A Schottky defect is an excitation of the site occupations in a crystal lattice leading to point defects named after Walter H. In ionic crystals, this defect forms when oppositely charged ions leave their lattice sites and become incorporated for instance at the surface, creating oppositely charged vacancies.
Types of stoichiometric defects:
- Vacancy Defects.
- Interstitial defects.
- Frenkel Defects.
- Schottky Defects.
There are 3 types of point defects:
- Stoichiometric defect.
- Frenkel defect.
- Schottky defect.
Point defects are lattice defects of zero dimensionality, i.e., they do not possess lattice structure in any dimension. Typical point defects are impurity atoms in a pure metal, vacancies and self-interstitials.
An F-center, Farbe center or color center (from the original German Farbzentrum, where Farbe means color and zentrum means center) is a type of crystallographic defect in which an anionic vacancy in a crystal lattice is occupied by one or more unpaired electrons.
Answer : (i) Schottky defect : When an atom or ion is missing from its normal site, a lattice vacancy (Schottky defect) is created. Equal number of cations and anions are missing in ionic solids, e.g., in NaCl, there is Schottky defect. (iv) F-centres : Electrons trapped in anion vacancies are called F-centres.
A defect is a physical, functional, or aesthetic attribute of a product or service that exhibits that the product or service failed to meet one of the desired specifications.
In a perfect crystal, all lattice sites are occupied by atoms so that no vacancies are present. If Efis the energy required to create a vacancy by removing an atom from the lattice site (Fig. 4.2) and placing it in a normal site on the crystal surface.
AgBr shows both Frenkel and Schottky defects because the radius ratio for AgBr is intermediate.
Frenkel defect is shown by ionic substance in which there is a large difference in the size of ions, for example, ZnS, AgCl, AgBr and AgI shows this effect due to small size of Zn2+ and Ag+ ions. For example, NaCl, KCl, CsCl and AgBr. It may be noted that AgBr shows both, Frenkel as well as Schottky defects.
We distinguish different kinds of crystal defects which are most easily classified according to their dimension; vacancies and interstitials (zero dimensional point defects), dislocations (one dimensional line defects), and grain and phase boundaries (two dimensional planar defects).
Point defects include the Frenkel type, the Schottky type, and the impurity type. The Frenkel defect involves a single ion, which is displaced from its normal lattice point and shifts to a nearby interstice, or space, between atoms in the lattice.
Non-stoichiometric defects: The defects which disturb the stoichiometry of the compounds are called non-stoichiometry defects. These defects are either due to the presence of excess metal ions or deficiency of metal ions.
Schottky defect arise due to missing of ions from their lattice point and Frenkel defect arise when the missing ions occupy interstitial sites. In AgBr, Ag+ ion is small in size and when removed from lattice point they can occupy interstitial sites and hence show both Frenkel and Schottky defects.
This defect does not have any impact on the density of the solid as it involves only the migration of ions within the crystal thus preserving both the volume as well as mass.
vacancy in solidsIn the so-called Schottky defect, an atom moves from the inside of the crystal to its surface, leaving behind an isolated vacancy. In the Frenkel defect, an atom moves to a new position between other atoms of the solid.
Finally, the Ag+ ion is small in size in AgBr and therefore when removed from lattice point they can occupy interstitial sites and hence show both Frenkel and Schottky defects.
(1) Yes, Schottky defects can exist in K2O; each defect will consist of one O2- vacancy and two K+ vacancies. (2) No, in the strict sense, Schottky cannot exist in K2O if we consider this type of defect to consist of a cation-anion pair; for every O2- vacancy created there must exist two K+ vacancies.
A metal excess defect is one of the defects seen in the crystal structures. These are the non-stoichiometric inorganic solids that contain constituent elements in non-stoichiometric ration because of the defects in their crystal structures.
Normally these defects will lead to a decrease in the density of the crystal. Schottky defects is a vacancy defect which lowers the density of the solid crystal.In this an equal number of cation and anion is missing from the interstitial site and this leads to the lowering of the density.
What is the effect of Frenkel defect on electrical conductivity of the solid? Compounds having frenkel defect conduct electricity to a small extent. When electric field is applied, an ion moves from its lattice site to occupy a hole, it creates a new hole. In this way, a hole moves from one end to the other.
