Characteristics and Classifications of Defects in Solids

To determine the correct statement(s), let us analyze each of the given options:

1. Option (A):

   • A Frenkel defect occurs when an ion (usually the smaller cation) is displaced from its regular lattice site to an interstitial site.
   • This defect is favoured in ionic compounds with a low coordination number and where there is a large difference in the sizes of the cation and anion (i.e., $r_{\text{cation}} \ll r_{\text{anion}}$), so that the smaller cation can comfortably fit into the interstitial space.
   • Conversely, a very small difference in ionic sizes favours a Schottky defect.
   • Therefore, statement (A) is incorrect.

2. Option (B):

   • In a Frenkel defect, the cation is dislocated from its normal lattice position to an interstitial site, leaving a vacancy at its original site and creating an interstitial defect at the new site.
   • For this reason, the Frenkel defect is also formally referred to as a dislocation defect.
   • Therefore, statement (B) is correct.

3. Option (C):

   • In metal-excess non-stoichiometric defects, an anion vacancy is created when an anion leaves its lattice site, and an electron is trapped in this vacant anionic site to maintain electrical neutrality.
   • These anionic sites containing trapped unpaired electrons are called F-centres (from the German word Farbe, meaning colour), and they are responsible for imparting colour and paramagnetic behaviour to the crystal.
   • Therefore, statement (C) is correct.

4. Option (D):

   • A Schottky defect involves the missing of an equal number of cations and anions from their normal lattice sites, leading to vacancy creation.
   • Because vacancies are created without changing the volume of the crystal, the mass decreases, which causes a decrease in the density of the solid. It also increases electrical conductivity due to vacancy hopping. Thus, Schottky defects significantly alter the physical properties of solids.
   • Therefore, statement (D) is incorrect.

Hence, the correct statements are B and C.