key Concepts in Crystal Defects
| Defect Type | Mechanism | Effect on Density | Favored By | Example |
|---|---|---|---|---|
| Frenkel Defect | Ion moves to interstitial site, leaving a vacancy | No change (or slight increase due to expansion) | Large size difference between ions, low coordination number | $\text{AgCl}$, $\text{ZnS}$ |
| Schottky Defect | Equal number of cation and anion vacancies created | Decreases | Similar size of ions, high coordination number | $\text{NaCl}$, $\text{KCl}$, $\text{CsCl}$ |
Additional Information: Types of Point Defects
Frenkel and Schottky defects are examples of stoichiometric point defects in ionic crystals, meaning they do not alter the overall stoichiometry of the compound. Point defects can be categorized further:
- Stoichiometric Defects: These are defects that do not disturb the stoichiometry of the solid. Frenkel and Schottky defects fall into this category for ionic compounds. In non-ionic solids, vacancies and interstitial atoms (different atom occupying an interstitial site, not from the lattice) are stoichiometric point defects.
- Non-stoichiometric Defects: These defects cause the ratio of cations to anions to deviate from the ideal stoichiometric formula. Examples include metal excess defects (due to anion vacancies or extra cations in interstitial sites) and metal deficiency defects (due to cation vacancies or extra anions in interstitial sites).
- Impurity Defects: These occur when foreign atoms are present in the crystal lattice. They can replace host atoms (substitutional impurity) or occupy interstitial sites (interstitial impurity). Doping semiconductors is an application of creating controlled impurity defects.
Understanding these different types of defects is crucial for explaining many solid-state phenomena, including electrical conductivity, diffusion, and mechanical strength.
Important Questions from Electrochemistry
Identify transition metal complexes which are not octahedral in shape.
(A) [Co(NH₃)₆]³⁺
(B) [Ni(CO)₄]
(C) [CoCl(NH₃)₅]²⁺
(D) [CoCl₂(NH₃)₄]⁺
(E) [PtCl₄]²⁻
Choose the correct answer from the options given below:
The product of complete hydrolysis of XeF₆ in the following reaction is:
XeF₆ + H₂O → ? HF
In a reaction A and B react to form product. The initial rate of reaction (ro) was determined using different initial concentrations of A and B as shown below:
A/mol L-1 B/mol L-1 ro/mol L-1 s-1 0.10 0.30 6.81 × 10-4 0.10 0.10 2.27 × 10-4 0.20 0.30 13.62 × 10-4 What is the initial rate of reaction (ro) when the critical concentration of A and B is 0.50 mol/L and 0.50 mol/L, respectively?
In which of the following actinoid elements 6d subshell is vacant?
The role of a catalyst is to change:
