The appearance of colour in solid alkali metal halides is generally due to

The correct statement$(s)$ regarding defects in solids is(are):
$A$. Frenkel defect is usually favoured by a very small difference in the sizes of cation and anion.
$B$. Frenkel defect is a dislocation defect.
$C$. Trapping of an electron in the lattice leads to the formation of $F$-center.
$D$. Schottky defects have no effect on the physical properties of solids.

Frenkel defect is generally observed in which of the following?

When $ZnO$ is heated,it turns from white to yellow because it ................

What type of defect can arise when a solid is heated? Which physical property is affected by it and in what way?

$F$-centres are formed in $NaCl$ crystal if it is heated with $Na$ vapour. The above is due to