Solid state physics (schottkey and frenkel)

schottkey and frenkel defects with explanation

1. IMPERFECTIONS IN CRYSTALS An important feature of crystal is the regular atomic arrangement but no crystal is perfectly regular. Any deviation from this perfect atomic periodicity is an imperfection which also called as lattice defect. Lattice defect – a state in which the atomic arrangement(it`s structure) in smaller region of a crystal has departed from regularity.

2. CLASSIFICATION OF STRUCTURAL DEFECTS

3. SCHOTTKY AND FRENKEL DEFECTS

4. SCHOTTKY DEFECT A  Schottky defect  is a type of point defect in a crystal lattice named after  Walter H. Schottky . In non-ionic crystals it means a lattice vacancy defect. In ionic crystals, the defect forms when oppositely charged ions leave their lattice sites, creating vacancies. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid.

5. If in an ionic crystal of the type A +  B - , equal number of cations and anions are missing from their lattice. It is called Schottky defect.    Just because the equal no. of anion and cation are missing, the whole crystal arrangement is maintained as electrically neutral. This occurs only when there is small difference in size of cations and anions.

6. This type of defect is shown by highly ionic compounds such as NaCl, KCl, CsCl, AgBr, KBr And it also have a high co – ordination number.

7. Consequences of Schottky defect As the number of ions decreases as a result of this defect, the mass decreases whereas the volume remains the same. Hence density of the solid decreases, The crystal begins to conduct electricity to a small extent by ionic mechanism, The presence of too many voids lowers lattice energy and the stability of the crystal.

8. SCHOTTKY DEFECT

9. FRENKEL DEFECT The phenomenon is named after the   Soviet physicist YakovFrenkel , who discovered it in 1926. A Frenkel defect is a type of defect in crystalline solids wherein an atom is displaced from its lattice position to an interstitial site, creating a vacancy at the original site and an interstitial defect at the new location within the same element without any changes in chemical properties.

10. DEFINITION A  Frenkel defect ,  Frenkel pair , or  Frenkel disorder  is a type of point defect in a crystal lattice. The defect forms when an atom or smaller ion (usually cation) leaves its place in the lattice, creating a vacancy, and becomes an interstitial by lodging in a nearby location.   Since cations are usually smaller it is more common to find the cations occupying interstitial sites.      So it is a combination of vacancy and interstitial atom.

11. FRENKEL DEFECT

12. This in ionic compounds which have Low co ordinations number, Larger difference in size of cation and anions, Compounds having highly polarising cation and easily polarisable anion. A few examples of ionic compounds exhibiting this defect are AgCl, AgBr, AgI, ZnS etc.

13. Consequences of Frenkel defect As no ions are missing from the crystal lattice as a whole, therefore density of the solid remains the same, The closeness of like charges tends to increases the dielectric constant of the crystal, The crystal conducts electricity to a small extent by ionic mechanism.

14. Schottky Defect Vs Frenkel Defect Schottky defect Frenkel defect Equal number of cations and anions are missing from the lattice sites . Found in highly ionic compounds with high coordination numbers and where the cations and anions are of similar size . Density of the solid decreases. A cation leaves the normal lattice site and occupies the interstitial sites. Found in ionic compounds with low coordination numbers and where the anions are much larger in size than cations . Density of the solid remains the same.

15. REFERENCE Solid state physics by Guptha and Kumar, Solid state physics by S.O.Pillai , https://en.wikipedia.org/wiki/Schottky_defect https://en.wikipedia.org/wiki/Frenkel_defect

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