What does stoichiometric defect mean?

i) Stoichiometric Defects: – It is a type of point defects which does not disturb the stoichiometry of solid. This is also known as Intrinsic or Thermodynamic Defects. Types of stoichiometric defects: Vacancy Defects, Interstitial defects, Frenkel Defects, Schottky Defects. Vacancy defects and Interstitial defects are found in non-ionic compounds while similar defects found in ionic compounds are known as Frenkel Defects and Schottky Defects. (a) Vacancy Defects: When some lattice sites left vacant while the formation of crystal, the defect is called Vacancy Defects. In vacancy defects, an atom is missing from its regular atomic site. Because of missing of atom the density of substance decreases, i.e. because of vacancy defects. The vacancy defect develops on heating of substance. (b) Interstitial Defects: - Sometime in the formation of lattice structure some of the atoms occupy interstitial site, the defect arising because of this is called Interstitial Defects. In interstitial defect, some atoms occupy sites at which; generally there is no atom in the crystal structure. Because of the interstitial defects, the number of atoms becomes larger than the number of lattice sites. Increase in number of atoms increases the density of substance, i.e. interstitial defects increase the density of substance. The vacancy defects and interstitial defects are found only in non-ionic compounds. Such defects found in ionic compounds are known as Frenkel Defects and Schottky Defects. (c) Frenkel Defects: It is a type of vacancy defect. In ionic compounds, some of the ions (usually smaller in size) get dislocated from their original site and create defect. This defect is known as Frenkel Defects. Since this defect arises because of dislocation of ions, thus it is also known as Dislocation Defects. As there are a number of cations and anions (which remain equal even because of defect); the density of the substance does not increase or decrease. Ionic compounds; having large difference in the size between their cations and anions; show Frenkel Defects, such as ZnS, AgCl, AgBr, AgI, etc. These compounds have smaller size of cations compared to anions. (d) Schottky Defects: Schottky Defect is type of simple vacancy defect and shown by ionic solids having cations and anions; almost similar in size, such as NaCl, KCl, CsCl, etc. AgBr shows both types of defects, i.e. Schottky and Frenkel Defects. When cations and anions both are missing from regular sites, the defect is called Schottky Defect. In Schottky Defects, the number of missing cations is equal to the number of missing anions in order to maintain the electrical neutrality of the ionic compound. Since, Schottky Defects arises because of mission of constituent particles, thus it decreases the density of ionic compound. read less