Correct option (B, C)
Explanation:
(A) Incorrect statement. A small difference in sizes of cation and anion favour Schottky defect while Frenkel defect is favoured by large difference in sizes of cation and anion.
(B) Correct statement. In Frenkel defect, the smaller atom or ion gets dislocated from its normal lattice positions and occupies the interstitial space.
(C) In F-centre defect, some anions leave the lattice and the vacant sites hold the electrons trapped in it maintaining the overall electro-neutrality of solid.
(D) Incorrect Statement. In Schottky defect, some of the atoms or ions remaining absent from their normal lattice points without distorting the original unit cell dimension. This lowers the density of solid.