Why stoichiometric defects are also called intrinsic defect?

Stoichiometric defects are also called intrinsic defects because they arise due to the inherent nature of the crystal lattice and do not involve the addition or removal of foreign atoms. These defects occur due to the irregularities in the arrangement of atoms or ions within the crystal structure.

For example, in the case of Frenkel defects, some of the ions leave their normal lattice sites and occupy interstitial positions, creating both vacancies and interstitials. These defects are intrinsic to the material itself and do not require the presence of impurities or external factors.

Similarly, Schottky defects involve the absence of a pair of ions from their regular lattice sites, maintaining the overall stoichiometry of the compound. These vacancies can occur naturally during the formation of the crystal lattice and do not require the introduction of foreign species.

Therefore, stoichiometric defects, such as Frenkel and Schottky defects, are referred to as intrinsic defects as they originate from the intrinsic properties of the material's crystal structure and do not involve external impurities.