Point Defects: Introduction, Explanations

Crystalline solids have a short-range as well as long-range order in the arrangement of their constituent particles, yet crystals are not perfect. Crystalline solids are formed by joining many small crystals. Different types of defects are found in crystals after the process of crystallization. Broadly, there are four types of defects: point defects, line defects, volume defects, and surface defects. Point defects are irregularities or deviations from ideal arrangements around a point or an atom in a crystalline substance. Line defects are irregularities or deviations from ideal arrangements in entire rows of lattice points.

Key Terms: Defects, Crystalline Solids, Stoichiometry, Crystals, Crystallization, Point defects, Surface defects, Atom, Lattice

---

Point Defects: Definition

[Click Here for Sample Questions]

The defects in the crystals which occur around an atom or particle are called point defects. These defects occur only at or around a single lattice point. They do not extend in space in any dimension hence leading to be termed also as zero-dimensional (0-D) defects. These are the smallest possible defects in any crystalline solid material. Point defects occur when:

• One or more atoms of the crystal are missing from their corresponding lattice site.
• Atom(s) is shifted from its corresponding lattice site to the interstitial position in the crystal.
• Foreign atom(s) occupy the interstitial position in the crystal lattice.
• The original atom of the crystal is replaced by a foreign atom.

Also Read:

---

Types of Point Defects

[Click Here for Sample Questions]

There are three types of point defects:

1. Stoichiometric Defects: These are the point defects in a crystal that do not disturb the stoichiometry of the solid. These compounds obey the law of definite proportions, the law of constant composition, and the law of conservation of mass. They are also called intrinsic or thermodynamic defects.

The stoichiometric defect is further classified into:

• Vacancy Defect: When some of the lattice sites are vacant, the crystal is said to have a vacancy defect. This results in a decrease in the density of the substance. The number of vacancy defects depends on the temperature of the crystal. This occurs due to imperfect packing during crystallization.
• Interstitial Defect: When some constituent particles (atoms or molecules) occupy an interstitial site, the crystal is said to have an interstitial defect. If the atom is of the same crystal, then the defect is called a self-interstitial defect. This defect increases the density of the crystal and causes atomic distortion.

Rather than simple vacancy or interstitial defects, these defects are shown as Frenkel and Schottky defects.

2. Schottky Defect: It is a vacancy defect in ionic solids. To maintain electrical neutrality, the number of missing cations and anions is equal. Like a simple vacancy defect, the Schottky defect also decreases the density of the substance. The number of such defects in ionic solids is quite significant.

For example, in NaCl, there are approximately 10⁶ Schottky pairs per cm³ at room temperature. In 1 cm³ there are about 10²² ions. Thus, there is one Schottky defect per 10¹⁶ ions. The Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. For example, NaCl, KCl, CsCl, and AgBr.

3. Frenkel Defect: This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location. Frenkel defect is also called dislocation defect. It does not change the density of the solid. Frenkel defect is shown by an ionic substance in which there is a large difference in the size of ions, for example, ZnS, AgCl, AgBr, and AgI due to the small size of Zn₂⁺ and Ag⁺ ions.
4. Impurity Defect: If molten NaCl containing a little amount of SrCl₂ is crystallized, some of the sites of Na+ ions are occupied by Sr₂⁺. Each Sr₂⁺ replaces two Na⁺ ions. It occupies the site of one ion and the other site remains vacant. The cationic vacancies thus produced are equal in number to that of Sr₂⁺ ions. Another similar example is the solid solution of CdCl₂ and AgCl.
5. Non-Stoichiometric Defect: The disturbance in cations and anions ratio either because of adding or removing of ions of the crystal are called stoichiometric defects. The non-Stoichiometric defect is of the following two types:

• Metal excess defect: In this defect metal ions occur in excess in the lattice of the crystal. There are two types of metal excess defect:
  • Metal excess defect due to anionic vacancies: This occurs due to the absence of anions from its original lattice site in crystals. Therefore, instead of anions, electrons occupy their position.
  • Metal excess defect due to the presence of extra cations at interstitial sites: Here, on heating the compound, it releases extra cations. These cations occupy the interstitial sites in crystals and the same number of electrons goes to neighboring interstitial sites.
• Metal deficiency defect: In this, the solids have a smaller number of metals relative to the described stoichiometric proportion. It is normally found in transition elements as they possess multiple valences.

Also Read:

---

Things to Remember

Following are some important points:

• In the 12th board exam, the weightage of this topic may be around 2 to 3 marks.
• For JEE Mains, the weightage is around 2 marks.
• For NEET, the weightage percent is 3%.
• You should Prepare well.
• You should Practice accordingly.
• You can solve various question banks for a clear picture of the type of questions asked.