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 Vedantu 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:
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.
A vacancy will hence be created in the original space of the cation which is left for an interstitial space.
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 –
NaCl (Sodium Chloride) (Frenkel Defect is shown in NaCl in the image above)
Zinc sulfide
Silver (I) Chloride
Silver (I) Bromide
Silver (I) Iodide
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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FAQs on Frenkel Defect
1. What are the characteristics of the Frenkel defect?
The characteristics of the Frenkel defect can be provided as follows:
This defect occurs only when the cations are small when they are compared to the size of the anions. There will be no change in the chemical properties of the solid that consist of the Frenkel defect.
The density of the solid is not affected when a Frenkel defect is observed in a solid hence making both the volume and mass remain constant.
There is also electrical neutrality being observed in the solids when Frenkel defect is present
The entropy of the solid with Frenkel increases.
2. Why is the Frenkel defect not observed in alkali metal halides?
Frenkel defect is also called the interstitial defect as the cation which is quite small in size tends to change its place from its original position to an interstitial site. It is hence a vacancy in the solid structure. This vacancy that occurs needs the ions to be very small. If the cations are not small then there is no chance for them to change places. In the case of alkali metal halides, it is seen that the cation is too large when it is compared to the anion. This is why there is no Frenkel defect observed in alkali metal halides.
3. How does anion Frenkel defect in fluorite occur?
Cation Frenkel defects are very common due to the small size that is observed in cations when compared to anions. However, the anions that are present in a fluorite structure tend to have lower electrical power than that seen in the cations and hence don’t find it very difficult to move near each other. The fluorite structure ccp cations has all the tetrahedral holes occupied by anions hence all octahedral holes will remain unoccupied. There is hence the anion Frenkel defect that is observed in the Fluorite structures. Examples of fluorite structures involve CaF2, PbF2, etc.
4. Does the Frenkel defect affect the ionic conductivity of solids?
Ionic conduction in a solid is possible when the solid consists of free electrons that can conduct electricity. This phenomenon when applied to solids with Frenkel defect it is observed that the ionic conductivity is definitely affected. It is hence that the interstitial cations that are present in the solid help in increasing conductivity. Due to the Frenkel defect the lattice energy and the stability of the solid keep on decreasing. This in turn influences the chemical properties of ionic compounds. The more Frenkel defects seen in a solid the higher will be the ionic conductivity.
5. What are the consequences of the Frenkel defect?
The consequences of the Frenkel defect can be provided as follows:
There is an increase in the dielectric constant as the charged ions come closer.
The crystal that contains the Frenkel defect tends to conduct electricity to a small extent by using the ionic mechanism.
The density of the solid with Frenkel defect will remain the same.
Frenkel defect while it hardly affects the chemical properties it will increase the entropy of the solid in general due to the randomness that is observed in the interstitial occupying cations. To know more regarding the Frenkel defects students can also check out Vedantu NCERT Solutions for Chemistry which provides more details on the same.
