Question

\(\mathrm{ZnO}\) is white when cold and yellow when heated. It is due to the development of (a) metal excess defect (b) Frenkel defect (c) Schottky defect (d) both (b) and (c)

Short answer

The color change in ZnO is due to the metal excess defect.

Step-by-step solution

Understanding the Problem

We need to find out why ZnO changes color when heated. It involves a type of defect in its crystal structure.

Identifying the Defect

ZnO is originally white but turns yellow upon heating due to the presence of a defect that absorbs specific wavelengths of light.

Exploring Defect Types

Metal excess defect involves an imbalance in the stoichiometric ratio due to metal atoms or electrons in excess. Frenkel and Schottky defects usually don't cause color changes because they involve vacancies without excess electrons contributing to absorption of light.

Analyzing Changes

When ZnO is heated, it loses some oxygen, leaving behind free electrons. This increases the electron density in the crystal structure, which can cause absorption of specific wavelengths of light, leading to the yellow color.

Final Conclusion

The yellow color when ZnO is heated is due to metal excess defect because excess electrons from oxygen vacancies increase light absorption.

Key concepts

Metal Excess Defect

The metal excess defect is a fascinating phenomenon where there is an imbalance in the stoichiometric ratio of a solid due to an excess of metal ions or electrons. In simpler terms, it means that the crystal contains more metal atoms than it ideally should, according to its chemical formula. This defect is often the result of the loss of non-metal ions, like oxygen, leading to vacancies occupied by electrons. These excess electrons provide additional electronic states that can absorb visible light, causing noticeable color changes.

For example, when ZnO is heated, it loses some oxygen, creating oxygen vacancies. These vacancies are often filled by electrons. These extra electrons increase the electron concentration in the crystal lattice. As a result, the crystal can absorb certain wavelengths of light, and this absorption results in the yellow coloration of ZnO when heated. It is fascinating how such tiny changes at the atomic level can lead to dramatic changes in appearance!

• Increase in electron concentration due to oxygen loss
• Absorption of specific light wavelengths
• Change in color (e.g., ZnO turning yellow upon heating)

Frenkel Defect

The Frenkel defect is another kind of imperfection in solids, but it is quite different from the metal excess defect. It comprises two main components: a vacancy and an interstitial defect. This happens when an atom or ion moves from its original lattice site to an interstitial site, creating a vacancy where it once was and an extra atom on another part of the lattice. This does not alter the overall stoichiometry of the compound since the number of atoms and ions remains the same.

In general, Frenkel defects are common in materials with small, highly charged ions that can easily jump to nearby interstitial sites. However, these defects typically do not affect the color of the material because they do not involve a significant number of free electrons that could absorb light. In other words, a Frenkel defect does not cause color changes like the yellowing of ZnO.

• Involves both vacancy and interstitial defects
• Preserves stoichiometry
• Does not typically cause color changes

Schottky Defect

The Schottky defect is another type of point defect in crystalline materials, primarily occurring in ionic compounds. This defect arises when equal numbers of cations and anions vacate their lattice sites, creating pairs of vacancies. Because both types of ions leave the crystal structure, the charge neutrality and the stoichiometry of the compound are preserved.

While Schottky defects can affect the density of a material—since there's a reduction in the number of particles within the volume of the crystal—they typically do not result in color changes. This is because, similar to Frenkel defects, there are no excess electrons or other changes that would affect light absorption significantly. This defect doesn't cause the yellow color in ZnO when it's heated, making it distinct from the metal excess defect.

• Equal number of cation and anion vacancies
• Preservation of stoichiometry and charge balance
• Does not lead to color change