Imperfections in Solids Point Defects

Introduction

In crystalline solids, any irregularities or distortion observed around a particular point, or an atom is called a point defect. Defects occur when crystallisation takes place at a very fast or intermediate rate. As a result of which the particles or atoms do not get enough time to arrange themselves in a regular pattern. Point defects are further classified into three categories, and they are − Stoichiometric defects (st.), Impurity defects and Non-Stoichiometric defects. In Stoichiometric defects, the ratio of cations (positive charge) and anions (negative charge) remains unchanged. An impurity defect occurs when an external (foreign) atom occupies the space in the crystal lattice. Non-stoichiometric defect

What are Point Defects in Solids?

A point defect (p.d) occurs when one or more atoms in a solid (crystalline) leave their lattice position or a foreign atom occupies its original position or takes the interstitial position in the solid crystal.

Causes of Point defects in a Solid Crystal

Some of the major causes of Point defects are as follows −

• Irregularity in the size and shape of the crystalline solid.

• High energy (e) radiations (like x-rays) strike or hit the solid surface.

• Rapid or continuous cooling from very high temperatures.

Types of Point Defects in Solids

Point defects are further classified into three categories, are

• Stoichiometric defects

• Impurity defects

• Non-stoichiometric defects.

DaniFeri, Point defects in crystal structures, CC BY-SA 3.0

What is a Stoichiometric Defect

The stoichiometric defect is a type of point defect that does not disturb (affect) the stoichiometry of the solid. This is also called intrinsic defects in which the ratio (/) of the cations (+ve) to the anions (-ve) remains the same. This type of defect occurs due to the deviation from the regular (general) pattern of atoms or ions in the solid (s) system. There are two types of Stoichiometric defects- Vacancy defects and interstitial defects.

Types of Stoichiometric defects

There are broadly or mainly two types of Stoichiometric defects −

• Vacancy defects − Here, the atom is not found at its lattice sites causing the lattice site to be vacant or empty creating a vacancy defect.

• Interstitial defects − Here, an atom (a) or molecule covers the intermolecular spaces or sites in the crystals.

What is the Frenkel Defect?

Frenkel defect is a form or type of point defect present in the crystalline solid. This defect takes place when an atom (a) or any smaller ion (cation) leaves its original place in the lattice, creating a vacancy (blank space) there, and becomes an interstitial by locating itself in a nearby location.

Naclfrenkeldefect.svg: Gabriel Rosser, Oxford University Gabs / derivative work: Sponk, AgCl Frenkel defect, CC BY-SA 3.0

What is a Schottky Defect?

The Schottky defect is also a type of point defect that takes place because of the site occupying excitation in the crystal lattice. This type of defect forms when the oppositely charged ions leave their lattice sites and for instance reaches the surface of the lattice, creating oppositely charged vacancies.

VladVD, NaCl - Schottky defect, CC BY-SA 3.0

What is Impurity Defect?

Impurity defects are defined as defects, foreign atoms that replace some of the atoms (a) that are used for the formation of that particular solid or that squeeze into the interstices. Impurity defect changes the properties (behaviours) of the original crystalline solids. This type of defect is useful in the formation of semiconductors.

What is a Non-Stoichiometric Defect?

If due to the imperfections in the sold crystal, the ratio (/) of the cations (+ve) to that of the anions (-ve) becomes different from that indicated (shown) by the ideal chemical formula, then such defects are termed or called as non-stoichiometric defects.

Forms of Non-Stoichiometric defect

• Metal (m) excess defect due to anion (-ve) vacancy − A compound may contain excess metal ions if an anion (-ve) is missing from the lattice site, leaving behind a positively charged hole.

• Metal (m) excess defect due to interstitial cation (+ve) − A compound may contain excess metal ions if an extra positive (+ve) ion is present (found) in the interstitial site.

• Metal (m) deficiency (lack) due to cation vacancy − In this type of defect, the cation (+ve) is missing (absent) from its lattice site.

• Metal (m) deficiency (lack) due to extra anion (-ve) in interstitial site − In this type of defect, an anion (-ve) is present (found) at the interstitial position.

Applications of Point Defects

• Point defects are capable of influencing the electrical (e) and magnetic (m) properties of the materials.

• Point defects can also influence (affect) the rate of corrosion in the materials.

Applications of Interstitial Defects

• This type of defect is found in several materials like alloys, insulators, and semiconductors.

• This defect can increase the susceptibility rate of corrosion.

Applications of Defects of Deformation

• This defect can cause the material to deform (break) or change its shape or size.

• This defect causes decreased strength or decreased functionality of the lattice.

Applications of Substitutional Defects

• This defect is used to understand how the materials fail mechanically.

• It is also used to understand the rate of corrosion of the materials.

Difference between Schottky and Frenkel defects

Frenkel defects	Schottky defects
In this type of defect, the density (d) of the lattice remains the same.	In this type of defect, the density (d) of the lattice reduces.
Mass (m) of the lattice also remains constant.	Mass (m) of the lattice decreases or reduces.
Atoms (a) or ions leave their positions but remain present in the lattice.	Atoms (a) or ions completely leave the lattice.

Conclusion

In crystalline solids, when the ideal (proper) arrangement of the solids (s) is distorted (damaged) around a particular point or atom then it is called a point defect. Point defects are further classified into three defects and those are −Stoichiometric defects (st.), Frenkel defects (f.d), and Schottky defects. There are several reasons for the crystal defects. Like deformation of the crystal (solid), continuous or rapid cooling (cool) from high temperature (t), or high-energy (e) radiation striking the crystal or solid.

FAQs

1.What do you mean by crystalline solid?

Crystalline solid is a type of solid in which the constituent particles are arranged in a highly ordered manner in all the directions of the lattice.

2.What are foreign atoms?

Foreign atoms are the external atoms that are mixed with the crystal and are of a different kind from the constituent atoms of the crystal.

3.Give some examples of Frenkel defects.

ZnS,AgBr,AgCl,and AgI are some examples of Frenkel defects.

4.Name the types of stoichiometric defects.

• Vacancy defects

• Interstitial defects

5.In which type of defect mass of the lattice decreases?

In Schottky defect mass of the lattice decreases.