Crystals as such are never perfect and in general have some defect, which simply refers…

Crystals as such are never perfect and in general have some defect, which simply refers to a disruption in the periodic order of a crystalline material. Based on their nature, these are classified into three types : stoichiometric, impurity and non-stoichiometric defects. In non-stoichiometric defects, the ratio of the number of atoms of one kind to the number of atoms of the other kind does not correspond exactly to the ideal whole number ratio implied by the formula. For example, the ideal formula of ferrous oxide should be FeO, but it is actually found to be Fe_0.93O in a sample, because some ferric ions have replaced ferrous ions in the crystal. These defects affect the properties of the crystals. Sometimes such defects are introduced to improve the properties of the crystals for a specific use, such as in the field of electronics. Elements of Group 14 are commonly doped with impurities of elements of Group 13 or 15 . In ionic crystals, impurities are introduced with cations having higher valence than the cations in the pure crystal structure.
NaCl was doped with 10^–3 mol % SrCl₂. The concentration of cation vacancies is