physics of materials: symmetry and bravais...
TRANSCRIPT
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Physics of Materials
Dr. Anurag SrivastavaAtal Bihari Vajpayee
Indian Institute of Information Technology and
Manegement, Gwalior
Physics of Materials:
Symmetry and Bravais LatticeTo understand Crystal Plane/Face
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ABV- IIITM-Gwalior (MP) IndiaPhysics of Materials
Couldn’t
find his
photo
ML Frankenheim
15 lattices
1811-1863
Auguste Bravais
14 lattices
Your photo
13 lattices
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
A state in which parts on opposite sides of a plane, line,
or point display arrangements that are related to one
another via a symmetry operation such as
translation, rotation, reflection or inversion.
Application of the symmetry operators leaves the entire
crystal unchanged.
Symmetry
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
The first point is repeated at equal distances
along a line by a translation uT, where T is
the translation vector and u is an integer
Symmetry Elements:
Translation
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Symmetry Elements:
Translation
moves all the points in the
asymmetric unit the same distance
in the same direction. This has no
effect on the handedness of
figures in the plane. There are no
invariant points (points that map
onto themselves) under a
translation.
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
A rotation can be applied on the translation vector T in alldirections, clock or anti-clock wise, through equal angles α,Because of the regular pattern, the translation between these
two points will be some multiple of T (pT)
Symmetry Elements:
Rotation
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
turns all the points in the asymmetric unit
around one axis, the center of rotation. A
rotation does not change the handedness of
figures. The center of rotation is the only
invariant point (point that maps onto itself).
Symmetry Elements:
Rotation
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Symmetry Elements:
Rotation
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Symmetry Elements:
Rotation
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Symmetry Elements
Screw axes (rotation + translation)
rotation about the axis of
symmetry by 360/n, followed
by a translation parallel to the
axis by r/n of the unit cell length
in that direction. (r < n)
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
120 rotation
1/3 unit cell translation
Symmetry Elements
Screw axes (rotation + translation)
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Symmetry Elements
Inversion, or center of symmetry
every point on one side of a
center of symmetry has a similar
point at an equal distance on the
opposite side of the center of
symmetry.
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Symmetry ElementsReflection or Mirror symmetry
An object with a reflection symmetry will be a
mirror image of itself across a plane called
mirror plane ( m).
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
flips all points in the asymmetric unit
over a line, which is called the mirror,
and thereby changes the handedness of
any figures in the asymmetric unit.
The points along the mirror line
are all invariant points (points that map
onto themselves) under a reflection.
Symmetry ElementsReflection or Mirror symmetry
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
In this operation, every part of the object is
reflected through an inversion center called center
of symmetry which is denoted as i. The object is
reproduced inverted from its original position.
Symmetry ElementsInversion symmetry
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Symmetry elements:
mirror plane and inversion center
The handedness is changed.
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Symmetry Elements
Glide reflection (mirror plane + translation)
reflects the asymmetric unit
across a mirror and then
translates parallel to the mirror.
A glide plane changes the
handedness of figures in the
asymmetric unit. There are no
invariant points (points that map
onto themselves) under a glide
reflection.
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Example: consider the bcc structure. Assume the atoms can be represented as
hard spheres with the closest atoms touching each other and the lattice
constant is 0.5 nm.
Calculate the surface density of atoms on the (110) plane.
Surface Density of Atoms on a Particular Plane
Consider the atoms on the (110) plane. The atom at each corner is shared by
four similar rectangles. So one fourth of each atom at the corner contributes
to the shaded rectangle. The four corner atoms effectively contribute one
atom to the shaded rectangle. The atom at the center is not shared by any
other rectangle. It is entirely included in the shaded rectangle. Therefore, the
shaded rectangle contains two atoms.
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Surface Density of Atoms on a Particular Plane
Description of directions in a lattice:
In addition to lattice planes, we also want to describe a particular direction in
the crystal. The direction can be expressed as a set of three integers that are
the components of a vector in that direction. For example, the body diagonal in
the sc lattice has vector components of 1,1,1. The body diagonal is then
described as the [111] direction. The brackets are used to designate direction
as distinct from the parentheses used for crystal planes.
Solution: the surface density is found by dividing the number of lattice atoms
by the surface area.
The surface density of atoms is a function of the particular crystal plane and
generally varies from one crystal plane to another.
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Silicon is the most common semiconductor material. Both silicon and
germanium have a diamond crystal structure.
The basic building block of the diamond structure is the tetrahedral
structure.
The Diamond Structure
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
An important characteristic of the diamond structure is that any atom in the
structure has four nearest neighboring atoms.
All atoms in the diamond structure are of the same species, such as silicon or
germanium.
The Diamond Structure
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
The Zincblende (Sphalerite) Structure
The zincblende (sphalerite) structure differs from the diamond structure only
in that there are two different types of atoms in the structure.
Compound semiconductors, such as GaAs, have the zincblende structure.
The important feature of both the diamond and zincblende structure is that
the atoms are joined together to form tetrahedrons.
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Classification of lattice
The Seven Crystal System
And
The Fourteen Bravais Lattices
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Lattices
In 1848, Auguste Bravais demonstrated that in a 3-dimensional system there are fourteen possible lattices
A Bravais lattice is an infinite array of discrete pointswith identical environment
seven crystal systems + four lattice centering types =14 Bravais lattices
Lattices are characterized by translation symmetry
Auguste Bravais
(1811-1863)
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Crystal System External Minimum Symmetry Unit Cell Properties
Triclinic None a, b, c, al, be, ga,
Monoclinic One 2-fold axis, || to b (b unique) a, b, c, 90, be, 90
Orthorhombic Three perpendicular 2-foldsa, b, c, 90, 90, 90
Tetragonal One 4-fold axis, parallel c a, a, c, 90, 90, 90
Trigonal One 3-fold axis a, a, c, 90, 90, 120
Hexagonal One 6-fold axis a, a, c, 90, 90, 120
Cubic Four 3-folds along space diagonal a, a, ,a, 90, 90, 90
triclinictrigonal
hexagonal
cubic tetragonal
monoclinicorthorhombic
7 Crystal Systems
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ABV- IIITM-Gwalior (MP) IndiaSemiconductor Physics
No. Type Description
1 Primitive Lattice points on corners only. Symbol: P.
2 Face Centered Lattice points on corners as well as centered on faces. Symbols: A (bc faces); B (ac faces); C (ab faces).
3 All-Face Centered Lattice points on corners as well as in the centers of all faces. Symbol: F.
4 Body-Centered Lattice points on corners as well as in the center of the unit cell body. Symbol: I.
Four lattice centering types
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
NotationP: Primitive (lattice points only at the corners of the unit cell)
I: Body-centred (lattice points at the corners + one lattice point at the centre of
the unit cell)
F: Face-centred (lattice points at the corners + lattice points at centres of all faces
of the unit cell)
C: End-centred or base-centred (lattice points at the corners + two lattice points at
the centres of a pair of opposite faces)
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
14 Bravais lattices divided into seven
crystal systems
Crystal system Bravais lattices
1. Cubic P I F
Simple cubic
Primitive cubic
Cubic P
Body-centred cubic
Cubic I
Face-centred cubic
Cubic F
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
14 Bravais lattices divided into seven
crystal systems
Crystal system Bravais lattices
1. Cubic P I F
2. Tetragonal P I
3. Orthorhombic P I F C
4. Hexagonal P
5. Trigonal P
6. Monoclinic P C
7. Triclinic P
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Orthorhombic C
End-centred orthorhombic
Base-centred orthorhombic
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
14 Bravais lattices divided into seven
crystal systems
Crystal system Bravais lattices
1. Cubic P I F
2. Tetragonal P I
3. Orthorhombic P I F C
4. Hexagonal P
5. Trigonal P
6. Monoclinic P C
7. Triclinic P
?
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
End-centred cubic not in the Bravais list ?
End-centred cubic = Simple Tetragonal
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
14 Bravais lattices divided into seven
crystal systems
Crystal system Bravais lattices
1. Cubic P I F C
2. Tetragonal P I
3. Orthorhombic P I F C
4. Hexagonal P
5. Trigonal P
6. Monoclinic P C
7. Triclinic P
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Face-centred cubic in the Bravais list ?
Cubic F = Tetragonal I
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Basis for classification of lattices
into
7 crystal systems
and
14 Bravais lattices?
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Symmetry classification of lattices
Based on rotational and reflection symmetry alone
7 types of lattices
7 crystal systems
Based on complete symmetry, i.e., rotational, reflection and translational symmetry
14 types of lattices
14 Bravais lattices
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Coordination Number
Coordination number is the number of
nearest neighbor to a particular atom in the
crystal
For example Coordination number for
different unit cells are:
Simple cubic – 6
Body centered – 8
Face centered --12
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Coordination Number of
Simple Cubic
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Atomic Packing efficiency
Packing efficiency indicates how closely atoms
are packed in a unit cell
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Atomic Packing efficiency of Simple
Cubic
rno. of atoms per unit cell = (1/8)*8) =1.
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Atomic Packing efficiency of BCC
no. of atoms per unit cell=
(1/8)*8(corner atoms)+1(body centre)=2.
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Atomic Packing efficiency of FCC
no. of atoms per unit cell is=(1/8)*8(corner
atoms)+(1/2)*6(atoms at face)=4
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Solid State
Packing Efficiency of Close Packed Structure - 1Both ccp and hcp are highly efficient lattice; in terms of packing. The packing efficiency of both types of close packed structure is
74%, i.e. 74% of the space in hcp and ccp is filled. The hcp and ccp structure are equally efficient; in terms of packing.
The packing efficiency of simple cubic lattice
is 52.4%. And the packing efficiency of body
centered cubic lattice (bcc) is 68%.Calculation of pacing efficiency in hcp and ccp structure:The packing efficiency can be calculated by the percent of space occupied by spheres present in a unit cell.
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
And diagonal AC = b
Now, in ∆ ABC,
AB is perpendicular, DC is base and AC is diagonal
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
Thus, packing efficiency of hcp or ccp structure=74%
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
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ABV- IIITM-Gwalior (MP) IndiaCrystal Physics
What is the difference between CCP, FCC,
and HCP?
CCP stands for cubic closed packing , FCC is for face centered cubic
structure…
Now , HCP and CCP are one of the forms in which a cubic lattice is
arranged and FCC is one of the types of unit cells(in general). When we
place the atoms in the octahedral voids the packing is of ABCABC..
type.This is known as CCP. Now sometimes we also call it as face
centered closed structure (FCC)But remember FCC can also stand for
the unit cell (Face centered cell).
*Hexagonal close packed (hcp). 12:12 coordination. ABAB layers.
Cubic close packed(ccp). 12:12 coordination. ABCABC layers.
Body centred cubic (bcc) 8:8 coordination.
Face centred cubic (fcc) is just an alternate name for ccp.*