To identify the compound exhibiting metal excess defect caused by an anionic vacancy, an analysis of the defect types in each option is required:
- ZnO: When heated, zinc oxide (ZnO) releases oxygen, creating an oxygen vacancy. This leads to interstitial zinc ions (Zn2+), resulting in a metal excess defect attributed to anionic (oxygen) vacancies.
- NaCl: Sodium chloride (NaCl) typically displays Schottky defects, and does not exhibit metal excess defects due to anionic vacancies.
- FeO: Iron(II) oxide (FeO) predominantly shows non-stoichiometry stemming from metal deficiencies, not excesses.
- CdO: Analogous to ZnO, cadmium oxide (CdO) can also present metal excess defects from anionic vacancies, though this phenomenon is less prevalent than in ZnO.
Consequently, ZnO is the compound that exhibits metal excess defect due to an anionic vacancy.