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Which of the following statements about defects in ionic solids are correct? (A) Frenkel defect is typically favoured when the cation and anion sizes are very similar (small difference in radii). (B) Frenkel defect is a dislocation defect, where an ion is displaced from its lattice site to an interstitial site. (C) F-centres (colour centres) are formed when electrons are trapped at anion vacancies, and this constitutes a metal-excess defect. (D) Schottky defects do not affect any physical properties of the solid.

1. (A) and (B) only
2. (B) and (C) only
3. (A), (B) and (C) only
4. (A), (C) and (D) only

Correct answer: (B) and (C) only

Solution

Statement (A) is incorrect: Frenkel defects are favoured when there is a LARGE difference in the sizes of cation and anion (small cations can fit into interstitial voids). Statement (B) is correct: Frenkel defect is indeed a dislocation defect. Statement (C) is correct: trapping of electrons in anion vacancies forms F-centres, causing metal-excess defect. Statement (D) is incorrect: Schottky defects reduce the density and affect physical properties.

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