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 havecrystals 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