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A '''Schottky defect''' is a type of [[Crystallographic defects#Point defects|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 [[ion]]s leave their lattice sites, creating [[Vacancy defect|vacancies]]. These vacancies are formed in [[Stoichiometry|stoichiometric]] units, to maintain an overall neutral charge in the ionic solid. The vacancies are then free to move about as their own entities. Normally these defects will lead to a decrease in the [[density]] of the crystal. The following are the chemical equations in [[Kröger-Vink Notation]] for the formation of Schottky defects in TiO<sub>2</sub> and BaTiO<sub>3</sub>. | |||
Ø <math> \Leftrightarrow V_{Ti}'''' + 2V_O^{\bullet \bullet}</math> | |||
Ø <math> \Leftrightarrow V_{Ba}'' + V_{Ti}'''' + 3V_O^{\bullet \bullet}</math> | |||
This can be illustrated schematically with a two-dimensional diagram of a [[sodium chloride]] crystal lattice: | |||
[[Image:Nacllattice.svg|thumb|left|The defect-free NaCl structure]] | |||
[[Image:Naclschottkydefect.svg|thumb|left|Schottky defects within the NaCl structure]] | |||
<br style="clear:both;"/> | |||
==Definition== | |||
If in an ionic crystal of type A<sup>+</sup>B<sup>-</sup> an equal number of cations and anions are missing from their lattice sites so that electrical neutrality as well as stoichiometry is maintained this is called a Schottky Defect. | |||
It is a vacancy defect (due to missing ions) and also a stoichiometric defect, as the ratio of the number of cations and anions remains the same. | |||
It occurs only when there is small difference in size between cations and anions. This is produced as the result of the thermal incorporation of unoccupied lattice sites from the exterior of the crystal. The lattice undergoes thermal vibration and thermal expansion when the temperature is raised above 0K. When it happens the pair of vacancies are incorporated in the crystal. | |||
==Examples== | |||
This type of defect is shown in highly ionic compounds, highly coordinated compounds, and where there is only a small difference in sizes of cations and anions of which the compound lattice is composed. | |||
* [[Sodium chloride|NaCl]] | |||
* [[Potassium chloride|KCl]] | |||
* [[Potassium bromide|KBr]] | |||
* [[Caesium chloride|CsCl]] | |||
* [[Silver chloride|AgCl]] | |||
==Effect on Density== | |||
The total number of ions in a crystal with this defect is less than the theoretical value of ions, thus, the density of the solid crystal is less than normal. | |||
==See also== | |||
*[[Frenkel defect]] | |||
*[[Wigner effect]] | |||
*[[Crystallographic defects]] | |||
==References== | |||
{{reflist}} | |||
Kittel, Charles, Introduction to Solid State Physics - 8th ed. Wiley, 2005. ISBN 0-471-41526-X. | |||
==External links== | |||
{{condensedmatter-stub}} | |||
[[Category:Crystallographic defects]] | |||
Revision as of 04:45, 23 October 2013
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. The vacancies are then free to move about as their own entities. Normally these defects will lead to a decrease in the density of the crystal. The following are the chemical equations in Kröger-Vink Notation for the formation of Schottky defects in TiO2 and BaTiO3.
Ø
Ø
This can be illustrated schematically with a two-dimensional diagram of a sodium chloride crystal lattice:
Definition
If in an ionic crystal of type A+B- an equal number of cations and anions are missing from their lattice sites so that electrical neutrality as well as stoichiometry is maintained this is called a Schottky Defect.
It is a vacancy defect (due to missing ions) and also a stoichiometric defect, as the ratio of the number of cations and anions remains the same.
It occurs only when there is small difference in size between cations and anions. This is produced as the result of the thermal incorporation of unoccupied lattice sites from the exterior of the crystal. The lattice undergoes thermal vibration and thermal expansion when the temperature is raised above 0K. When it happens the pair of vacancies are incorporated in the crystal.
Examples
This type of defect is shown in highly ionic compounds, highly coordinated compounds, and where there is only a small difference in sizes of cations and anions of which the compound lattice is composed.
Effect on Density
The total number of ions in a crystal with this defect is less than the theoretical value of ions, thus, the density of the solid crystal is less than normal.
See also
References
43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro. Kittel, Charles, Introduction to Solid State Physics - 8th ed. Wiley, 2005. ISBN 0-471-41526-X.