Download - SAED Patterns of Single Crystal, Polycrystalline and Amorphous Samples abc r1r1 r2r2 200 020 110
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SAED Patterns of Single Crystal, Polycrystalline and Amorphous Samples
a b c
r1 r2200
020
110
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Electron Diffraction
Specimen foil
T D
e-
L 2
r
dhkl
[hkl] SAED pattern
L -camera lengthr -distance between T and D spots1/d -reciprocal of interplanar distance(Å-1)SAED –selected area electron diffraction
Geometry fore-diffraction
Bragg’s Law: = 2dsin
=0.037Å (at 100kV)=0.26o if d=4Å
= 2dr/L=sin2as 0r/L = 2
r/L = /d or
r = Lx1
d
hkl
Reciprocal lattice
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Ewald’s Sphere
Ewaldcircle
0
incident beam
diffracted beam
2
C
ki Gg
130
H
Ewald’s sphere is built for interpreting diffraction
CG-C0=0G or kd-ki=g Laue equation
lkl=1/
kd
Wherever a reciprocal lattice point touches the circle, e.g., at G, Bragg's Law is obeyed and a diffracted beam will occur. At H, no diffraction.
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Convergent Beam Electron Diffraction (CBED)
CBED uses a conver-gent beam of elec-trons to limit area of specimen which con-tributes to diffractionpattern.Each spot in SAED thenbecomes a disc withinwhich variations inintensity can be seen.CBED patterns containa wealth of informationabout symmetry andthickness of specimen.Big advantage of CBEDis that the informationis generated from smallregions beyond reachof other techniques.
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SAED vs CBED
spots disks
lens
sample
Parallel beam Convergent beam
T D T D
Convergence angle
objective
SAED CBED
Spatial resolution >0.5m
Spatial resolutionbeam size
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CBED-example 1
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CBED-example 2
HOLZ
HOLZ - High Order Laue Zone
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Applications of CBED
•Phase identification•Symmetry determination-point and
space group•Phase fingerprinting•Thickness measurement•Strain and lattice parameter measurement •Structure factor determination
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Symmetry Deviations
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Phase Identification in BaAl2Si2O8
200oC 400oC 800oC
<0001>
Hexagonal Orthorhombic Hexagonal
6mm 2mm 6mm
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Phase Fingerprinting By CBED
[001] CBEDpatterns of anantiferroelectricPbZrO3 singlecrystal specimenat (a) 20oC, (b)280oC, (c)220oC.(d) [001] CBEDpattern of a rhombohedral ferroelectric Pb(ZrTi)O3 Specimen at 20oC.
Orthorhombic AFE Cubic PE
Rhombohedral FE Rhombohedral FE
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Symmetry and Lattice Parameter Determination
BFEDS
[143]
[100]
[111]
CBED
CBED-HOLZ
Experimental simulated
Latticeparameters
SAED
010
001
0.2m
NbA
B
A
A
A
A
B
B
B
B