Nicola Döbelin
RMS Foundation, Bettlach, Switzerland
Good Diffraction Data
BGMN/Profex User Meeting 2019
Good Diffraction Data2
Good Diffraction Data3
Good Diffraction Data4
• Very poor sample preparation
(a few grains on a piece of glass?)
• Problematic instrument setup
for low-angle range
• Texture
• «Refinable»
Good Diffraction Data
Complex phase composition
Large number of phases
Low-symmetry phases
Low crystallinity / glass / organic content
Fluorescence
Sample preparation
Graininess
Texture
Height displacements
Instrument setup
S/N ratio
Background
Angular resolution(peak width / asymmetry)
5
Inherent to the sample
Can (should)be minimized
Digital Diffractometers6
Transmission GeometryDebye-Scherrer Geometry
Glass Capillary
FoilFluid Cell
Capillaries are ideal for:• Light atoms (Polymers, Pharmaceuticals)• Small amounts• Hazardous materials• Air-sensitive materials
Use characteristic radiation withlow absorption coefficient
Flat powder sample
Reflective GeometryBragg-Brentano Geometry
Reflective Geometry is ideal for:• Absorbing materials (Ceramics, Metals)• Thin films• Texture analysis
Use characteristic radiation withhigh absorption coefficient
Bragg-Brentano Parafocusing Diffractometer7
Sample
X-raytube
Detector
Goniometercircle
Divergenceslit
Sollerslit
Sollerslit
Anti-Scatterslit
Beammask
Anti-Scatterslit
Receivingslit
Focusingcircle
Kβ Filter
Typical Configuration(with Kβ filter)
Bragg-Brentano Parafocusing Diffractometer8
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»
Instrument Configuration
Many optical elements = many options to optimize data quality
How to find the best configuration?
9
Tube
Soller Slits
ProgrammableDivergence Slit
Beam Mask
Sample Stage«Spinner»
Anti-ScatterSlit
ProgrammableAnti-Scatter Slit
Soller Slits
Ni-Filter
Detector
Divergence Slit and Beam Mask10
Sample
X-raytube
Detector
Goniometercircle
Divergenceslit
Sollerslit
Sollerslit
Anti-Scatterslit
Beammask
Anti-Scatterslit
Receivingslit
Focusingcircle
Kβ Filter
Optimum Settings: Divergence Slit & Beam Mask11
Irradiated length
Beam divergence
Beam width
Controlled byDivergence slit
Controlled byBeam mask
θ
Optimum Settings: Divergence Slit12
20° 2θ
80° 2θ
Fix divergence slit:
Variable / Automatic divergence slit:
80° 2θ
- Constant intensity+ No mechanical parts
+ Gain of intensity at high 2θ- Backlash in slit mechanics
Fixed vs. Variable Divergence Slit13
20 30 40 50 60 70 80 90
Inte
nsity [a
.u.]
Diffraction Angle [°2]
fix
variable
More than 2x higher intensityat 90° 2θ with variable DS
Divergence Slit: Irradiated Length14
30.2 30.3 30.4 30.5 30.6 30.7
0
500
1000
1500
2000
2500
3000
Inte
nsity [co
un
ts]
Diffraction Angle [°2]
5mm
10mm
15mm
Soller Slits: 0.02 rad, Beam Mask: 10mm
30.2 30.3 30.4 30.5 30.6 30.7
0
20
40
60
80
100
Inte
nsity [%
]
Diffraction Angle [°2]
5mm
10mm
15mm
15 mm10 mm5 mm
Beam Mask15
30.2 30.3 30.4 30.5 30.6 30.7
0
500
1000
1500
2000
2500
3000
3500
Inte
nsity [co
un
ts]
Diffraction Angle [°2]
5mm
10mm
20mm
Soller Slits: 0.02 rad, Irradiated Length: 10mm
30.2 30.3 30.4 30.5 30.6 30.7
0
20
40
60
80
100
Inte
nsity [%
]
Diffraction Angle [°2]
5mm
10mm
20mm
20 mm10 mm5 mm
Optimum Settings: Divergence Slit & Beam Mask16
Sample holder
Sample surface
For phase quantifications:- Variable divergence slit- Adjust «irradiated length» and beam
mask for maximum illumination- Avoid beam spill-over!
Irradiated area
Correct! Wrong! Wrong!
Reduce «irradiated length»of divergence slit
Use a smaller Beam Mask
For structure refinements:- Fix divergence slit- Adjust «slit size» and beam mask for
maximum illumination at lowest 2θ- Avoid beam spill-over!
Optimum Settings: Divergence Slit & Beam Mask17
Using sample holders of various sizes?
Match your Divergence Slit and Beam Mask!
Or else: Waste of intensity Beam spill-overor
Soller Slits / Collimators18
Sample
X-raytube
Detector
Goniometercircle
Sollerslit
Sollerslit
Anti-Scatterslit
Anti-Scatterslit
Receivingslit
Focusingcircle
Kβ Filter
30.2 30.3 30.4 30.5 30.6 30.7
0
20
40
60
80
100
Inte
nsity [%
]
Diffraction Angle [°2]
0.02rad
0.04rad
Soller Slits / Collimators19
30.2 30.3 30.4 30.5 30.6 30.7
0
1000
2000
3000
4000
5000
6000
7000
Inte
nsity [co
un
ts]
Diffraction Angle [°2]
0.02rad
0.04rad
In primary & secondary beam, Beam Mask: 10mm, Irradiated Length: 10mm
0.04 rad0.02 rad
Asymmetry more pronounced at low 2θ, less at high 2θ
Anti-Scatter Slits20
Sample
X-raytube
Detector
Goniometercircle
Anti-Scatterslit
Anti-Scatterslit
Receivingslit
Focusingcircle
Kβ Filter
Anti-Scatter Slits and Beam Knifes21
Sample
Divergenceslit
Anti-Scatterslit
Removes radiation scatteredat air molecules
=Lower background signal
In primary beam:Same settings as divergence slit
Beam knife
Anti-Scatter Slits22
Secondary beam
Point detector (0D):Same settings as divergence slit
Linear / area detector (1D / 2D):Wide open or removed
Anti-Scatter Slits23
34.8 34.9 35.0 35.1 35.2 35.3 35.4 35.5
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
Inte
nsity (
Co
un
ts)
Diffraction Angle (°2theta)
9.00 mm
4.50 mm
2.25 mm
34.8 34.9 35.0 35.1 35.2 35.3 35.4 35.5
0
20
40
60
80
100
Inte
nsity (
%)
Diffraction Angle (°2theta)
9.00 mm
4.50 mm
2.25 mm
Soller Slits: 2.5 deg, Irradiated Length: 15mm, LynxEye XE in 1D mode
Blocking detectorchannels
Receiving Slit24
Sample
X-raytube
Detector
Goniometercircle
Receivingslit
Focusingcircle
Kβ Filter
Receiving Slit25
34.8 34.9 35.0 35.1 35.2 35.3 35.4 35.5
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
Inte
nsity (
Co
un
ts)
Diffraction Angle (°2theta)
0.075 mm
0.225 mm
0.525 mm
34.8 34.9 35.0 35.1 35.2 35.3 35.4 35.5
0
20
40
60
80
100
Inte
nsity (
%)
Diffraction Angle (°2theta)
Soller Slits: 2.5 deg, Irradiated Length: 15mm, LynxEye XE in 0D mode
Detector Opening (PSD Opening)26
Sample
X-raytube
Detector
Goniometercircle
Focusingcircle
Kβ Filter
Detector Opening (PSD Opening)27
Maximum PSD opening
Openingangle
Minimum PSD opening
Position-sensitive detectors (1D, 2D)
Detector Opening (PSD Opening)28
34.8 34.9 35.0 35.1 35.2 35.3 35.4 35.5
0
20
40
60
80
100
Inte
nsity [%
]
Diffraction Angle [°2theta]
2.36°
1.00°
0.10°
0.01°
34.8 34.9 35.0 35.1 35.2 35.3 35.4 35.5
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
Inte
nsity [C
ou
nts
]
Diffraction Angle [°2theta]
2.36°
1.00°
0.10°
0.01°
Soller Slits: 2.5 deg, Irradiated Length: 15mm, LynxEye XE in 1D mode
Summary: Optical Elements29
Optical Element Effect (Too) Small (Too) Large
Divergence Slit Adjusts beam length on the sample
Loss of intensity Beam spills oversample
Beam Mask Adjusts beam width on the sample
Loss of intensity Beam spills oversample
Soller Slit Reduces peak asymmetry Loss of intensity,Better resolution
More asymmetry,Less resolution
Anti-Scatter Slit Reduces backgroundsignal
Loss of intensity High background
Receiving Slit Adjusts peak width / resolution
Loss of intensityBetter resolution
Loss of resolutionHigher intensity
PSD Opening Adjusts the number ofactive detector channels
Loss of intensity -
IntensityResolution
Filters and Monochromators30
Sample
X-raytube
Detector
Goniometercircle
Focusingcircle
Kβ Filter
Filters and Monochromators31
Secondarymonochromator
Kβ Filter
Kβ Filter
Energy-dispersivedetector
Summary: Filters and Monochromators32
Optical Element Filter effect Effect on Kα Intensity Comment
Primary beammonochromator
Eliminates Kβ Strong lossWorks with 1D / 2D
detectors
Secondary beam monochromator
Eliminates Kβ peaksEliminates Fluorescence
Strong loss 0D detectors only
Kβ Filter Reduces Kβ peaks Moderate loss
Can be combinedEnergy dispersive Detector
Reduces Kβ peaksEliminates Fluorescence
Little loss
31.5 34.5 35.0 35.5
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Diffraction Angle (°2theta)
Kbeta Filter 0.0125 mm
31.5 34.5 35.0 35.5
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Diffraction Angle (°2theta)
Digitally filtered
31.5 34.5 35.0 35.5
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Diffraction Angle (°2theta)
Digitally filtered + Kbeta Filter 0.0125 mm
+ =
Sample Preparation33
Potential Problems:- Graininess- Micro-absorption- Texture- Crystallite size- Sample height displacement- Surface roughness- Sample transparency
Graininess34
http://people.physics.anu.edu.au/~web107/
Single crystals generatespotty diffracted rays.
Fine powders generatesmooth diffraction rings.
Graininess35
Spotty diffraction rings
Very weak peak
Very strong peak
Grainy samples:- non-reproducible intensities- «phantom» peaks- «missing» peaks
The same sample, atthe same 2θ position,but different intensities!
Graininess: Rocks in Dust36
«Rocks in Dust»:A few large crystals ina fine matrix
Usually invisible, but ifscanned: Strong peaksout of nowhere!
http://www.diamond.ac.uk/
Graininess: Rocks in Dust37
Graininess38
Reducing graininess:- Grinding / milling- Adjust divergence slit and beam mask for largest possible irradiated area
(= more particles contribute to diffraction pattern)- Use spinning sample stage
(= better randomisation)- Counting time per step ≥ 1 revolution of samples stage spinner
Few diffracting crystallites Many diffracting crystallites
Graininess: Rocks in Dust39
topaz bearing granite, samples ball-milled < 63 µm and hand-ground < 20 µm
25 30 35 40
0
100
200
300
2 Theta / ° ( Scan Axis: 2:1 sym. const. area )
Inte
nsity /
cp
s
Comparison of 2 Scans
kfsp 002/040preferredorientation!
plag 002/040topaz 230
„rock in the dust“
< 63 µm
< 20 µm
Courtesy of R. Kleeberg
Micro-absorption40
Phase 1: High absorptioncoefficient for X-radiation
Phase 2: Low absorptioncoefficient for X-radiation
X-rays
X-rays
Micro-absorption41
Small particles absorb insignificantpart of the radiation. Volumes of interaction with
phases 1 & 2 are representativefor phase composition
Strong attenuation by phase 1Large particles absorb significantpart of the radiation. Small volume of interaction
Weak attenuation by phase 2 Large volume of interaction
Micro-absorption42
Reducing micro-absorption:- Grinding / milling to reduce particle size
Micro-absorption occurs in samples with…- … large particles (not crystallites!)- … phases with large contrast in absorption coefficients
Texture, Preferred Orientation43
SEM Images: L. Galea, RMS Foundation
Platelets, Needles, Fibers, Whiskers
Random orientation Preferred orientation
Texture, Preferred Orientation44
Try to avoid orientation at the surface of the sample:
- Press powder without «rubbing» the surface- Use back- or side-loading sample holder- Disorder surface with textured stamp
- Various creative solutions can befound on the internet (involvingVaseline, hair spray, spray pistols, …)
PO can be corrected mathematically,but phase quantification will be biased.
Photo: Kristin Unger
Photo: Aron Knoblich
Summary: The Perfect Sample45
The perfect sample for Bragg-Brentano diffractometers:- Crystallites and particles of 1-5 µm size- Perfectly random orientation- Perfectly flat surface- Surface precisely centered in the goniometer- Not transparent (absorb all primary radiation)
Industry standard for automated XRD sample milling:McCrone Micronizing Mill (now distributed by Retsch)
General Rules for Quantitative Phase Analysis
Instrumental background should be low and linear, no “bumps” or even edges
Force the peak/background ratio as high as you can
Peak/noise ratio on maximum as well, but a noise better than the quality of profile description doesn’t make sense
Irradiate maximum of sample surface (improve grain statistics and intensity)
Register as much diffracted intensity as you can get without significant loss of resolution
Parallel beam techniques and high-resolution configuration are bad choices for QPA purpose
Don’t measure/evaluate angular ranges containing no or potentially biased phase Intensity information (very low angles, high angles)
Choose step width of approximately 1/5 of FWHM of the sharpest peak
46
Courtesy of R. Kleeberg
Good Diffraction Data47
Good sample preparation…
…and good instrument settings…
…are crucial for successful
Rietveld refinements