Imperfections in Solids: Causes, Types and Examples

Crystalline solids are not always perfect; there are defects in them too. Irrespective of the rate of crystallization, crystals are not free of imperfections. Here, by defects, we mean irregularities in the arrangement of constituent particles. There are two types of defects in Solids – Point defects and line defects. Point defects are the ones that arise with respect to a particular point or atom. Line defects, on the other hand, are observed in an entire row or line of atoms in the solid. In this article, we will learn about point defects specifically.

Key Terms: Atoms, Point defects, Solids, Types of point defect, Metal excess defect, Metal Deficiency Defect

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Point defects

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As discussed above, point defects are the irregularities in an ideal arrangement due to a point or an atom and the areas surrounding it, in a crystalline substance. It is classified into three types –

1. Stoichiometric defects
2. Non – stoichiometric defects
3. Impurity defects

1. Stoichiometric defects

Stoichiometric compounds maintain their stoichiometry, which means the ratios of cations and anions remain the same as represented by their chemical formula. Such defects or imperfections that do not disturb the ratio of cations and anions are known as stoichiometric defects. They are further divided into-

• Vacancy defect
• Interstitial defect
• Schottky defect
• Frenkel defect

Vacancy defect

This kind of stoichiometric defect exists in crystals where some lattice sites are vacant. In this case, a particle will be missing in the solid. This implies that the density of the solid will decrease. Even if the surrounding particles try to fill the gap, the vacant place will only shift its position. The solid structure of the crystal does not allow the particles around the vacant spot to collapse.

Interstitial defect

When eight spheres of a unit cell meet at the center of a solid structure it tends to leave a little space which is called the interstitial site. It is known as an interstitial defect when another particle occupies this space or the interstitial site. As a matter of fact, the density of the solid increases herewith.

Schottky defect

Unlike vacancy and interstitial defects, Schottky defect occurs when more than one particle is missing in the solid. One important thing that must be noted is that the number of missing cations is equal to that of missing anions. So, there is no change in the overall electrical composition of the solid. However, the mass of the object will decrease. The volume remains unchanged but the density will reduce. In case of too many particles being missing, the lattice structure may be disrupted, which in turn, might affect the stability of the solid. Such a thing can be observed in NaCl, KCl and the like.

Frenkel defect

A cation when missing from its original position and instead, occupying an interstitial site gives rise to Frenkel defect. It generally happens when the cation is smaller in size and can easily fit into the interstitial site. Electrical neutrality is not hampered in this type of defect. No atom is missing as such; there is just a change in the position of the particle. This phenomenon is usually observed in compounds in which there is a noticeable size difference between cations and anions, for example, AgCl.

2. Non – stoichiometric defects

1. let’s look into the concept of the non – stoichiometric defects, the ones that appear without hampering the stoichiometry of the crystalline substance. These defects can change the ratio of cations and anions in the solid. This imperfection is found in a large number of inorganic compounds. There are two kinds of non – stoichiometric defects.

• Metal excess defects
• Metal deficiency defects

Metal excess defect

The substance may have a surplus of metal ions in its space lattice which is eventually occupied by electrons to maintain neutrality. This generally occurs in two ways,

(a) Metal excess defect due to anionic vacancy

In a compound with excess metal ions, a vacancy is formed when an anion is missing from its position. An electron occupies this space to maintain electrical neutrality. The space occupied by the electron is called the F – center. This F – center is the reason behind the coloration of the compound. For instance, NaCl becomes yellow when heated.

(b) Metal excess defects due to extra cations

Many compounds with non – stoichiometric defects often release extra cations when heated. These cations can fit into interstitial sites. To balance out, an equal number of electrons do the same. The ultimate result is the abundance of metal in the solid. This defect is mostly found in transition elements. For example, Zinc oxide on being heated losses oxygen.

Metal deficiency defects

Some compounds may have lesser metal than their ideal stoichiometric proportions, making it difficult to prepare. In such compounds, sometimes a cation goes missing from the structure. However, the neutrality of the solid is not compromised as a neighboring ion takes up two charges. This defect is also found in transition elements that can have multiple valencies.

3. Impurity defect

When foreign atoms invade the solid structure by replacing some of the constituent atoms it is known as an impurity defect. The foreign atoms occupy the interstitial site of the parent atom.

The video below explains this:

Imperfections in Electrical and Magnetic properties of Solids Detailed Video Explanation:

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Things to Remember

• There are three types of defects - Point defects, Non-Stoichiometric defects and Impurity defect.
• Vacancy defect, Interstitial defect, Schottky defect, and Frenkel defect are some of the stoichiometric defects.
• Metal excess defect occurs due to anionic vacancy and metal deficiency defect occurs due to cationic vacancy.
• Impurity fect occurs when any foreign element replaces a constituent element.

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Sample Questions

Ques. Which kind of stoichiometric defect is exhibited by AgBr? (1 mark)
(a) Schottky defect
(b) Frenkel defect
(c) Both of the above
(d) None of the above

Ques. How does a Schottky defect affect the properties of the solid? (1 mark)
(a) mass decreases, density increases
(b) mass decreases, density decreases
(c) mass increases, density increases
(d) mass increases, density decreases

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