[Chemistry Class Notes] Frenkel Defect Pdf for Exam

Introduction

Frenkel defect which was named after the scientist Yakov Frenkel is the defect that is observed in the formation of lattice crystals. These lattice crystals will consist of cations that have a small size which is usually displaced from their original place and hence a vacant site is seen in the lattice. This topic that is given under the CBSE Class 12 Chemistry syllabus is of high importance as it helps students understand why solids tend to change shape due to various defects. Here is a detailed outline on the Frenkel Defect – Definition, Example, Reason, and Number Calculation which is provided by that helps them to understand the topics and the concepts that are involved.

Formation of Frenkel Defect

There is various ways in which the Frenkel defect can be formed. These ways through which the Frenkel defect is formed can be given as follows:

1. A cation tends to leave the lattice site’s original position and leave to another interstitial site depending on its size and the amount of space that is present.

2. A vacancy will hence be created in the original space of the cation which is left for an interstitial space.

3. This cation that is present between the interstitial space tends to occupy such a place where it gets lesser repulsion and hence is able to occupy the interstitial space.

Occurrence of the Imperfections in Solid Crystals

One of the most important characteristics of crystals that sets them apart from amorphous solids is the perfect alignment of ions. However no solid is perfectly aligned and hence the defects are observed which are called the lattice defects.

The meaning of defect is shortcomings or imperfections. Solid crystals also show some kinds of defects which we study in solid-state chemistry. Some units of the crystals may have one or more atoms less than other ideal units of crystals. These imperfections of crystals are called defects in crystals. In other words, the interruptions in regular patterns in crystalline solids are called crystallographic defects. There are many types of crystallographic defects such as point defects, line defects, planar defects etc. Frenkel defects are point defects. We will discuss the Frenkel defect in this article in detail.

Frenkel Defect Definition 

When an atom or smaller ion (generally a cation) leaves its place in the lattice, creating a vacancy, and becomes an interstitial by lodging in a nearby location. Thus, a vacancy is created in the lattice. This type of defect is called Frenkel defect. It was discovered by soviet physicist Yakov Frenkel. It is a type of point defect which is also known as dislocation defect.

Frenkel Defect Example 

Following are examples of Frenkel Defect –

Reason of Frenkel Defect 

When the ionic compound size of anions is much larger than cations then Frenkel Defect occurs. Due to the size difference in ions, ions occupy an interstitial position in the lattice. Ionic crystals having Frenkel defects also remain neutral in nature. As the number of cations and anions remain equal. Thus, it can be said that Frenkel defects are shown by those ionic solids which have large size differences between the cation and anion. 

Frenkel defect doesn’t have any impact on density, mass, or volume of the crystal as in this defect migration of ions takes place within the crystal. So, the density, mass, and volume of the ionic crystal remain the same even after having a Frenkel defect.

Calculation of Number of Frenkel Defects 

Number of Frenkel Defects can be calculated by the following formula – 

[sqrt{N{N}’} e^{- -}frac{Delta H}{2RT}]

Where N = normally occupied positions 

N’ = the number of available positions

∆H = enthalpy released by 1 Frenkel defect       

R= gas constant 

T = temperature

Like the Frenkel defect, Schottky defect is also a point defect in crystalline solids. Even a few crystalline ionic compounds such as silver bromide exhibit both Schottky and Frenkel defects as well. 

Difference between Schottky and Frenkel Defect

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