defects in crystals by deepika
TRANSCRIPT
Defects in crystals
UNIT CELL : A crystal is a solid in which the constituent
atoms, molecules,ions are packed in a regularly ordered, repeating pattern of ‘building blocks’, extending in all three spatial dimensions.-the ‘building block’ is known as the unit cell.
The line of dots is called the lattice, and each lattice point (dot)must have identical surroundings
LATTICE
Seven Crystal Systems
CRISTAL DEFECTS
POINT DEFECT
LINE DEFECT
Defect will be on a certain point or position.
If ions or atoms are misplaced or replaced by other atoms along a line.
POINT DEFECT
STOICHIOMETRIC
NON-STOICHIOMETRI
C
STOICHIOMETRIC
IN IONIC SOLIDS
SCHOTTKY DEFECT
FRENKEL DEFECT
IN NON-IONIC
SOLIDS
VACANCY DEFECT
INTERSTITIAL DEFECT
Missing of Cation
Anion missing
SCHOTTKY DEFECT
Mass of crystal decreases.
In an ionic crystal of NaCl type equal number of cations & anions are missing from their lattice sites.
Missing ion present in interstitial site
Missing cation
FRENKEL DEFECT
An ion is missing from its lattice site and occupies the interstitial site.
Mass and volume of crystal are not changed.
Density of solid remains unaffected.
Particle in the interstitial site
INTERSTITIAL DEFECT
some foreign atoms are trapped in interstitial voids of lattice .
Mass of crystal increases.
NON-STOICHIOMETRI
C
BY METAL EXCESS
Anion vacancies
By the presence of extra cation in interstitial sites
BY METAL DEFICIENCY
By removal of metal
cation from their lattice
site
Metal excess defect by Anion vacancies
If no. of missing anions are more than no. of missing cation .
Anion may be missing from its lattice site leaving a hole which occupied by an electron to maintain electrical neutrality .
The location of electron where it is trapped i.e. anion vacancies called F – Centers
F - Centers
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F-center
F = farbe a German word means ‘colour’
F centers are responsible for imparting color to the crystal.
Example ; at room temperature ZnO is white in color when it heated some O ions get released in the form of O2 & electrons are trapped at their location due to this it appears yellow .
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