What type of defect is shown by \(ZnO\)?

Metal excess defect due to the presence of extra cations at interstitial site.

The correct answer is option 2. Metal excess defect due to the presence of extra cations at interstitial site.

Crystalline solids often deviate from their perfect, ideal structures due to the presence of defects or imperfections. These defects can be broadly classified into:

Point Defects: These involve deviations at or around a single lattice point. Point defects are further categorized into:

Vacancy Defects: Absence of an atom in the lattice site.

Interstitial Defects: Presence of an extra atom in an interstitial site.

Substitutional Defects: Replacement of an atom in the lattice with a different atom.

Line Defects: These involve imperfections along a line, such as dislocations.

Surface Defects: These occur on the surface of the material, where the structure might be less ordered.

Defects in Zinc Oxide (\( \text{ZnO} \))

Zinc oxide, a white powder at room temperature, can show different types of point defects depending on the conditions, especially when heated. A metal excess defect occurs when there are more metal atoms than the stoichiometric ratio requires. This can happen in two main ways:

a. Due to Anionic Vacancies (Option 4):

In some materials, when an anion is missing from its lattice site, a metal ion might occupy an interstitial position instead. This leaves behind a "hole" where the anion should be, and free electrons are left in the lattice to maintain electrical neutrality. This type of defect is common in halides but is not the primary defect in \( \text{ZnO} \).

b. Due to Extra Cations at Interstitial Sites (Option 2):

This is the defect observed in zinc oxide. When \( \text{ZnO} \) is heated, it loses oxygen atoms, which creates oxygen vacancies. To maintain charge balance, some \( \text{Zn}^{2+} \) ions move from their regular lattice positions into interstitial sites (positions between the usual lattice points). These interstitial zinc ions are accompanied by free electrons, which are also trapped in the crystal. These free electrons are delocalized and can move through the lattice, giving the material conductivity.

Detailed Process:

Oxygen Loss: Heating \( \text{ZnO} \) can cause the loss of oxygen atoms from the lattice, resulting in vacancies where oxygen ions (\( \text{O}^{2-} \)) were originally located.

\(\text{ZnO (s)} \overset{{\text{heat}}}{\longrightarrow} \text{Zn}^{2+}_{\text{interstitial}} + \text{O}_2(g) + 2e^-\)

Formation of Interstitial Zinc Ions: To compensate for the missing negative charge (due to the oxygen vacancies), zinc ions move into interstitial sites in the crystal lattice.

Electron Presence: Free electrons are generated to maintain electrical neutrality, which can move through the crystal, contributing to the conductivity and imparting a yellow color to the heated \( \text{ZnO} \).

The yellow color is due to the absorption of certain wavelengths of light by these free electrons, which gives \( \text{ZnO} \) a yellow appearance when heated.

Summary

The correct description of the defect in \( \text{ZnO} \) is a metal excess defect due to the presence of extra cations at interstitial sites. When heated, \( \text{ZnO} \) loses oxygen, and some \( \text{Zn}^{2+} \) ions move into interstitial sites. Free electrons are also generated, contributing to the defect and the material's properties.

This phenomenon is significant in semiconductors and is an example of how defects can drastically alter the physical properties of a material.