In case of ionic solids the opposite ions balance the charge of each other. The amount of cations and anions is mostly same in case of charge.
But when we add a cation with higher charge , the cation can replace the other cations in lattice and can create vacancy because to balance the charge, the cation with lower charge will leave the lattice.
for example if we add Sr²+ in NaCl crystal . The Sr²+ ion will replace two Na+ ions . To balance the charge of Sr²+ , two Na+ cations will leave the place and create vacancy.
NaCl crystal without impurity Formation of vacancy on addition of Sr²+
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