Crystalline defects refer to irregularities in a crystal lattice that can impact a material's properties. There are several types of defects including point defects, line defects, planar defects, and volume defects. Point defects are zero-dimensional and include vacancies, interstitials, and impurities. Vacancies occur when an atom is missing from its lattice site, while interstitials are atoms that occupy spaces between normal lattice sites. Defects can be either desirable or undesirable, as they allow for strengthening materials through properties like dislocations or alloying, but can also introduce weaknesses.

1. Defect in Solids

2. Issues to address …
o What is a crystalline defect?
o What types of defects arise in solids?
o How defects affect materials’ properties?
o Are defects undesirable?
Crystalline Defects

3. • A crystalline defect refers to a lattice irregularity
• The properties of the solids are not only controlled by
their chemical composition and chemical bonding of
constituent atoms but also controlled by the defects or
faults present in their structure.
Crystalline
Defects

4.  There is no such thing as a perfect crystal!
o Vacancies and impurities to some extent are alwaysthere
 Defect does not necessarily imply a bad thing
o introduction of grain boundaries to strengthen materials
o addition of C to Fe to make steel
o addition of Cr to Fe for corrosion resistance
 “Defect” can be either desirable or undesirable

5. Types of Defects
Point Defects (0-D defects)
Line Defects (1-D defects)
Planer Defects (2-D defects)
Volume Defects (3-D defects)

6. Point Defects
 V
acancies
 Interstitials
 Impurity defect
 Schottky Defect
 Frankel defect
Line Defects
 Edge Dislocation
 Screw dislocation
Mixed Dislocations
Planer
Defects
Grain Boundaries
Stacking Faults
External Surfaces
Volume Defects
 Voids
Precipitates
Inclusions
Cracks

7.  When an atom is missing from its lattice site, then a vacancy is
created
 Vacancy is an intrinsic point defect
 The presence of vacancies increases the randomness (entropy)
of the crystal
 Vacancies can be used to control the properties of materials
Vacancy Defect

8. Vacancy Defect

9. Nv=N.𝒆 𝒌𝑻
For a given quantity of a material, the number of vacancies
(Nv) depends on the temperature as (T)
(−𝑸𝒗)
N= total number of atomic sites
Qv= energy required to form a vacancy
T= Temperature
k= Boltzmann constant
Vacancy Defect

10. Vacancies in solids can be produced
 during crystal growth process
 by plastic deformation
 by atomic rearrangement in materials due to atoms mobility
 by irradiation with energetic ions
Vacancy Defect

11. Interstitials
• occupies the non-lattice site in it.
• This type of interstitial is called self-interstitial.
• Self-interstitial is also an intrinsic point defect
• Interstitials exist in small
concentrations as compared to vacancies
• An interstitial defect occurs in a crystal when an atom

12. • In metals a self-interstitial introduces relatively large
distortion in the surrounding lattice because the atom is
substantially larger than the interstitial position in which
it is situated.

13. Interstitials

14. Impurities in Solids
Types of Impurity Defects
• Substitutional Impurity Defect
• Interstitial Impurity Defect

15. It the impurity atom replaces the host atom in a solid then this type
of defect is called substitutional point defect.
Interstitial Impurity Defect
If impurity atom occupies an interstitial position in a lattice
then it is called interstitial point defect.
Substitutional Impurity Defect

16. Substitutional impurity defect depends upon the following;
1) Atomic Size Factor
2) Crystal Structure
3) Electronegativity
4) Valencies

17.  For the formation of substitutional point defect the
difference between the atomic radii between the solvent and
the solute atoms should be less than 15%.
 Crystal Structure
The crystal structures of both metals (solvent and solute) must be
the same
 Electronegativity
The electronegativity difference between host and impurity atoms
should be small
 Valancies
A metal having a higher number of valancies shall have a tendency to dissolve more of
the other metal as compared to one of a lower valance
Atomic Size Factor

18. Interstitial Point Defect
• If impurity atom occupies an interstitial position in a lattice then
it is called interstitial point defect.
• For metallic materials which have relatively high atomic
packing factor, these interstitial positions are relatively small.
• The atomic diameter of an interstitial impurity must be
smaller than that of the host atoms.

20. Schottky Defect
• Schottky defect refers to a point defect
that occurs in an ionic solids when
oppositely charged atoms (cation and
anion) are missing from their lattice
sites.
• This creates a di-vacancy in the
crystal.
• The crystal remains electrically neutral

21. Frenkel Defect
When an atom leaves
its position and
occupies an interstitial
position in a solid then
a vacancy-interstitial
pair is formed. This
type of defect is called
Frenkel defect.

22. Concentration of schottky defects in thermal equilibrium (T#0)

25. Configrational entropy

28. Concentration of Frenkel Defects in Monatomic
Crystal in Thermal Equilibrium

30. • Point defect is a Zero-dimensional in a crystal.
• Two Common Types: Vacancy and Interstitial
• Metal Vacancy is represented as VM
• Metal interstitial in metal M is represented as Mi
Point Defects
Notations

32. Point Defects
Notations

33. For any compound MX, (M=Metal and X= Non-Metal)
Metal vacancy is represented as VM
Non-Metal Vacancy is represented as VX
Metal Interstitial is represented as Mi
Non-Metal Interstitial is represented as Xi
Point Defects
Notations

34. Point Defects
Notations

35. If impurity atom A in a crystal of a metal Mthen,
Substitutional impurity atom is represented as AM
Interstitial impurity atom is represented as Ai
Point Defects
Notations

39. Antisite
Defect
• Antisite defects occur in an alloy or compound when atoms
of different types exchange their positions
• For example, in an alloy AB, the antisite defect occurs when
Atom A occupies onAtom B site
OR
Atom B occupies onAtomAsite
• If both types of atoms exchange their positions then the
composition of alloy remains unchanged.

42. 𝑴𝒈𝑴𝒈 + 𝑨𝒍𝑨𝒍 → 𝑴𝒈𝑨𝒍 + 𝑨𝒍𝑴𝒈
If Mg atom occupies on Al site and Al occupies on Mg
site then it can be represented by the following equation

43. Note:
• Antisite defects can occur during crystal growth, when
atoms are misplaced on the surface of the growing
crystal.
• Alternatively, they can be created by internal mechanisms
once the crystal is formed, provided that sufficient energy
is applied to allow for atom movement.

44. DEFECT FORMATION
& REACTION
EQUATIONS

45. Defect Formation and Reaction
Equations
• Defects are introduced in a solid to modify its physical or
chemical properties
• Normally, when we write chemical equations then we ignore
defects in the balance of equations.
• Defects formation can be included while writing chemical
equations.

46. Addition and Subtraction
of Atoms
• When writing defect formation equations, the strategy involved is
always to add or subtract elements to or from a crystal via
electrically neutral atoms.
• When ionic crystal is involved, this requires that electrons are
considered separately.
• For example, In the case of NiO, there is a possibility of
formation of either Ni vacancy (VNi) or the oxygen vacancy (VO).

47. • If NiO is considered to be ionic then formation of a
VNi means that the removal of neutral Ni atom or
Ni2+ ion together with twoelectrons.
• Similarly, formation of VO means removal of neutral
oxygen atom or O2- ion, followed by the addition of
two electrons to the crystal.
• Similarly, only neutral atoms are added to interstitial
positions. If ions are considered to be present then
the necessary number of electrons must be added or
subtracted as well.

48. Equation Formalism
Rules
• The number of metal atom sites must always be in correct
proportion to the number of nonmetal atom sites in the crystal.
• The total number of atoms on one side of the equation must
balance the total number of atoms on the other side.
• The crystal must be electrically neutral.

49. For Example
• In the case of NiO, Ni is metal and O is non
metal. There must always be equal numbers
of metal and nonmetal atom positions in the
equation
• In TiO2, Ti is a metal and O is non metal. There must always
be twice as many O sites than that of the Ti sites.

50. • In general, for a compound, MaXb, there must be an
‘a’ metal atom sites for every ‘b’ nonmetal atom sites
• This means not only that the total charge on one side
of the equation must be equal to the total charge on
the other side, but also that the sum of the charges on
each side of the equation must equal zero.

51. Formation of Antisite Defects
Consider two atoms A and B. If atom A occupies the site of
atom B then it can be represented as AB and if atom B
occupies the atom A site then it can be written as BA. Both
AB and BA are antisite defect in the crystal. The defect
Equation can be written as;
AA + BB → AB + BA

52. Antisite defects can also be created via the intermediate
formation of a Frenkel defects by the following rules;
Assume atom A is in its normal position ( AA). If it goes to
interstitial site (Ai) then a vacancy atAsite is created (VA).
This represented as;
AA→ Ai + VA (Vacancy-Interstitial Pair)
Formation of Antisite Defects

53. Similarly, assume, atom A is at interstitial position (Ai) and atom
B is at its normal position (B). If atom A moves from interstitial
position to atom B position (AB) then we can write it as;
Ai + BB → AB + Bi

54. The interstitial Bi atom if occupies on atom A sitethen:
Bi + VA → BA
If all these equations are added, the result is:
AA + BB → AB + BA
Formation of Antisite Defects

55. Combination of Point Defects in
Pure Materials
In materials, various kinds of point defects may be present
simultaneously. However, the formation energy of each defect type is
different.
For example, the formation of an intrinsic interstitial defect requires
the simultaneous creation of a vacancy.
𝐒𝐢𝐒𝐢 → 𝐒𝐢𝐢 + 𝐕𝐒𝐢

56. Similarly, in Fe Al alloy, antisite defects occur that consists
of Fe atoms on Al sites and Al atoms on Fesites

57. Structural Consequence of Point Defects

58. Line
Defects
• The defects that extend along a line are called line defects.
• These are also called dislocations
• These are one-dimensional defects
• These defects cause lattice distortion around a line
• Line defects or dislocations are created during solidification of
crystals.
• These are also formed during the plastic deformation

59. Dislocation: It is Line Defect that extend along One Dimension
Types of Dislocations
1) Edge dislocation
2) Screw dislocation
3) Mixed dislocations

60. Edge Dislocation
• If one of the vertical plane does not
extend to full length but ends in between
the crystal then it is called edge
dislocation.
• It is created in a crystal by the insertion
of an extra half plane of atoms
• Edge dislocation is represented by ⊥ or
┬
• In edge dislocation, the burger vector ‘b’
is perpendicular to dislocation line.

61. Perfect Crystal Edge Dislocation

62. Movement of an Edge Dislocation

64. displacement distance
The
of atoms around the
dislocation is called slip or
Burger vector ‘b’.
In edge dislocation, the
burger vector ‘b’ is
perpendicular to dislocation
line.
Edge dislocation is
represented by ⊥ or ┬

65. Burger Vector:
It is a vector that determines the magnitude as well as
direction of slip
Berger Circuit

66. Burgers vector
A vector which represents the magnitude and direction of the
lattice distortion resulting from a dislocation in a crystal lattice.

67. Screw
Dislocation
• In this
dislocation,
type of
the
atoms of crystal are
displaced in two
separate planes
such that it forms a
screw-shape around
the dislocation.

68. Mixed Dislocations
Combination of
Edge and Screw
dislocations

69. Planer
Defects
These are two dimensional defects in crystalline solids and are
also called surface or interfacial defects.
Classification
o Grain boundaries
o Tilt and Twist boundaries
o Twin boundaries
o Stacking faults
o External surfaces

70. Grain
Boundary
• Grain boundary is another interfacial defect
• It is a boundary separating two small grains or crystals
• Grain boundaries are usually the result of uneven growth when
the solid is crystallizing
• At low temperatures, the grain boundaries strengthen metals by
restricting dislocations movement
• When grain size increases, the total number of grain boundaries
are reduced

71. Schematic View of Grain Boundaries

72. Classification of Grain Boundaries
1) Low Angle Grain Boundaries (Tilt and TwistBoundaries
2) High Angle Grain Boundaries (Twin Boundaries)

73. • When the angle between two crystals is less than 11o ,then the
grain boundary is called low angle grain boundary.
• It is also called Tilt grain boundary
• It can be described as set of parallel, equally spaced edge
dislocations of same sign located one above other.
• A Tilt Boundary, between two slightly mis-aligned grains
appears as an array of edge dislocations.
Tilt Boundaries

74. Tilt Boundaries

75. Twist
Boundaries
Twist boundary is
described as an
array of screw
dislocations.

76. • A twin boundary is formed when
atoms in crystal on one side of the
boundary becomes the mirror image
of the atoms on the other side of the
boundary.
• The region of material between the
boundaries is called twins.
Twin
Boundaries

77. Stacking
Faults
Stacking fault is a disruption in long range stacking sequence of
atomic planes.
It is a surface defect that occurs that often occurs in closed packed
structures
For example, in FCC, the stacking is described as
ABCABCABC…….
If the stacking sequence changes as ABCA CABC…. then there
exists a fault of missing a layer of atoms, which is called stacking
fault.

78. Stacking Faults

79. External Surfaces
• External surfaces are imperfection in the material and are
considered as planer defects
• Atoms are the surface are higher in energy than that of the bulk
• External surface produce high surface energy of atoms and thus a
defect occurs in crystal

80. External Surfaces

81. Volume defects
• These are Three dimensional defects
• These are also known as bulk defects
• These occur on much bigger scale than rest of
the crystal defects.
• These defects are normally introduced during
processing and fabrication steps.
• Examples are: voids, precipitates, inclusions,
cracks, pores, bubbles

82. • Voids are regions where there are a large number of atoms
missing from the lattice.
Voids

83. When voids occur due to trapping of air bubbles during
material’s solidification then this is called porosity
Voids

84. Pores: are small holes or openings produces due to the absence
of atoms. They can effect optical, thermal and mechanical
properties of the materials
Pores

85. Precipitates: Another type of bulk defect occurs when impurity
atoms cluster together to form small regions of a different
phase.
Precipitates

86. Inclusions: Foreign particles or large precipitates particles,
undesirable, can affect electrical, mechanical, optical
properties
Cracks: spit different parts of the material without breaking
and can affect the mechanical properties of the solid