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Reciprocal Lattice
• Points to discuss• Reciprocal lattice• Definition and examples
• First Brillouin
Zone
• Lattice planes• Miller Indices
Reciprocal Lattice
• A diffraction pattern is not a direct representation of the crystal lattice
• The diffraction pattern is a representation of the reciprocal lattice.
• But what is a reciprocal lattice?
Reciprocal Lattice
• For every real lattice there is an equivalent reciprocal lattice. A two dimension (2‐D) real
lattice is defined by two unit cell vectors, say and
inclined at an angle. The equivalent
reciprocal lattice in reciprocal space is defined by two reciprocal vectors, say
and .
• Each point in the reciprocal lattice represents a set of planes.
Reciprocal Lattice
• The set of all wave vectors that yield plane waves with the periodicity of a given Bravais
lattice is known as its reciprocal lattice.
• Analytically, belongs to the reciprocal lattice of a Bravais
lattice of points , provided that
the relation holds for any and for all in the Bravais
lattice.
• Factoring out we can characterize the reciprocal lattice as the set of wave vectors
satisfying =1 for all in Bravais
lattice.
Reciprocal Lattice
• The Bravais
lattice that determines a given reciprocal lattice is often referred to as the direct lattice when viewed in relation to its reciprocal lattice.
• Reciprocal lattice is a Bravais
lattice. We shall prove it in next slides.
• Now question is that how reciprocal lattice vectors can be chosen.
Reciprocal Lattice vectors in 2D
Real lattice
Reciprocal lattice
a1
a2
b1
b2
Reciprocal lattice vectors can also be denoted by a*,b* etc. or sometime by g1 , g2 etc.
Defining the reciprocal lattice vector
Take two sets of 2D planes:
Draw directions normal:These lines define the orientation but not the length
We use 1d
to define the lengths
These are called reciprocal lattice vectors b1 and b2
b1
b2
Dimensions = 1/length
a1
a2
Reciprocal Lattice vectors• The reciprocal vectors are defined as follows:• is of magnitude 1/d1
where d1
is the spacing of the vertical planes, and is perpendicular to
.
• is of magnitude 1/d2
where d2
is the spacing of the horizontal planes, and is perpendicular to
.
• A
reciprocal
lattice
can
be
built
using
reciprocal vectors.
Both
the
real
and
reciprocal
constructions
show
the
same
lattice,
using different but equivalent descriptions.
• Consider the following animation.
The animation can be envisaged as follows:
• The real lattice is described at the left, the reciprocal lattice is described at the right.
• g is the reciprocal lattice vector.• The absolute value of g
is equal to and
the direction is that of the normal N
to the appropriate set of parallel atomic planes of the real lattice separated by distance d.
The animation can be envisaged as follows:
• Imagine yourself to be an atom.
• When you fly around the crystal formed from two parallel 7 x 7 planes, you would see the
picture at the bottom.
• If someone watches you from above and tries to figure out what you are seeing, he would
draw the pictures at the top of the screen.
• One picture is a the real image, the other is an imaginary picture.
Reciprocal lattice vectors
• From examples it is obvious that for 3D.
i.e. is perpendicular to both and .
Similarly is perpendicular to both andand is perpendicular to both and
.
Reciprocal lattice vectors
• The cross product defines a vector parallel to
with modulus of the area
defined by and .
• The volume of the unit cell is thus given by
• We can define the reciprocal lattice vectors
, and in terms of direct lattice
vectors , and as follows.
Reciprocal lattice vectors
Reciprocal Lattice is a Bravais
Lattice
• Reciprocal lattice vectors satisfy,
can be written as a linear combination of
Where
Reciprocal Lattice is a Bravais
Lattice
• If is any direct lattice vector, then
and thus,
Since is an integer, are also integers. Thus
is times an integer. Thus reciprocallattice is a Bravais lattice.
The Reciprocal Of The Reciprocal Lattice
• Since Reciprocal lattice is itself a Bravais lattice, one can construct its reciprocal lattice.
• Let reciprocal of the reciprocal lattice is the set of all vectors G satisfying for
all in the reciprocal lattice.
• Since any direct lattice vector has this property, all direct lattice vectors are in the
lattice reciprocal to the reciprocal lattice.
The Reciprocal Of The Reciprocal Lattice
• No other vectors can be, for a vector not in the direct lattice has the form,
with at least one non-integral for that value of , and the above condition is violated for the reciprocal lattice vectors .
Reciprocal Lattice Of Simple Cubic Bravais
Lattice
• The simple cubic Bravais
lattice with cubic primitive cell of side has as its reciprocal
lattice a simple cubic lattice with cubic primitive cell of side . Where is
the crystallographer's definition.
• The cubic lattice is therefore said to be self‐ dual, having the same symmetry in reciprocal
space as in real space.
Reciprocal Lattice Of SC Bravais
Lattice
If
then
Reciprocal Lattice Of fcc
Bravais
Lattice
• The reciprocal lattice to a fcc
lattice is the bcc lattice.
• Consider a fcc
compound unit cell.
• Locate a primitive unit cell of the fcc, i.e., a unit cell with one lattice point.
• Take one of the vertices of the primitive unit cell as the origin.
• Give the basis vectors of the real lattice.
Reciprocal Lattice Of fcc
Bravais
Lattice• Then from the known formulae you can
calculate the basis vectors of the reciprocal lattice.
• These reciprocal lattice vectors of the fcc represent the basis vectors of a bcc real
lattice.
• Note that the basis vectors of a real bcc lattice and the reciprocal lattice of an fcc
resemble
each other in direction but not in magnitude.
Reciprocal Lattice Of fcc
Bravais
Lattice• The fcc
Bravais
lattice with conventional cubic
cell of side has as its reciprocal a bcc lattice with conventional cubic cell of side
. i.e.
This has precisely the form of the bcc primitive vectors provided that the side of the cubic cell is taken to be .
Reciprocal Lattice Of bcc Bravais Lattice
• The reciprocal lattice to a bcc lattice is the fcc lattice.
• Only the Bravais
lattices which have 90 degrees between (cubic,
tetragonal, orthorhombic) have parallel to their real‐space vectors.
Reciprocal Lattice Of bcc Bravais Lattice
• The bcc Bravais
lattice with conventional cubic cell of side has as its reciprocal a fcc
lattice with conventional cubic cell of side
. i.e.
• Reciprocal of bcc is fcc
and reciprocal of fcc
is bcc this proves that the reciprocal of the
reciprocal is the original lattice.
Reciprocal Of Simple Hexagonal Bravais
lattice
• A simple Hexagonal Bravais
lattice with lattice constants c and a has its reciprocal another
simple Hexagonal lattice with lattice constants and rotated through
about the c-axis with respect to the direct lattice.
Reciprocal Of Simple Hexagonal Bravais
lattice
Reciprocal Lattices to SC
Direct lattice Volume
3/2 a
Reciprocal lattice
Reciprocal Lattices to fcc
Direct lattice Reciprocal lattice
Volume
3/22 a
Reciprocal Lattices to bcc
Direct lattice Reciprocal lattice
Volume
3/24 a
Volume Of The Reciprocal Lattice Primitive Cell
• If is the volume of the primitive cell with side in the direct lattice the primitive cell of
the reciprocal lattice has a volume .
• The simple cubic Bravais
lattice, with cubic primitive cell of side a, has for its reciprocal a
simple cubic lattice with a cubic primitive cell of side ( in the crystallographer's definition).
The cubic lattice is therefore said to be self‐ dual, having the same symmetry in reciprocal
space as in real space.
Brillouin
zone
• In the propagation of any type of wave motion through a crystal lattice, the
frequency is a periodic function of wave vector k.
• In order to simplify the treatment of wave motion in a crystal, a zone in k‐space is
defined which forms the fundamental periodic region, such that the frequency or energy for
a k
outside this region may be determined from one of those in it.
Brillouin
zone
• This region is known as the Brillouin
zone sometimes called the first or the central
Brillouin
zone.
• It is usually possible to restrict attention to k values inside the zone.
• Discontinuities occur only on the boundaries.• The central Brillouin
zone for a particular solid
type is a solid which has the same volume as the primitive unit cell in reciprocal space.
Construction of first Brillouin
zone
Draw lines connecting the origin point to its nearest neighbors.
Draw perpendicular bisectors to these lines. These perpendicular bisectors are Bragg Planes.
Taking the smallest polyhedron containing the point bounded by these planes is first Brillouin zone..
First Brillouin
Zone
Higher Brillouin
Zones• The second Brillouin
zone is the set of points
that can be reached from the first zone by crossing only one Bragg plane.
• The (n
+ 1)th Brillouin
zone is the set of points not in the (n
‐
1)th zone that can be reached
from the nth zone by crossing n
‐
1 Bragg planes.
• The nth Brillouin
zone can be defined as the set of points that can be reached from the
origin by crossing n
‐
1 Bragg planes, but no fewer.
The locus of points in
reciprocal space that
have no Bragg Planes
between them and
the origin defines the
first Brillouin
Zone. It
is equivalent to the
Wigner‐Seitz unit cell
of the reciprocal
lattice. Small black
dots represent point of intersection of
Bragg planes
The second Brillouin
Zone is the region of
reciprocal space in
which a point has one
Bragg Plane between
it and the origin. This
area is shaded yellow
in the picture below.
Note that the areas of
the first and second
Brillouin
Zones are
the same.Small black dots
represent point of intersection of
Bragg planes
The construction can
quite rapidly become
complicated as you
move beyond the
first few zones, and it
is important to be
systematic so as to
avoid missing out
important Bragg
Planes.
Small black dots represent point
of intersection of Bragg planes
http://www.doitpoms.ac.uk/tlplib/brillouin_zones/printall.php
First Brillouin
zone of bcc lattice• The reciprocal of the bcc lattice is the fcc
lattice. The first Brillouin
zone of the bcc lattice is just the fcc
Wigner Seitz cell.
First Brillouin
zone of bcc lattice
First Brillouin
zone of fcc
lattice
• The first Brillouin
zone of the fcc
lattice is just the bcc Wigner Seitz cell.
First Three Brillouin Zones Of bcc and fcc lattices
Lattice planes
• Any plane containing at least three non‐ collinear Bravais
lattice points.
• Because of the translational symmetry of the Bravais
lattice, any such plane will actually
contain infinitely many lattice points which form a 2D Bravais
lattice within the plane.
Lattice Planes Of simple Cubic Bravais Lattice
Family Of Lattice Planes
• A set of parallel equally spaced lattice planes, which together contains all the points of the
three dimensional Bravais
lattice.
• Any lattice plane is a member of such family.
• Resolution of a Bravais
lattice into a family of lattice planes is not unique.
Theorem of possible families of lattice planes
• For any family of lattice planes separated by a distance d, there are reciprocal lattice vectors perpendicular to the planes the shortest of
which have a length of .
• Conversely, for any reciprocal lattice vector there is a family of lattice planes normal to
and separated by a distance d, where
is the length of the shortest reciprocal lattice vector parallel to .
Proof Of First Part Of Theorem• Given a family of lattice planes.
• be a unit vector normal to the planes.
• is a reciprocal lattice vector.
• The plane wave is constant in planes perpendicular to and has the same value in
planes separated by .
• One of the lattice planes contains the Bravais lattice point , must be unity for any point r in any of the planes.
Proof Of First Part Of Theorem• The planes contain all Bravais
lattice points
=1 for all , so that is indeed a reciprocal lattice vector.
is the shortest reciprocal lattice vector normal to the planes.
• For any wave vector shorter than will give a plane wave with wave length greater than .
• Such a plane wave cannot have the same value on all planes in the family, and cannot give a plane wave
that is unity at all Bravais
lattice points.
Proof Of Second Part Of Theorem• Given a reciprocal lattice vector.• Let be the shortest parallel reciprocal
lattice vector.
• Consider a set of real space planes on which the plane wave has the value unity.
• These planes are perpendicular to and separated by a distance .
• All Bravais
lattice vectors satisfy
for any reciprocal lattice vector they must all lie within these planes.
Proof Of Second Part Of Theorem• The spacing between the lattice planes is also
d but not an integral multiple of d, for if only every nth plane in the family contained
Bravais
lattice points.
• Then according to the first part of the theorem, the vector normal to the planes of
length i.e. the vector , would be a reciprocal lattice vector.
• This would contradict our original assumption that no reciprocal lattice vector parallel to
is shorter than .
Miller Indices Of lattice Planes• The correspondence between reciprocal
lattice vectors and families of lattice planes provides a convenient way to specify the
orientation of a lattice plane.• In general we describe the orientation of a
lattice plane by giving a vector normal to that plane.
• There are reciprocal lattice vectors normal to any family of planes, we pick a reciprocal
lattice vector to represent the normal.
Miller Indices Of lattice Planes• To make the choice we should use the shortest such
reciprocal lattice vector. In this way we arrive at the Miller indices of the plane.
• Miller indices of a lattice plane are the coordinates of the shortest reciprocal lattice vectors normal to that
plane, with respect to the specified set of primitive reciprocal lattice vectors.
• A plane with Miller indices is normal to the reciprocal lattice vector .
• are integers.• They have no common factor.• They depend on particular choice of primitive
vectors.
Miller Indices Of lattice Planes• A set of three integers that designate
crystallographic planes, as determined from reciprocals of fractional axial intercepts.
• In any kind of repeating pattern, it is useful to have a convenient way of specifying the
orientation of elements relative to the unit cell. This is done by assigning to each such
element a set of integer numbers known as its Miller index.
Miller Indices Of Cubic Bravais
Lattice• In simple cubic Bravais
lattice the reciprocal
lattice is also simple cubic and the Miller indices are the coordinates of a vector normal
to the plane in the obvious cubic coordinate system.
• Fcc
and bcc Bravais
lattices are described in terms of a conventional cubic cell .Any lattice
plane in a fcc
or bcc lattice is also a lattice plane in the underlying simple cubic lattice,
the same elementary cubic indexing can be used to specify lattice planes.
Miller Indices Of Cubic Bravais
Lattice• It is only in the description of non‐cubic crystal
that we must remember that the Miller indices are the coordinates of the normal in a
system given by the reciprocal lattice, rather than the direct lattice.
• A lattice plane with Miller indices is perpendicular to the reciprocal lattice vector
= , it will be contained in the continuous plane for suitable
choice of the constant A.
Miller Indices• This plane intersects the axis determined by
the direct lattice primitive vectors , at the points and , where the are determined by the coordination that
indeed satisfy the equation of the plane
.
• Since
and
, it follows that,
Miller Indices• Intercepts with the crystal axes of a lattice plane are
inversely proportional to the Miller indices of the plane.
• Miller indices is a set of integers with no common factors, inversely proportional to the intercepts of the
crystal plane along the crystal axes.
Figure shows Miller indices of a lattice plane. The shaded plane can be a portion of the continuous planes in which the points of the lattice plane lie. The Miller indices are inversely proportional to the xi.
Examples of Miller Indices
Indexing planes in three‐dimensions• We proceed in exactly the same way, except
that we now have 3‐digit Miller indices corresponding to the axes a, b
and c.
The indices may denote a single plane or a set of parallel planes.
Some examples of planes
Some More Examples
Some More Examples
Lattice planes are usually specified by giving their Miller indices in parentheses (h,k,l). In cubic system a plane with a normal (4,-2,1) is called a (4,-2,1) plane. The commas are eliminated without confusion by writing n instead of –n, simplifying the description to (421)
Conventions For Specifying Directions• A similar convention is used to specify
directions in the direct lattice, but to avoid confusion with the Miller indices (directions in
the reciprocal lattice) square brackets are used instead of parenthesis.
Some directions
Notation of specifying family of planes
• (100), (010) and (001) planes are equivalent in a cubic crystal.
• We refer to them collectively as the {100} planes, and in general we use {hkl} to refer to
the (hkl) planes and all those that are equivalent to them by virtue of the crystal
symmetry.
Notation of specifying family of directions
• The [100], [010], [001], [100], [010] and [001] directions in the cubic crystal are referred to , collectively, as the <100> directions.
Symmetry Operations
3-tetrad axes 4-triad axes
6-diad axes