No crystal is found to be prefect at room temperature . The defects present in the crystals can be stoichiometric or non-stoichiometric. Due to non-stoichiometric defects, the formula of the ionic compound is different from the ideal formula. For example , the ideal formula of ferrous oxide should be FeO but actually in one sample , it was found to be `Fe_0.93 O`. This is because the crystal may have some ferric ions in place of ferrous ions. These defects change the properties of the crystals. In some cases , defects are introduced to have crystals of desired properties as required in the field of electronics . Doping of elements of Group 14 with those of Group 13 or 15 is most common. In ionic compounds , usually impurities are introduced in which the cation has higher valency than the cation of the parent crystal , e.g. of `SrCl_2` into NaCl
NaCl was doped with `10^(-3)` mol % `SrCl_2`. The concentration of cation vacancies is
A. `6.02xx10^18 "mol"^(-1)`
B. `6.02xx10^15 "mol"^(-1)`
C. `6.02xx10^21 "mol"^(-1)`
D. `6.02xx10^12 "mol"^(-1)`
