Question

The white $ZnO$ turns yellow on heating because of

• A. Frenkel defect
• B. Metal excess defect
• C. Metal deficiency defect
• D. Schottky defect

Answer 1

Answer: (B)

$ZnO$, on heating, loses oxygen and thus, the zinc metal becomes in excess in $ZnO$ crystal. Hence, the formula of $ZnO$ becomes $Z{n}_{1+x}O$ The excess $Z{n}^{2+}$ ions move to interstitial sites and the electrons to neighbouring interstitial sites. The electrons are responsible for the yellow colour of $ZnO$
$ZnO\stackrel{\Delta }{\to }Z{n}^{2+}+\frac{1}{2}{O}_2+2e^{-}$
Thus, yellow colour of $znO$ is due to metal excess defect