In which crystal is the Schottky defect observed?

Write a short note on impurity defects.

When $KCl$ is heated in an atmosphere of $K$ vapor,it appears violet. This is because .................

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.

Schottky defect and Frenkel defect are ....... .

Which of the following compounds is likely to show both Frenkel and Schottky defects in its crystalline form?