Question

The yellow colour of \(\mathrm{ZnO}\) and conducting nature produced in heating is due to (a) Metal excess of defects due to interstitial cation (b) Extra positive ions present in an interstitial site (c) Trapped electrons (d) All

Short answer

The yellow coloration and conducting nature of heated ZnO is due to trapped electrons, hence option (c) is correct.

Step-by-step solution

Understanding the Color Change in Heated ZnO

The color change in zinc oxide (ZnO) when heated is commonly attributed to defects in its crystal structure. These defects can be formed in different ways, including excess metal or trapped electrons. Understanding the nature of these defects can help in identifying the reason for the color change.

Analyzing Each Given Option

(a) Metal excess defects due to metallic cations occupying interstitial spaces in the crystal would create additional positive charges.(b) Extra positive ions in an interstitial site would also contribute to metal excess defects.(c) Trapped electrons, known as F-centers, are electrons trapped in a vacancy where an anion would be in the crystal lattice. These trapped electrons can absorb visible light and re-emit it, which can be seen as a color change.(d) The question implies considering whether all given options could be correct.

Determining the Correct Option

The characteristic yellow color of heated ZnO is primarily due to trapped electrons that create color centers within the material. These electrons provide the material with free carriers, thereby increasing its conductivity and causing the yellow coloration. Hence, the correct answer would be related to trapped electrons.

Key concepts

Crystal Structure Defects

Understanding why heated zinc oxide (ZnO) changes color involves exploring the concept of crystal structure defects. Defects in crystal structures are irregularities that disrupt the regular ordering of atoms or ions. There are several types of defects, including vacancies, interstitial defects, and substitutional defects.

ZnO, a white powder at room temperature, can incorporate defects during processes like heating. These defects often involve either excess metal atoms or trapped electrons that distort the crystal's electronic structure.

When metal cations occupy interstitial spaces, not their usual lattice sites, they create metal excess defects. If an anion is missing from the lattice and an electron fills this void, it is known as an F-center, which affects how the crystal interacts with light. These imperfections are not merely errors but can dramatically alter a material's properties, such as its color, through the absorption and re-emission of light.

F-Centers

The term F-centers stands for 'color centers' or 'Farbzentren' in German, which are specific types of crystal structural defects. F-centers are created when an anion vacancy in the crystal lattice captures an electron.

The presence of an F-center is responsible for the absorption of certain wavelengths of light that lead to the appearance of color in an otherwise colorless or differently colored material. For instance, in the case of ZnO, F-centers absorb visible light and can re-emit it, leading to the observed yellow color upon heating.

F-centers are significant because they provide an electron that can move more freely within the crystal lattice. This mobility of electrons is connected to the material's conductivity and the possibility of it exhibiting semiconducting or even metallic properties when subjected to conditions such as heating.

Conductivity in Materials

The conductivity of materials is a critical property that determines their ability to conduct electric current. Conductivity is influenced by the presence of charge carriers—electrons or holes that can move under the influence of an electric field.

In a perfect crystal lattice, such as that of an unheated ZnO, there are very few free charge carriers at room temperature, making it an insulator. However, when defects are introduced, as occurs when ZnO is heated, the electron configuration changes, increasing the number of free carriers.

These additional charge carriers can move through the lattice more easily, reducing the material's resistance to current flow and thus enhancing conductivity. The relationship between crystal structure defects, F-centers, and the electronic band structure is fundamental to understanding conductivity changes in materials like ZnO during processes such as heating.