1. A method of introducing defects in ionic solids by adding impurity ions.
2. If the impurity ions are in different valence state from that of host, vaccancies are creited in the crystal lattice of the host.
3. For cg., addition of CdCl2 to silver chloride yields solid solutions where the divalent cation Cd2+ occupies the position of Ag2+
4. This will disturb the electrical neutrality of the crystal. In order to maintain the same, proportional number of Ag ions leave the lattice. This produces a cation vaccancy in the lattice, such kind of crystal defects are called impurity defects.