Schottky Defect – Definition, Explanation, Properties, Examples

Schottky defect is a type of point defect that occurs in ionic crystals. It involves the creation of vacancies in both the cation and anion position of crystal lattice. It is commonly observed in ionic compounds with similar sizes of cations and anions.

In this article, we will learn in detail about Schottky defect, its examples, properties and formula to calculate Schottky defect. We will also compare Schottky defect with Frenkel defect.

What is Schottky Defect?

Schottky Defect is a point defect in which an equal number of cations and anions are missing from their lattice sites. Since, there is vacancy at lattice sites, Schottky defects are also called vacancy defects. In this defect, the ions do not move to interstitial site or replaced by another atoms or ions, they are simply missing or absent from their lattice site, thus creating a defect in the lattice structure the. Schottky defect is predominantly found in ionic compounds.

Schottky Defect Examples

Example of a Schottky defect can be seen in ionic compounds like sodium chloride (NaCl), where a sodium ion (Na⁺) and a chlorine ion (Cl^–) are missing from their regular positions in the crystal lattice, leaving behind a vacancy in their place. This creates a pair of Schottky defects, one for the cation (Na⁺) and one for the anion (Cl^–).

Some other common examples of salts where the Schottky defect is prominent include Potassium Chloride (KCl), Potassium Bromide (KBr), Caesium Chloride (CsCl), and Silver Bromide (AgBr).

Properties of Schottky Defect

The characteristics of Schottky Defect is mentioned below:

• Cations and Anions are of comparable size in the Schottky defect.
• An equal number of cations and anions are missing from their normal lattice sites, maintaining the overall charge neutrality of the crystal.
• Schottky defects tend to occur in crystals with high coordination numbers.
• The concentration of Schottky defects increases with temperature, as higher temperatures provide more energy for ions to move from their lattice sites.
• Atoms also move out of the crystal permanently.

Causes of Schottky Defect

The causes of Schottky Defect is tabulated below:

• High degree of ionic compound.
• High coordination number of anion.
• Small difference between size of cation and anion.
• Schottky defect can be created due to high temperature or radiation
• Densely packed structure is also responsible for Schottky defect because of easy vacancy creation

Effects of Schottky Defect

The consequences of Schottky defect is required to understand for processing the materials.

• As the number of ions decreases, mass decreases. However, volume remains unchanged . Hence the density of substances decreases.
• The number of missing cations and anions is equal electrical neutrality of the compound is preserved.
• This defect is found in ionic crystals such NaCl, AgBr and KCl.
• Schottky defects can decrease the density of a crystal because atoms are missing from their normal positions, leading to a slight decrease in the overall mass of the crystal per unit volume.
• Schottky defects do not introduce any new charge into the crystal, as an equal number of cations and anions are missing. Therefore, they do not affect the overall neutrality of the crystal.

Schottky Defect Calculation

The number of defects for an MX crystal can be calculated by the given formula

[Tex]n_s = Ne^-\frac{\Delta H}{2RT}[/Tex]

where,

• n_s = number of Schottky defects per unit volume
• N = Number of ions
• ΔH = enthalpy of defect formation,
• T = absolute temperature (in K),
• R = gas constant.

We can calculate N by using the formula;

N = (density of the ionic crystal compound) × N_A/ Molar mass of the ionic crystal compound

Differences Between Schottky and Frenkel Defect

Schottky Defect and Frenkel defects are two types of point defects that exists in ionic solids. The difference between Schottky defect and Frenkel defect is tabulated below:

Schottky Defect	Frenkel Defect
Schottky defect in ionic crystals arises when equal number of cations and anions are missing from their normal sites, resulting in equal number of cations and anions vacancies.	Frenkel defect in ionic crystal arises when some ions (generally cations) leave their normal lattice sites and occupy the interstitial sites.
Decrease in density of crystal.	The density of the crystal does not change.
Mass decreases.	Mass remains same.
It is observed in ionic solids having high coordination number.	It is observed in ionic solids having low coordination number.
Size of cation is equal to size of anion.	Size of cation is lesser than size of anion.
Example: CsCl, KCl, NaCl, CsBr, etc.	Example: ZnS, AgCl, etc.
Electrical conductivity increases.	Electrical conductivity remains same.

Schottky Defect Frequently Asked Questions

Where does Schottky defect occurs ?

Schottky defects occur mainly in ionic compounds where positive and negative ions are of the same size. For example, NaCl, KCl show this defect.

How to identify Schottky defect?

Schottky defect in crystals is observed when equal numbers of cations and anions are missing from the lattice.

How does Schottky defect affect charge neutrality of a crystal?

The charge neutrality of a crystal remain unchanged in case of Schottky defect as equal number of positive and negative ions are missing from the crystal

How is Schottky defect different from Frenkel defect?

In case of Schottky defect equal number of cations and anions are missing from their site while in Frenkel defect cation moves from its lattice site to interstitial site

What are examples of Schottky defect?

Examples of Schottky defect includes NaCl, KBr, MgO, AgBr etc.