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Explain defects caused by impurity in ionic solids. |
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Answer» Solution : When in a pure crystal STRUCTURE, if the defect is arised by addition of foreign substance in SMALL amount it is known as impurity defect. In case of ionic solids, the impurity DEFECTS can be introduced by addition of IONS. The defect is arised only if the ions to be ADDED are in higher oxidatio state than the ions of crystal lattice. If a molten NaCl containing a little amount of `SrCl_2` is crystallised, some of the sites of `Na^+` ions are occupied by `Sr^(2+)`. Each `Sr^(2+)` replaces two `Na^(+)` ions and it occupies the site of one Nat ion and other site remains vacant. Thus cationic vacancies thus produced are equal in number to that of `Sr^(2+)` ions. Another similar example is the solid solution of `CdCl_2` and AgCl`. 
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