lesson 2 diffractometers & phase...
TRANSCRIPT
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Lesson 2Diffractometers &
Phase Identification
Nicola DöbelinRMS Foundation, Bettlach, Switzerland
February 11 – 14, 2013, Riga, Latvia
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2
Repetition: Generation of X-rays
Target (Cu, Mo, Fe, Co, ...)
e‒
Vac
uum
Wavelength (nm)In
tens
ity0.00 0.05 0.10 0.15 0.20 0.25 0.30
Cu
Kα1
Kα2
Kβ
Cu Radiation
Ni filter
Ni-filtered Cu Radiation
Graphite Monochromator
CuKα1/2Radiation
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Repetition: Powder Diffraction
3
n · λ = 2 · d · sin(θ)
d
λ
θθ 2θ
(120)
(100)
(010)
Powder sample
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Repetition: Powder Diffractometer
4
X-ray tube
Primary Beam
PowderSample
Diffraction Cones«Secondary Beams»
X-ray Detectorscanning X-ray intensity
vs. 2θ angle
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Analogue Cameras
5
http://adias-uae.com
Debye-Scherrer Camera:
Powder in Glass Capillary
Diffraction pattern recordedon photographic film
2θ angle
Various alternative setups:
Gandolfi …
Guinier …
Straumanis …
Bradley …
Seemann-Bohlin …
…Camera
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Digital Diffractometer
6
Debye-Scherrer Configuration
Transmission Geometry
Glass Capillary
FoilFluid Cell
Capillaries are ideal for:
• Light atoms (Polymers, Pharmaceuticals)
• Small amounts
• Hazardous materials
• Air-sensitive materials
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Bragg-Brentano Diffractometer
7
Flat powder sample
Bragg-Brentano Configuration
Reflective Geometry
Reflective Geometry is ideal for:
• Absorbing materials (Ceramics, Metals)
• Thin films
• Texture analysis
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Instruments
8
Lab Instrument Monochromator Configuration
RTU Rigaku Ultima+ Graphite
Monochromator
Bragg-Brentano
(Reflection)
RTU Panalytical X’Pert Ni-Filter Bragg-Brentano
(Reflection)
LU Bruker D8 Ni-Filter Bragg-Brentano
(Reflection)
Uppsala Uni Bruker D8 Ni-Filter Bragg-Brentano
(Reflection)
RTU Salaspils Bruker D8 Energy-dispersive
Detector
Bragg-Brentano
(Reflection)
RMS (Uni Bern) Panalytical X’Pert Ni-Filter Bragg-Brentano
(Reflection)
RMS (Uni Bern) Panalytical CubiX Graphite
Monochromator
Bragg-Brentano
(Reflection)
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Bragg-Brentano Diffractometer
9
Flat powder sample
More optical elements are requiredto control the beam pattern.
X-ray tube
Detector
Kβ filter
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Beam Divergence
10
Angle of Divergence Point focus
Line Focus
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Beam Divergence
11
Limiting vertical divergencewith a «divergence slit» (DS)
Limiting horizontal divergencewith a «Collimator»
Collimator for Line focus= «Soller Slits» (SS):
Non-parallel rays areblocked by lamellae
Beam Mask:Limits the beam width
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Beam Divergence
12
Divergence Slit
Soller Slit
Beam Masks
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Soller Slits
13
Poster
Viewed throughSoller Slit
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Bragg-Brentano Parafocusing Diffractometer
14
Sample
X-raytube
Detector
Goniometercircle
Divergenceslit
Sollerslit
Sollerslit
Anti-Scatterslit
Beammask
Anti-Scatterslit
Receivingslit
Focusingcircle
Kβ Filter
Typical Configuration(with Kβ filter)
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Bragg-Brentano Parafocusing Diffractometer
15
Sample
Detector
Sollerslit Anti-Scatter
slit
Receivingslit
Secondarymonochromator
Typical Configuration(with secondary monochromator)
Modern instruments are modular.
Configuration can be changed easily.
PANalytical: «PreFIX»
Bruker: «SNAP-LOCK»
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Example: PANalytical X’Pert Pro MPD
16
Tube
Soller Slits
ProgrammableDivergence Slit
Beam Mask
Sample Stage«Spinner»
Anti-ScatterSlit
ProgrammableAnti-Scatter Slit
Soller Slit
Ni-Filter
Detector
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Optimum Settings: Divergence Slit
17
80° 2θ
20° 2θ
20° 2θ
Wide divergence slit:Beam spills over sample at low 2θ angles.
Use narrower divergence slit.
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Optimum Settings: Divergence Slit
18
20° 2θ 80° 2θ
Low incident angle:
- Low penetration depth
- Large illuminated area
High incident angle:
- Deep penetration depth
- Small illuminated area
Fixed divergence slit:
Irradiated Volumeis constant
Constant intensity ofdiffraction pattern
20° 2θ
Low incident angle:
- Narrow divergence slit
- Low penetration depth
Variable divergence slit:
80° 2θ
High incident angle:
- Wide divergence slit
- Deep penetration depth
Irradiated Areais constant
Higher diffracted intensityat high 2θ angle
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Fixed vs. Variable Divergence Slit
19
20 30 40 50 60 70 80 90
Inte
nsity
[a.u
.]
Diffraction Angle [°2θ]
fixed
variable
More than 2x higherintensity at 90° 2θwith variable DS
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Optimum Settings: Divergence Slit
20
Sample holder
Sample surface
Recommendation:
- Set divergence slit to «variable»
- Adjust «irradiated length» and beam maskfor maximum illumination
- But avoid beam spill-over!
Irradiated area
Primarybeam
Correct! Wrong! Wrong!Reduce «irradiated length»
of divergence slitUse a smaller Beam Mask
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Optimum Settings: Divergence Slit
21
Using sample holders of various sizes?
Match your Divergence Slit and Beam Mask!
Or else: Waste of intensity Beam spill-overor
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Variable Divergence Slit: Irradiated Length
22
30.2 30.3 30.4 30.5 30.6 30.70
500
1000
1500
2000
2500
3000In
tens
ity [c
ount
s]
Diffraction Angle [°2θ]
5mm 10mm 15mm
Soller Slits: 0.02 rad
Beam Mask: 10mm
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Variable Divergence Slit: Irradiated Length
23
30.2 30.3 30.4 30.5 30.6 30.70
20
40
60
80
100In
tens
ity [%
]
Diffraction Angle [°2θ]
5mm 10mm 15mm
Soller Slits: 0.02 rad
Beam Mask: 10mm
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Beam Mask
24
30.2 30.3 30.4 30.5 30.6 30.70
500
1000
1500
2000
2500
3000
3500In
tens
ity [c
ount
s]
Diffraction Angle [°2θ]
5mm 10mm 20mm
Soller Slits: 0.02 rad
Irradiated Length: 10mm
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Beam Mask
25
30.2 30.3 30.4 30.5 30.6 30.70
20
40
60
80
100In
tens
ity [%
]
Diffraction Angle [°2θ]
5mm 10mm 20mm
Soller Slits: 0.02 rad
Irradiated Length: 10mm
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Soller Slits
26
30.2 30.3 30.4 30.5 30.6 30.70
1000
2000
3000
4000
5000
6000
7000In
tens
ity [c
ount
s]
Diffraction Angle [°2θ]
0.02rad 0.04rad
Beam Mask: 10mm
Irradiated Length: 10mm
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Soller Slits
27
30.2 30.3 30.4 30.5 30.6 30.70
20
40
60
80
100In
tens
ity [%
]
Diffraction Angle [°2θ]
0.02rad 0.04rad
Beam Mask: 10mm
Irradiated Length: 10mm
Affects resolution atlow 2θ angles
(effect is less pronouncedat high 2θ angles)
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Receiving Slit
28
Strongly affects theresolution.
(Not all instrumentshave a receiving slit)
Parrish, W. Advances in X-ray diffractometry of clay minerals. X-ray analysis papers(W. Parrish, ed.), pp. 105-129. Centrex, Eindhoven, The Netherlands, 1965.
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Summary: Optical Elements
29
Optical Element Effect Too Small Too Large
Divergence Slit Adjusts beam length
on the sample
Loss of intensity Beam spills over
sample
Soller Slit Reduces peak
asymmetry
Loss of intensity,
Better resolution
More asymmetry,
Less resolution
Anti-Scatter Slit Reduces background
signal
Loss of intensity High background
Beam Mask Adjusts beam width
on the sample
Loss of intensity Beam spills over
sample
Receiving Slit Adjusts peak width /
resolution
Loss of intensity
Better resolution
Loss of resolution
Higher intensity
Kβ Filter Reduces Kβ peaks - -
Graphite
Monochromator
Eliminates Kβ peaks - -
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Detectors
30
Detector’swindow
Receivingslit
Point Detector (0D) Linear Detector (1D) Area Detector (2D)
Receiving slitdetermines
active height
Linear array ofsolid statedetectors
2D array ofsolid statedetectors
Scintillation counterSOL-XE
X’CeleratorLynxEye
D/teX Ultra
PIXcelVÅNTEC
SOL-XE: Energydispersive
Fast
Can be set to«0D mode»
2D image ofDebye rings
Can be set to «1D»and «0D» mode
DetectorType
Example
KeyFeatures
Needs receiving slit!
Position-sensitive«bins»
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Instruments
31
Lab Instrument Monochr. Detector
RTU Rigaku Ultima+ Graphite 0D
RTU Panalytical X’Pert Ni-Filter 1D X’Celerator
LU Bruker D8 Ni-Filter 1D LynxEye
Uppsala Uni Bruker D8 Ni-Filter 1D LynxEye
RTU Salaspils Bruker D8 Energy-disp.
Detector
0D SOL-XE
RMS (Uni Bern) Panalytical X’Pert Ni-Filter 1D X’Celerator
RMS (Uni Bern) Panalytical CubiX Graphite 0D
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Phase Identification
32
10 20 30 40 50 60
0
200
400
600
800
1000
Inte
nsity
[cou
nts]
Diffraction Angle [°2θ]
A crystal structure will generatea characteristic XRD pattern. Feature Origin
Peak positions - Symmetry of the unit
cell
- Dimensions of the
unit cell
Relative peak intensities - Coordinates of atoms
in unit cell
- Species of atoms
Absolute peak intensities - Abundance of phase
- Primary beam
intensity
Peak width - Crystallite size
- Stress/Strain in crystal
lattice
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Phase Identification
33
10 20 30 40 50 60
0
200
400
600
800
1000
Inte
nsity
[cou
nts]
Diffraction Angle [°2θ]
1. Extract peak positions
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Phase Identification
34
10 20 30 40 50 60
0
200
400
600
800
1000
Inte
nsity
[cou
nts]
Diffraction Angle [°2θ]
Sample
Corundum
Fluorite
1. Extract peak positions
2. Compare withdata base
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35
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Databases
36
Database Publisher # of Entries Data sets
PDF-2 ICDD(http://www.icdd.com)
250’182 All
PDF-4+ ICDD(http://www.icdd.com)
328’660 Inorganic
PDF-4/Minerals ICDD(http://www.icdd.com)
39’410 Minerals(Subset of PDF-4+)
PDF-4/Organics ICDD(http://www.icdd.com)
471’257 Organics
COD CODhttp://www.crystallography.net
215’708 All(excl. biopolymers)
Databases containing powder diffraction data (line positions)
Commercial
Open Access
(Received funding by Research Council of Lithuania (2010-2011))
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Programmes for Search / Match
37
Programme Publisher Supported Databases*
HighScore PANalytical PDF-2/4
COD
EVA Search/Match Bruker PDF-2/4
PDXL2 Rigaku PDF-2
COD
RayfleX GE PDF-2/4
Sleve ICDD PDF-2/4
Match! Crystal Impact PDF-2/4
COD
CSM Oxford Cryosystems PDF-2/4
Jade MDI PDF-2/4
+ many more(see http://www.ccp14.ac.uk/solution/search-match.htm)
*incomprehensive
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Search / Match: Restrictions
38
By chemical Composition By Subfile
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Summary: Phase Identification I
39
- Phases are identified from XRD patterns by comparing peakpositions with database entries
- Search/Match software & database are required
- Various commercial / open programmes and databases
- Qualitative (sometimes semi-quantitative) results are obtained
- Phase identification is independent of Rietveld refinement(must be done before)
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Question I: Polytypes
40
Is powder XRD the ideal tool to distinguishand identify the following phases?
Phase Composition Space Group
Calcite CaCO3 R-3c
Magnesite MgCO3 R-3c
Siderite FeCO3 R-3c
Structurally very similar (polytypes)
They generate similar diffraction patterns
XRD provides no direct information on Ca/Mg/Fe content
Only changes in unit cell dimensions.
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Question I: Polytypes
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Calcite CaCO3
Magnesite MgCO3
- Similar diffraction patterns(mostly peak shifts)
- Some information on Mg/Ca/Fecontens from unit celldimensions
Solution:Combine XRD with chemicalanalysis (ICP, XRF, EDX, XPS…)
Siderite FeCO3
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Question II: Polymorphs
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Is powder XRD the ideal tool to distinguishand identify the following phases?
Phase Composition Space Group
Calcite CaCO3 R-3c
Vaterite CaCO3 P63/mmc
Aragonite CaCO3 Pnam
Structurally different (polymorphs)
Chemical analyses not able to distinguish (chem. identical)
XRD can easily distinguish
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Question II: Polymorphs
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Calcite CaCO3
Aragonite CaCO3
Vaterite CaCO3
- Strongly different diffractionpatterns.
- Easily identified by XRD
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Summary: Phase identification II
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- XRD is mostly sensitive to structural differences
- Only little information on chemical differences
- Chemical analyses (XRF, ICP, EDX,…) providecomplementary information
- Sometimes additional chemical information can be veryhelpful for phase identification
- For a comprehensive material characterization, combineXRD with chemical analysis
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Typical Slit Configuration
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X-ray Mirrors
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• Collimators and Divergence Slits cut off intensity
• There are no lenses for X-rays(Index of refraction for all materials ~1)
• Bragg diffraction can be used to construct mirrors
• Single crystal with parabolic surface: All beams coming from the tube focusare in diffraction condition for Kα1/2
Parallel Kα1/2 beam
Goebel Mirror
Focused Kα1/2 beam
Focusing Mirror
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Polycapillary Optics
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Latest generation:
Polycapillary glass fiber opticsConserves most of the primary beam intensity