What type of defect is shown by ZnO ?

The correct answer is Option (2) → Metal excess defect due to the presence of extra cations at interstitial site.

Zinc oxide is white at room temperature. On heating it loses oxygen and turns yellow:

    ZnO → \( \ce{Zn^{2+}} \) + ½\( \ce{O2} \) + 2e⁻

The crystal now has an excess of zinc, Zn_1+xO. The excess Zn²⁺ ions move to interstitial sites and the electrons to neighbouring interstitial sites. This is a metal excess defect due to extra cations at interstitial sites.

Why the other options are incorrect

• Option (1) Frenkel defect — Incorrect. A stoichiometric defect in which a smaller ion (usually the cation) leaves its site for an interstitial site, as in ZnS and AgCl; the composition stays exact, unlike Zn_1+xO.

• Option (3) Schottky defect — Incorrect. Equal numbers of cations and anions are missing, as in NaCl and KCl; the density decreases but the formula stays stoichiometric.

• Option (4) Metal excess defect due to anionic vacancies — Incorrect. Shown by alkali halides such as NaCl heated in sodium vapour; electrons occupy the anion vacancies (F-centres). In ZnO the extra cations sit in interstitial sites instead.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Metal Excess Defect due to the Presence of Extra Cations at Interstitial Sites – “Zinc oxide is white in colour at room temperature. On heating it loses oxygen and turns yellow. ZnO → Zn²⁺ + ½O₂ + 2e⁻ (on heating). Now there is excess of zinc in the crystal and its formula becomes Zn_1+xO. The excess Zn²⁺ ions move to interstitial sites and the electrons to neighbouring interstitial sites.”