10. crystal diffraction - tu graz
TRANSCRIPT
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Institute of Solid State PhysicsTechnische Universität Graz
10. Crystal Diffraction
April 19, 2018
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crystal structure solution
"Guess" the crystal structure
From the atomic form factors, calculate the structure factors nG.Compare |SG|2 to
the measurements
position of atom j of the basisSum over basis
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x-ray diffraction
The shape and the dimensions of the unit cell can be deduced from the positions of the Bragg reflections; the content of the unit cell, on the other hand, must be determined from the intensities of the reflections.
Solid State Physics, Ibach and Lüth
The intensity of the peaks is proportional to the squared Fourier coefficients of the electron density.
k G
Diffraction condition:
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x-ray diffraction
2hkl
hkl
Gd
)(hklGhkl
k
kG
-k
for /2, 2|k| = |G| andconstructive interference takes place when 2dhkl = .
2 22 2hkl
hkl
Gk
d
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x-ray diffraction
2hkl
hkl
Gd
22 sinhklhkl
G k kd
2 sinhkld
)(hklGhkl
k
kG
-k
distance between the net planes2k
another formulation of the diffraction condition
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2 sind n
1 2 3k G hb kb lb
1 0012 002
nn
Bragg and Laue conditions
Bragg condition:
2 sinhkld Laue condition:
d001d002
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Powder diffraction
Powder diffraction is performed on a powder of many small crystals. Ideally, every possible crystalline orientation is represented equally in a powdered sample. The relative intensities of the diffraction peaks indicate which crystal structures are present.
d
X-rays: Bragg diffraction
2 sinhkld
k
k'
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PowderCell
fccbcc
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Powder diffraction
Phase identificationEvery crystal has a specific "fingerprint" given by the positions and intensities of the diffraction peaks. The composition of a multi-phase specimen can be determined by fitting its diffraction pattern to the diffraction patterns of pure crystals which can be looked up in a database.
International Centre for Diffraction Data www.icdd.com550,000 reference materials
Phase transitions, thermal expansion, piezoelectricity, piezomagnetism, bulk modulus, compliance tensor can be measured.
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Electron diffraction in a TEM
The wavelength of the electrons is typically much smaller than the lattice spacing. The diffraction peaks in the plane perpendicular to k are observed.
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LEEDLow Energy Electron Diffraction
Clean Pd (111) Pd (111) + 0.3 ML VOx
LEED is surface sensitive
100 V k ~ 5 × 1010 m-1
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LEEDht
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lam
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hadl
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s1/c
ryst
aldi
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tion/
atom
icfo
rmfa
ctor
s/LE
ED.p
hp
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Forbidden reflections
Low Energy Electron Diffraction
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Forbidden reflections
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Neutron diffraction
Typically a nuclear reactor is used as the neutron source
There are different atomic form factors for neutrons than for x-rays.
Determine the positions of H in biological samples.
Can for example distinguish between Fe and Co which have similar atomic form factors for x-rays.
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Atomic beams
Hydrogen and Helium are used for diffraction studies
2 2 2 221
2 22 2 2p k hE mvm m m
Low energies can be used for delicate samples.Measure the surface like LEED.