06-06-2005 interaction of x-rays with materials concepts and vocabularyp kidd
TRANSCRIPT
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Interaction of X-Rays with Materials
Concepts and Vocabulary P Kidd
04/20/23
Agenda• X-ray and Atom• Theories• Diffraction from Crystals• Bragg's Law• The Reciprocal Lattice in XRD• The Reciprocal Lattice and Bragg's Law• The Ewald Sphere• Reciprocal Lattice of a Single Crystal in 3D• Reciprocal Lattice of Powder or Polycrystalline Solid• 2Theta/Omega Powder Scans in Reciprocal Space• Polycrystalline Materials with Preferred Orientation and Texture• Pole Figure Measurement• Stress Measurement• SAXS and Reflectivity• Single Crystal Diffraction to Solve for Molecular Structure• Single Crystal Substrates and Thin Films• Thin Layers and Multilayers• Resolution• Summary
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X-ray and Atom
• An X-ray photon interacts with electrons
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Fluorescence and Scattering• Fluorescence
– Emission at a different wavelength (non-elastic)
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Fluorescence and Scattering• Scattering
– Emission at the same wavelength (elastic)
scattered
Transmitted + scattered
Dipole oscillations of electrons
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Diffraction = Sum of Scattering
• Scattered X-rays adding together in phase to give a diffracted beam (Interference)
Incident beamTransmitted beam
Diffracted Beam
Angle of diffraction
PANalytical Products
• An instrument– Provides X-rays– Aligns a sample– Detects diffraction pattern
• Analysis Software– Performs calculations with peak positions,
widths, intensities– Simulates and fits diffraction patterns
2
SDetector source
sample
Scattering Centres in Materials• Any material is a mass of scattering
centres (electric field distribution)
• Where any length scale is repeated sufficiently often there will be enhanced scattering intensity in some direction
• If the scattering centres are very ordered and periodic the peaks in scattered intensity form a diffraction pattern
Inhomogeneous electric field distributionGrain boundary
homogeneous electric field distributionRandom distribution of bond lengths
Amorphous solid or Liquid
homogeneous electric field distribution
Periodic array Crystal
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Theories: Common Names• Maxwell
– Equations describing interactions of electromagnetic waves (X-rays)
• Laue– Interference from 3D array of scattering centres in crystals
• Ewald– Worked with on general solution for 3D array
• Bragg– Simplified equation based on planes of scattering centres
• Fresnel– Optical model based on uniform refractive index of a material
(reflectivity)
• Dynamical Diffraction (Darwin, Prins, James)– Theory for scattering from perfect single crystals including multiple
scattering events within the crystal
• Kinematical Theory– Simplified theory for “small crystals” not including multiple scattering
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Diffraction From Crystals
• How to simplify diffraction from 3D arrays of atoms
• Principle (Laue/Ewald)– Add scattering from every
individual scattering centre
• Simplification (Bragg)– Describe a crystal as sets
of planes
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Bragg's Law
dhkl
2
Incident beam Diffracted beam
Crystal planes
n = 2dsin
Incident beam is inclined by with respect to crystal planesScattered beam is at 2 with respect to incident beamIncident beam, plane normal, diffracted beam are coplanar
Plane normal
Path difference = n
This is called a ‘reflection’
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Bragg's Law in Diffraction Patterns• Bragg’s law is used to identify the scattering
angle
• Work out the intensity in different directions:– “Form Factor”
• Consider the crystal structure (the density of scattering centres in a plane)
– “Structure factor”
• Consider the microstructure of the material– Polycrystalline powder– Polycrystalline solid– Single crystal
• Consider the Experimental set up
Analysis of diffraction pattern:
Different theoretical approaches are used depending upon these properties
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Crystals: Unit Cells (vectors)•Simplify repeat groups (molecules) to a Lattice (simple scattering point) .
•Define the crystal in terms of unit cells with vectors-|a|, |b| and |c| are unit cell dimensions given in Å
ab
c
•Give each scattering point a vector coordinate r
r = ua + vb + wc
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Unit Cells: Planes and Miller Indeces (hkl)
Classify the scattering points into planes
Designate the planes (hkl) by Miller indices
a
bc
nc/l
b/k
a/h
Plane (hkl)
Plane hkl cuts the unit cell a vector at a/hb vector at b/kc vector at c/l
Formulae for Calculating Plane Spacings and Angles
• Available in crystallography books, databases etc
C u b i c p l a n a r s p a c i n g s : 222
2
lkh
ad
H e x a g o n a l p l a n a r s p a c i n g s :
2
2
222
3
4)(
1
c
l
ahkkh
d
C u b i c i n t e r p l a n a r a n g l e s : 222222 '''
'''cos
lkhlkh
llkkhh
H e x a g o n a l i n t e r p l a n a r a n g l e s :
2221
'''
'
3
4''''cos
c
ll
akhhkkkhhdd lkhhkl
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The Reciprocal Lattice in XRD
• Why do we need this?
– Bragg’s law concept is a simplification that is useful in a limited number of situations
– XRD methods are advancing, we need a clear way of understanding them all.
– The competition are becoming educated in these areas. We need to stay ahead of them.
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The Reciprocal Lattice from Planes•Create reciprocal lattice (RL), where each point represents a set of planes (hkl)
-The points are generated from the RL origin where the vector, d*(hkl), from the origin to the RLP has the direction of the plane normal and length given by the reciprocal of the plane spacing.
000
001
002
110
111
112
d*(112
)
1/d112
001
002112
111110
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The Reciprocal “Lattice” of a 3D Array of Scattering Centres
Scattering centres in a real space crystal lattice
Reciprocal Lattice
Fourier transform
Scattering centres in a random group (e.g. amorphous material)
Fourier transform
The reciprocal of any repeated length scales give reciprocal “lattice” features
1/L
L
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Reciprocal Lattice and Scattering Vectors
000
d*hkl
Reciprocal lattice vector d*hkl
Length 1/dDirection, normal to hkl planes
Incident beam vector, k0,Length n/Direction, with respect to sample surface
Scattered beam vector, kH,Length n/ (user defined)Direction, 2 with respect to k0
2
Diffraction vector, S,S = kH – k0
d*hkl
k0
kH
S
k0kH
By rotating kH and ko the diffraction vector S can be made to scan through reciprocal space.
When S = d*hkl then Bragg diffraction occurs
S
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Scattering Vectors Related to a Real Experiment
PhiPsi
2
S
Detector source
sample
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Reciprocal Lattice and Bragg’s Law
000
d*hkl
= 2k0
kH
By rotating kH and ko and/or the sample we can achieve S = d*hkl
then Bragg diffraction occurs
S
|ko| = 1/
|d*hkl | = 1/d
sin = |d*hkl | /2 |ko|
= 2d sin
Trigonometry:
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The Ewald Sphere• This is a popular way of showing the reciprocal lattice and
scattering using vector algebra– Follows the same principle as previously
Ko
KH
S2d*hkl
Ko incident beam vectorKH diffracted beam vectorS scattering vectord*hkl reciprocal lattice vector
000
hkl
|KH| = |Ko| = radius of Ewald sphere = n/
KH -Ko = S
At maximum intensity:
S = d*hkl
Vector algebra:
KH
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Notation• Different people use different notation
– e.g.
Ko KI incident beam vector
KH Kd diffracted beam vector
S Q scattering vector
d*hkl r*hkl reciprocal lattice vector
|Ko| = |KH| = n/ Where n = 1 or 2 (for example)
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Reciprocal Lattice of a Single Crystal in 3D
004
113
224
115
440
-440
d*| d*| = 1/dhkl
Just a few points are shown for clarity
•There are families of planes
•All planes in the same family have the same length |d*|, but different directions
•The family members have the same 3 indices (in different orders e.g. 400,040,004 etc)
-2-24
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Why Sample Alignment is Important for Single Crystals
2
S 1/
1/
1/
• For n = 2dsin
•Use and to bring a rlp into the diffraction plane
•Use the right combination of and 2 so that S coincides with d*
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Psi and Phi Alignment
PhiPsi
2
S
Detector source
sample
Psi =
Phi =
Omega =
Theta =
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Reciprocal Lattice of Powder or Polycrystalline Solid
• Simultaneous illumination of many small crystals
– Random orientations
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Reciprocal Lattice of Powder
004
113
115
400
d*
• Add the reciprocal lattices of all the crystals
Single crystal lots of single crystals
Concentric Spherical Shells
• A sufficient number of randomly oriented crystals forms a reciprocal “lattice” of spherical shells
000
113
0 0 4
hkl
Just a part of the shells are shown for clarity
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Alignment of Powders or Polycrystalline Solids?!
• Bragg’s law can be satisfied for any and – Providing 2 is correct
2
S = 1/dhkl
S
Spherical shell radius 1/dhkl
1/dhkl
One hkl reflection
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2Theta/Omega “Powder” Scans in Reciprocal Space
2Theta/Omega scan
scattering vector S
Reciprocal lattice points
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2Theta/Omega “powder” scans in reciprocal space
111
2
2Theta/Omega scan
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2Theta/Omega “Powder” Scans in Reciprocal Space
111
220
311
2
2Theta/Omega scan
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2Theta/Omega “Powder” Scans in Reciprocal Space
111
220
311
004 331
2
2Theta/Omega scan
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2Theta/Omega “Powder” Scans in Reciprocal Space
111
220
311
004 331422
511
2
2Theta/Omega scan
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Preferred Orientation and Texture
• A polycrystalline solid may not have a truly random orientation of crystallitesA powder sample may have preferred orientation of not properly prepared.
• What happens to the reciprocal lattice?
Reciprocal Lattice of Non-Random Polycrystalline Material
• Non uniform reciprocal lattice– Different intensities at different directions
2
2
S = 1/dhkl
S
Spherical shell radius 1/dhkl
1/dhkl
Pole Figure Measurement
• A Pole figure maps out the intensity over part of the spherical shell
– 2 stays fixed, the sample is scanned over all at different positions
2
2S
One hkl reflection
Pole Figure Displayed
• Intensity displayed as a contour map, hemisphere is “flattened out”
Al 111 2 = 38o
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Reciprocal Lattice of Fibre Textured Material• Something between single
crystal and random polycrystalline
Sharp Fibre texture:Spots and ringsRandom in but not in
Weak Fibre texture:Arcs and ringsRandom in spread in
Nb 110Al111
38o
Two hkl reflections
Pole Figure
Residual Stress Analysis in Polycrystalline Materials
• Non uniform reciprocal lattice– Different d-spacings at different directions– Polycrystalline components subjected to external
mechanical stresses
Spherical shell distorted (not to scale!)
2
2
S = 1/dhkl
S
1/dhkl not constant
One hkl reflection
“Stress” Measurement
• A stress measurement determines dhkl at a series of Psi positions
– The sample is stepped to different positions, 2 scan at each position to obtain peak position
– Repeated for different positions as requiredSpherical shell distorted
2
2S
1/dhkl varies with position
One hkl reflection
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Amorphous Material
Fourier transform
1/LL
2
2Theta/Omega scan
Amorphous Halo
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Length Scales Other Than d(hkl)
• SAXS and Reflectivity– We have discussed scattering from atoms as
scattering centres
– Bundles of atoms, namely large molecules or particles can also form interference patterns
SAXS - particles Reflectivity – thin films
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Nano- Length Scales in Reciprocal Space: SAXS• Reciprocal lattice is very small• Scattering vector must be small• SAXS is for random array of
particles
Fourier transform
1/L
Ln
2
2Theta/Omega scan
Range of lengths
0o 3o
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Reflectivity• Reciprocal lattice is very small• Scattering vector must be small
Fourier transform
2
2Theta/Omega scan
0o 3o
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Back to Single Crystals
004
113
224
115
440
-440
d*| d*| = 1/dhkl
Just a few points are shown for clarity
Single Crystal Diffraction to Solve Molecular Structure
– Collect all Bragg reflections and analyse position and intensity
–We don’t do this!– We do solve for polycrystalline and powders
Single Crystal Substrates and Thin Films
• We investigate the fine structure of individual reciprocal lattice spots
004
113
224
115
440
-440
This requires high resolution instrumentation
“Reciprocal space map” “Scan”
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Thin Layers and Multi-layers
• The reciprocal lattices of the crystals and the multilayer combine
Fourier transform
004
113
224
115
-440
004
113
224
115
-440
Reflectivity is known as the 000 reflection
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Resolution
Single crystal silicon – measured in 0.001 to 0.01 degree steps 2
Textured Nb/Al multilayer peakmeasured in 0.01 to 1 degree steps in 2
Normal Resolution
High Resolution
Summary• An instrument
– Provides X-rays– Aligns a sample– Detects diffraction pattern
• A Material– Reciprocal “Lattice” Structure
• An Experiment – Designed to suit the material– Designed to answer the question
2
SDetector source
sample
When MRD?• When high resolution is
necessary– Investigate fine features in
reciprocal space
• When alignment is critical– Single crystals
• For reciprocal space mapping – Any Material
• Measurements using Psi and Phi – Texture, Stress
• For X-Y Sample mapping• Versatility
– Many different materials types in one lab