No crystal is found to be prefect at room temperature. The defects present in the crystals can be stoichimetric or non-stoichiometric. Due to non-stoichimetric defects, the formula of the ionic compound isdifferent from the ideal formula. For example , the ideal formula of ferrous oxide should be FeO but actually in one sample , it was found to beFe_(0.93)O.This is because the crystal may have some ferric ions in place of ferrous ions.These defects change the propeties of the crystals. In some cases, defects are introduced to have crystals of desired properties as required in the field of electronics. Dopingof elments of Group 14 with those of Group13 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,SrCl_(2)into NaCl.NaCl was doped with10^(-3) " mol % " SrCl_(2) . The concentreation of cation vancancies is