bravais lattice infinite array of discrete points arranged (and oriented) in such a way that it...
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BRAVAIS LATTICE
Infinite array of discrete points arranged (and oriented) in such a way that it looks exactly the same from whichever point the array is looked at.
BL describes the periodic nature of the atomic arrangements (units) in a X’l.
X’l structure is obtained when we attach a unit to every lattice point and repeat in space
Unit – Single atoms (metals) / group of atoms (NaCl) [BASIS]
Lattice + Basis = X’l structure
2-D honey comb net
P Q
R
Not a BL
Lattice arrangement looks the same from P and R, bur rotated through 180º when viewed from Q
Primitive Translation Vectors
If the lattice is a BL, then it is possible to find a set of 3 vectors a, b, c such that any point on the BL can be reached by a translation vector
R = n1a + n2b + n3c
where a, b, c are PTV and ni’s are integers
eg: 2D lattice
a1a2
a1
a2
a1
a2
(A) (B)(C)
(D)
a1
a2
(A), (B), (C) define PTV, but (D) is not PTV
F4
F6
F1
F2F3
F5
A
BC2
C1
All atoms are either corner points or face centers and are EQUIVALENT
Face centered cubic
a1
a2
a3
For Cube B, C1&C2 are Face centers; also F2&F3
)ˆˆ(2
a a
)ˆˆ(2
a a
)ˆˆ(2
a a
3
2
1
xz
zy
yx
PTV
(0,0,0)
(1,0,0)
(1,1,0)
(0,1,0)
(1,2,0)
(0,0,1)
(0,-1,2) (0,0,2)
(-1,0,2)(-1,-1,2)
(0,1,1)
(-1,-1,3)
a1
a2
a3
PTV
)ˆˆˆ(2
a a
ˆa a
ˆa a
3
2
1
zyx
y
x
A is the body center
A
B is the body center
All points have identical surrounding
Body-centered cubic:2 sc lattices displaced by (a/2,a/2,a/2)
B
PUC
2-D Lattice
a1
a260º
A lattice which is not a BL can be made into a BL by a proper choice of 2D lattice and a suitable BASIS
A
B
The original lattice which is not a BL can be made into a BL by selecting the 2D oblique lattice (blue color) and a 2-point BASIS A-B
DiamondStructure
(0,0,0)
(¼, ¼,¼)
x
y
z(¾, ¾,¼)
(¼, ¾, ¾) (¾, ¼, ¾)
No. of atoms/unit cell = 8Corners – 1Face centers – 3Inside the cube – 4
(000)
2π/λ
(001)
(002)
(00 -1)
(102)(202)(302)
(101)(201)(301)
(100)(200)(300)
(30 -1)
k = k´- k = G201
θ201
(201) plane
k´
Incident beam
k
Diffraction Intensities
• Scattering by electrons• Scattering by atoms• Scattering by a unit cell• Structure factors Powder diffraction intensity calculations– Multiplicity– Lorentz factor– Absorption, Debye-Scherrer and Bragg Brentano– Temperature factor
Scattering by atoms
• We can consider an atom to be a collection of electrons.• This electron density scatters radiation according to the Thomson
approach (classical Scattering). However, the radiation is coherent so we have to consider interference between x-rays scattered from different points within the atom
– This leads to a strong angle dependence of the scattering – FORM FACTOR.
Form factor (Atomic Scattering Factor)• We express the scattering power of an atom using a form factor (f)– Form factor is the ratio of scattering from the atom to whatwould be observed from a single electron
30
20
10
0
fCu
0 0.2 0.4 0.6 0.8 1.0
29
sinθ/λ
Form factor is expressed as a function of (sinθ)/λ as the interference depends on both λ and the scattering angle
Form factor is equivalent to the atomic number at low angles, but it drops rapidly at high(sinθ)/λ
X-ray and neutron form factor
The form factor is related to the scattering density distribution in an atoms- It is the Fourier transform of the scattering density- Neutrons are scattered by the nucleus not electrons and as the nucleus is very small, the neutron form factor shows no angular dependence
F-
C
Li+
f
sinθ/λ sinθ/λ
1H
7Li
3Heb
X-RAY
NEUTRON
Scattering by a Unit Cell – Structure Factor
The positions of the atoms in a unit cell determine the intensities of the reflections
Consider diffraction from (001) planes in (a) and (b)
If the path length between rays 1 and 2 differs by λ, the path length between rays 1 and 3 will differ by λ/2 and destructive interfe-rence in (b) will lead to no diffracted intensity
(a)
a
b
c
1
2
1
2
(b)
2
1
2
3 3
(b)(a)