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Question
What is an impurity defect? What are its types? Explain the formation of vacancies through aliovalent impurity with example.
Solution
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.
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