Defects in Solids

Description: Defects in solids are imperfections that disrupt the regular patterns in crystalline structures. They include point defects, line defects, and planar defects.

1.0Introduction

Any irregularity in the arrangement of crystals within a solid lattice is referred to as an imperfection in solids. Defects occur when crystallization (the process of forming crystals) happens at a very fast or intermediate rate, as particles do not have sufficient time to arrange themselves into a regular pattern.

2.0Causes of Crystal Defects

Crystal defects arise due to the following factors:

• Vacancies in the lattice
• Dislocation of particles in the lattice
• Nonstoichiometric proportions of ions
• Impurities within the lattice

3.0Types of defects

In general, defects in crystals can be classified into two types: point defects and line defects. 

• Point defects are irregularities or deviations from the ideal arrangement around a single point or atom in a crystalline substance. 
• On the other hand, line defects involve irregularities or deviations that affect entire rows of lattice points. 

4.0Types of Point Defects

Point defects in a crystal can be classified into three types:

• Stoichiometric Defects: Maintain the compound's stoichiometry, like vacancies, interstitials, and Frenkel defects.
• Non-Stoichiometric Defects: Alter the compound's stoichiometry, such as metal excess or deficiency defects.
• Impurity Defects: Foreign atoms replace or occupy lattice sites, introducing new properties to the material.

5.0Stoichiometric defect

It occurs when the ratio of cations to anions remains unchanged in a compound, maintaining its stoichiometry. It includes:

Vacancy defect 

A vacancy defect occurs when there is an absence of particles at certain positions in a lattice, creating a vacancy. This type of defect is commonly observed in non-ionic solids and metals. The presence of vacancy defects decreases the overall density of the crystal due to the missing particles.

Interstitial defect

An interstitial defect arises when additional constituent particles occupy the interstitial sites in a lattice. This defect is typically seen in non-ionic solids and metals. Unlike vacancy defects, interstitial defects increase the density of the crystal because the lattice accommodates extra particles.

Stoichiometric Defects in Ionic Solids

In ionic solids, stoichiometric defects do not disturb the overall stoichiometry of the compound. There are two main types of stoichiometric defects:

Schottky Defect:

• This defect occurs when an equal number of cations and anions are missing from their lattice sites, creating vacancies.
• It is common in ionic solids where the cations and anions are of similar size, such as sodium chloride (NaCl).
• Schottky defects reduce the density of the crystal because of the missing ions.

Consequences:

• Decrease in mass without affecting volume, leading to lower density.
• Enhanced electrical conductivity via ionic mechanism.
• Increased presence of voids reduces lattice energy, impacting crystal stability.

Frenkel Defect:

• This defect occurs when a cation leaves its regular lattice site and moves to an interstitial site, creating a vacancy at the original position and an interstitial defect at the new position.
• It is typical in ionic solids with a significant difference in size between the cations and anions, such as silver chloride (AgCl).
• Frenkel defects do not significantly change the density of the crystal, as the total number of ions remains the same but their positions are altered.

Consequences:

• Density of the solid remains unchanged.
• Increased dielectric constant due to closer like charges.
• Enhanced electrical conductivity via ionic mechanism.

6.0Non-stoichiometric defects

Non-stoichiometric defects occur when the ratio of cations to anions deviates from the ideal chemical formula. These defects result from an excess of metal or non-metal atoms, or the presence of impurities.

• Metal Excess Defects due to Anion Vacancies:

The compound has excess metal ions lacking negative ions from lattice sites in this defect. Electrons occupy these vacancies to maintain charge neutrality, forming 'F-centers' or colour centres. These centers impart color to the compound and exhibit unique properties.

• Metal excess defects due to interstitial cations

Metal excess defects can also occur when an extra cation occupies an interstitial site, with electrical neutrality maintained by an electron in another interstitial site. These defects are found in crystals with Frenkel defects.

• Metal deficiency due to cation vacancies

Metal deficiency defects arise from the absence of a metal ion from its lattice site, balanced by an adjacent ion with a higher positive charge. They are typically seen in compounds of transition elements.

• Point defects due to the presence of foreign atoms

Point defects due to foreign atoms occur when foreign atoms replace host atoms or occupy vacant interstitial sites. Depending on the electronic structure or size of the impurity, this leads to substitutional solid solutions or interstitial solid solutions.

7.0Impurity Defects

When molten NaCl containing a small amount of SrCl₂ is crystallized, some of the sites originally occupied by Na+ ions are replaced by Sr₂⁺ ions. Each Sr₂⁺ ion replaces two Na⁺ ions: one by occupying its site and the other leaving a vacant site. The number of cationic vacancies created is equal to the number of  Sr₂⁺ ions present.

   A similar example is seen in the solid solution of CdCl₂ and AgCl.