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JEE Main levelSingle correct optionDifficulty 2 of 3ss188

When white $ZnO$ is heated it turns yellow and on cooling becomes white again. The yellow colour is due to

ASchottky defects
Bexcess $Zn X^{2 +}$ ions in interstitial sites together with trapped electrons
Ccation vacancies compensated by $Zn X^{3 +}$
Danion vacancies filled with electrons (F-centres)

Answer: B

On heating,

ZnO

loses some oxygen:

ZnO Zn X^{2 +} + \frac{1}{2} O X_{2} + 2 e X^{-}

. The extra

Zn X^{2 +}

ions sit in interstitial sites and the electrons are trapped nearby; they absorb light to give a yellow colour. The process is reversible on cooling.

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