Answer:-

i. Impurity defect arises when foreign atoms, that is, atoms different from the host atoms, are present in the crystal lattice.

ii. There are two kinds of impurity defects: Substitutional and interstitial impurity defects.

iii. Formation of vacancy through aliovalent impurity:
Vacancies are created by the addition of impurities of aliovalent ions (that is, ions with oxidation state different from that of host ions) to an ionic solid.

e.g. Consider a small amount of SrCl² impurity added to NaCl during its crystallization. The added Sr²⁺ ions (O.S. = +2) occupy some of the regular sites of Na⁺ host ions (O.S. = +1). In order to maintain electrical neutrality, every Sr²⁺ ion removes two Na+ ions. One of the vacant lattice sites created by the removal of two Na⁺ ions is occupied by one Sr²⁺ ion. The other site of Na⁺ ion remains vacant as shown in the figure.