Question

Zinc oxide is normally white but turns yellow on heating and becomes white again on cooling because of:

• A. d - d transition spectra as well as a crystal defect
• B. the two-dimensional netweork structure of ZnO
• C. its high transition temperature $\left(T_C\right)$
• D. various types of lattice defects because of which the oxygen ion is lost during heating

Answer 1

Answer: (D)

various types of lattice defects because of which the oxygen ion is lost during heating

$ZnO$ is an example of non-stoichiometric defect under the category of metal excess defect (excess cations occupy interstitial site). In $ZnO$, $Z{n}^{2+}$ occupy at interstitial sites and hence, electrons are trapped at interstitial sites for neutralization. Crystals showing such type of defects are generally colored.

Zinc oxide is usually white in color but turns yellow on heating because it loses oxygen reversibly at high temperatures.

$ZnO\to Z{n}^{2+}+\frac{1}{2}{O}_2+2e^{-}$