Question

Stoichiometric defects are of two types such as vacancy defects and interstitial defects.

1. Which defect is basically a vacancy defect in ionic solids? 

2. Which stoichiometric defect causes the decrease in density of solid? 

3. On heating white ZnO it turns yellow. Which is the crystal involved here? Explain.

Answer 1

1. Schottky defect 

2. Schottky defect 

3. It is due to metal excess defect caused by the presence of extra cations at interstitial sites. ZnO is white in colour at room temperature. On heating it loses oxygen and turns yellow.

The excess Zn²⁺ ions move to interstitial sites and the electrons to neighbouring interstitial sites. The yellow colour of non-stoichiometric ZnO (Zn_1+x O) is due to these trapped electrons.