Defects in crystals
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Imperfections in crystals
Crystalline imperfections can be classified
on the basis of their geometry as follows
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A. vacancies/schottky defecfts
B. Interstitial Vacancies/Frenkel defecfts
Point Defects
0D
C. Compositional defecfts
Substitutional impurity
Interstitial impurity
D. Electronic impurity
Crystalline
Defects
Line Defects
1D
1. Edge Dislocation
2. Screw Dislocation
Surface
Defects
2D
a. Grain boundaries
b. Twin boundaries
c. Tilt boundaries
d. Stacking fault
Volume
Defects
3D
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Non Crystalline regions of dimensions at
least 10 to 30°A
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CLASSIFICATION OF DEFECTS BASED ON DIMENSIONALITY
0D
(Point defects)
1D
(Line defects)
2D
(Surface / Interface)
3D
(Volume defects)
Vacancy
Dislocation
Surface
Twins
Impurity
Disclination
Interphase
boundary
Precipitate
Frenkel
defect
Dispiration
Schottky
defect
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Grain
boundary
Faulted
region
Twin
boundary
Voids /
Cracks
Stacking
faults
Thermal
vibration
Anti-phase
boundaries
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Point Defects
• Vacancy: A Vacancy refers to an atomic site
from where the atom is missing.
Tensile Stress
Fields ?
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Point Defects
• Substitutional impurity: A Substitutional
impurity refers to a foreign atom that has
replaced a parent atom.
Compressive stress
fields
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Tensile Stress
Fields
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Point Defects
• Interstitial Impurity: An Interstitial impurity
refers to small sized atom occupying the void
space in the parent crystal without replacing
the parent atoms
Relative
size
Compressive
Stress
Fields
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Point Defects
• Frenkel imperfection: An ion displaced from a
regular site to an interstitial site is called
frenkel imperfection.
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Point Defects
• Schottky imperfection: A pair of cation and
anion missing from an ionic crystal resulting
in a pair of vacant ion sites is called schottky
imperfection
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Point Defects
• Electronic Defects: Errors in charge distribution
in solids are called electronic defects. These
defects are produced when the composition of an
ionic crystal does not correspond to the exact
stoichiometric formula.
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Line Defects
Line defects are one dimensional imperfections
in the geometrical sense. Line imperfections
are called dislocations.
Edge dislocation
In perfect crystal atoms are arranged in both
vertical and horizontal planes parallel to the
side faces. If one of these planes does not
extend to the full length but ends in between
within the crystal it is called edge dislocation.
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Edge dislocation
. Edge dislocation are caused by the termination
of a plane of atoms in the middle of the crystal.
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Burger’s Vector
• The presence of dislocation results in lattice
strain (distortion). The direction and
magnitude of such distortion is expressed in
terms of a Burger’s vector.
Or
The magnitude and the direction of the
displacement are defined by a vector called
the burger vector.
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4
4
3
3
3
3
4
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Above: Burger’s circuit for dislocation-free material.
note “compressed bonds” and “elongated bonds.”
To Right: Do same with dislocation and end up
“past” starting point.
Vector b = distance to get back to curcuit.
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• The Burgers vector(b) is perpendicular to the
edge dislocation line
• The Burgers vector(b) is parallel to the screw
dislocation line.
Screw Dislocation
Screw dislocation results from a displacement
of the atoms in one part of a crystal relative to
the rest of the crystal forming a spiral ramp
around the dislocation line.
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Screw
Dislocation
Slip Planes
Slip Direction
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Surface imperfections
• Surface imperfections refer to regions of distortions
that lie about a surface having thickness of a few
atomic diameters
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Surface imperfections
• Grain Boundaries (or) Crystal Boundary
At the interface between two crystals randomly oriented the
atoms held in between are attracted by crystals on either side
and depending on the forces the atoms occupy equilibrium
positions. This distorted region of a few atomic diameter
thickness is called grain boundary.
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Surface imperfections
• The orientation difference is usually greater than 10-15°. For
this reason the grain boundaries are also known as high angle
boundaries.
• When the orientation
difference between two
crystals is less than 10° the
distortion in the boundary
is not so drastic.
They are called low angle boundaries.
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Surface imperfections
• The sub grain boundaries are of two types they are
1. Tilt boundaries
2. Twist boundaries.
• An array of edge dislocations is called tilt boundary
• A cross grid of screw dislocation is called twist boundaries.
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Tilt
boundaries
Twist
bound
aries
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Twin Boundaries
A twin boundary happens when the
crystals on either side of a plane are
mirror images of each other. Two single
crystal sections are miss oriented but
joined together such that one plane is a
mirror image of the other. The boundary
common to the two planes is a twin
boundary. They are formed during the
growth of crystal
The boundary between the twinned
crystals will be a single plane of atoms.
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Stacking Faults
• Stacking faults are planar surface imperfections caused by
fault in the staking sequence of atomic planes in crystals
• In FCC crystal we have three different stacking layers
ABC while in HCP stacking we have only two different
layers BC hence when FCC crystal grows we have the
stacking as
………ABCABCABCABC……
While growing if the plane A indicated by arrow above
missing then we get the sequence
………ABCABCBCABC……
Thus we find that the stacking in the missing region be comes
HCP. This thin region is a surface imperfection and is
called a stocking fault.
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Stacking Faults
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Stacking Faults
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3D
(Volume defects)
Twins
Precipitate
Faulted region
Voids / Cracks
Thermal vibration
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3D
(Volume defects)
Twins
Precipitate
Faulted region
Voids / Cracks
Thermal vibration
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3D
(Volume defects)
Twins
Precipitate
Faulted region
Voids / Cracks
Thermal vibration
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Volume Defects
• Presence of a large vacancy (crakes) or void
such as clusters of atoms missing is also
considered as a volume defect. While crystal
is grown there is every possibility of inclusion
of non-crystalline regions of dimensions of at
least 10 to 30°A. This is also called Volume
imperfection.
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