Sample Preparation, Data Collection and
Phase-ID using Powder XRD
Pamela Whitfield
Canadian Powder Diffraction Workshop
Horses for Courses…
• Data quality required depends on what you want to do with it
• Phase-ID has less stringent requirements on both sample prep
and data collection
• Quantitative phase analysis, Rietveld analysis and structure
solution require careful sample prep but can require different
data collection regimes
• I’ll mostly cover requirements for phase ID but will touch on
considerations for other techniques…
– I did a presentation last week concentrating more on quantitative analysis;
if you’re interested just ask and you can have a copy
Questions to ask
• What is in your sample?
– Organics often better collected in transmission
– Fluorescence can cause problems in data quality
• How much have you got?
– Very small quantities
• capillary geometry? (not an option for many people)
• Smear mount
– We’ll assume conventional Bragg-Brentano reflection geometry
for most of the rest of this presentation
• What kind of instrument have you got access to?
– If you have a choice which is the best?
What matters for
phase-ID?
• Peak positions most important
• Relative intensities secondary
– but very important for Rietveld, etc….
• If wanting to do search-match it is useful if the phases exist in
the PDF database!
Where to start?
• What affects peak positions?
• What affects relative intensities?
• Preparing the samples
• Different types of sample holders
Peak positions
• Zero point error - is the system properly aligned?
• Sample displacement - is the sample too high/low? (0.1mm error
will shift peaks approx 0.045°)
 2θdeg  

2  180
π
 cos(θ)  
R
Note: convention is that –ve sample
displacement = sample too high
Not an issue for parallel beam systems
• Sample transparency
– if the X-rays penetrate a long way into the sample can get a
‘sample displacement’ even if the height is perfect
• again not an issue for parallel-beam systems
– if necessary use a thin sample to avoid transparency peak shifts
• relative intensities will be affected
Relative intensities
• Particle statistics (grain size)
• Preferential orientation
• Crystal structure
• Microabsorption (multiphase samples)
Sample-related
problems
• Grainy samples or ‘rocks in dust’
• Microabsorption
– a serious issue for quantitative analysis and could fill a talk by
itself!
• Preferential orientation
“Grainy” Samples
• Issue of graininess relates to particle statistics
• Particle statistics is what makes a powder a true powder!
• 600 mesh sieve = <20 mm
Comparison of the particle
statistics for samples with
different crystallite sizes
Crystallite
size range
Intensity
reproducibility
Diameter
40mm
10mm
1mm
Crystallites / 20mm3
5.97 × 105
3.82 × 107
3.82 × 1010
No. of diffracting
crystallites
12
760
38000
15-20mm
5-50mm
5-15mm
<5mm
18.2%
10.1%
2.1%
1.2%
Reproducibility of the intensity of the quartz (113) reflection with different crystallite sizes
“Seeing” Particle
Statistics
How to improve
particle statistics
• There are a number of potential ways to improve particle
statistics
– Reduce the particle size (without damaging crystallites!)
– Increase the area illuminated by X-rays
• Divergence angle
• Watch for beam overspill at low angles
– Rotate samples
– but not a replacement for proper sample prep!
McCrone mill =
good

Mortar and pestle = bad

How does it affect
your data?
• Reproducibility of data can be gauged by running repeat
samples after reloading sample each time
• Unmicronized sample: MgO only appears in 1 sample out of 3
periclase
Overlay of 3 repeat patterns from un-micronized cement
Overlay of 3 repeat patterns from micronized cement
Microabsorption
• Microabsorption is the thing that causes most nightmares for
analysts doing quantitative phase analysis
• Caused by a mixture of high and low absorbing phases
• High absorbers
•
•
•
•
beam absorbed at surface
only fraction of grain diffracting
relative intensity underestimated
QPA too low
• Low absorbers
•
•
•
•
beam penetrates deeper
more diffracting volume
relative intensity overestimated
QPA too high
What can you do
about it?
• Change radiation?
– Absorption contrast changes with energy
– Higher energy X-rays often less problematic
• Use neutrons?
– Not usually practical but a ‘gold standard’
• Use the Brindley correction?
– Can be dangerous
– Need to know absorption of each phase
– Need to know particle (not crystallite!) size for each phase
• But assumes spherical particles with a monodisperse size
distribution
– Usually unrealistic!
Effect of particle size
• Brindley proposed that a maximum acceptable particle size for
QPA can be calculated by:
tmax  1
100 m
m = linear absorption coefficient (LAC)
corundum
magnetite
zircon
CuKa LAC (cm-1)
125
1167
380
tmax (mm)
0.8
0.1
0.3
CoKa LAC (cm-1)
195
240
574
tmax (mm)
0.5
0.4
0.2
The scale of
escalating despair!
• Brindley also devised a criteria for whether you should be
‘concerned’ about microabsorption
– mD = linear absorption coefficient x particle diameter
• Fine powders
– mD < 0.01 negligible m-absorption
• Medium powders
– 0.01 < mD < 0.1 m-absorption present – Brindley model applies
• Coarse powders
– 0.1 < mD < 1 large mabsorption – Brindley model estimates the effect
• Very coarse powders
– mD > 1
severe m-absorption – forget it!
Preferential
Orientation
• Preferential orientation (PO) is most often seen in samples that
contain crystallites with a platey or needle-like morphology.
• Particular culprits
– Plates
• mica
• clays
• some carbonates, hydroxides e.g. Ca(OH)2
– Needles
• wollastonite
• many organics
• The extent of the orientation from a particular sample depends
greatly on how it is mounted
Different preparation
techniques
• Top-loading
• Flat-plate
• Back-loading
• Side-loading
• Capillary
Top-loading
• Simplest but most prone to inducing preferential orientation
• Sometimes orientation induced deliberately, e.g. ID of clays
Alternative holders such
as zero background
silicon or quartz usually
top-loading as well
Flat plate
aka: Smear mount
• Used with very small samples (phase-ID , Rietveld )
• Sample adhered to zero background plate using some form of
binder/adhesive that doesn’t have any Bragg peaks
– Hairspray! Spray ~12” from holder makes a sticky surface – my favourite
– PVA
– Slurry with ethanol or acetone – tricky to get right consistency
• Some quartz plates can show a sharp reflection when spun
Silicon zero
background plate
Quartz zero
background plate
Gem Dugout a commonly used source for zero background plates (www.thegemdugout.com)
Back-loading
Side-loading
• I don’t have one of these!
• Basic principle…..
plug
powder
sample
glass
slide
holder
Capillaries
• Probably best way to prevent orientation in platey materials
– not much good unless you have a capillary stage!
• Not 100% effective with needle-like materials though
• Capillaries range in diameter from 2mm to 0.1mm
• Made from either borosilicate or quartz glass
• Only useful where absorption is low
• Small diameters can be extremely fiddly to fill!
Example – hydrated
cement
• Hydrating cement produces beautiful plates of portlandite,
Ca(OH)2
• Breaking up these plates (changing their aspect ratio) will
reduce their tendency to lie flat, i.e. orientate
• What happens if you can’t…….?
15 day cement – toploaded and capillary
Capillary
14
1.25
12
1.20
10
1.15
8
Ca(OH)2 - reflection
1.10
Ca(OH)2 - capillary
6
Ca(OH)2 TI - reflection
4
1.05
Ca(OH)2 TI - capillary
2
1.00
0
0.95
0
30
60
90
Time (days)
120
150
180
Portlandite Texture Index
Refined Portlandite Content (wt%)
Top-loaded
• Portlandite orientation very obvious in
top-loaded sample
– wrong reflection is the 100% peak!
Effect on the QPA XRD results. Kinetics from
reflection data nonsensical.
N.B. Texture Index of 1 = perfect powder.
Corrections for PO in
Rietveld software
• Two different corrections exist in most software to correct
orientation during Rietveld analysis
– March-Dollase (MD)
• Single variable but an orientation direction must be supplied by the
analyst
– Spherical Harmonics (SH)
• VERY powerful approach – can increase SH ‘order’ to fit increasingly
complex behaviour
• Multiple variables but no orientation direction required
• Number of variables increase with reducing cell symmetry
• Be very careful in multiphase systems (e.g. cements, rocks) with
overlapping peaks
– Negative peaks are very common and very meaningless!
Data collection
strategies
• For Rietveld analysis guidelines were published by McCusker et
al in 1999 but still a good reference
• Choose beam divergence such that the beam doesn’t overspill
the sample at low angle
– remember the under-scan when a PSD is used!
– You’re first datapoint may be at 10° 2q but the instrument may
start at 8°!
(ENeqV1_0.xls very handy for working out correct divergence)
(http://ig.crystallography.org.uk/spreadsh/eneqv1_0.xls)
• Step size of approx FWHM/5
– Too small = wasting time and producing noisy data
– Too coarse = chopping intensity and peaks not modelled
properly
Experiment
optimization
• ‘Horses for courses’ – collect data fit for purpose
– Data for phase-ID does not have to be of the same quality as
for structure solution, etc
– Most common mistake among users
• too small step size for sample
0.01º step,
1s count
Rwp = 15.2%
9000
0.02º step, 2s
count
Rwp = 12.0%
8000
7000
Lin (Cps)
6000
2 different datasets from quartz stone
– both experiments took 25 seconds
5000
4000
3000
2000
1000
Smaller Rwp corresponds to a better fit.
0
25.5
26
27
2-Theta - Scale
28
Peak-to-background
• A number of things can affect the peak-to-background in a
pattern
– air-scatter at low angles
• use air-scatter sinks if needed
– nanoparticles have lower intrinsic peak heights
• not much you can do here
• eventually Rietveld results are no longer meaningful
– capillaries always have higher background
• subtracting capillary blank can improve this but careful not to
distort counting statistics
– fluorescence is the main cause of poor peak-to-background…
• Rietveld refinement round robin suggested a minimum P/B
value of 50 for accurate structural parameters….
Why does background
matter?
• With a high background the uncertainty in the background
parameters increase (often use more parameters as well)
– uncertainty in the peak intensities increases
→ greater uncertainty in structural parameters and quantitative phase
analysis
500
400
300
Which line would you
choose?
200
100
0
20.00
40.00
60.00
80.00

100.00
120.00
140.00
Fluorescence
• Fluorescence even adversely affects phase-ID detection limits
– secondary monochromator on conventional system is an effective filter
1300
CuKa - Li1.15Mn1.85O3.9F0.1
1200
1100
1000
Lin (Counts)
900
800
700
600
500
there is a real
peak here!
400
No
monochromator
300
200
100
0
15
20
30
40
50
2-Theta - Scale
60
70
80
Properly aligned
monochromator/mirror
Fluorescence – what
to do about it?
• With a PSD a monochromator not possible – Vantec data with
CoKa
50
Lin (Cps)
40
CoKa - LiMn1.5Ni0.5O4
30
20
Which dataset do you
prefer?
10
0
20
30
40
2-Theta - Scale
50
60
Fluorescence cont.
• Can improve data significantly by adjusting the detector
discriminator window
LL = 0.36
WW = 0.06
Rescaled to normalize
background
P/B = 13.4
10
0.1
0.2
0.3
0.4
0.5
0.6
PHA
9
8
LL = 0.28
WW = 0.34
P/B = 4.5
Sacrifice intensity
to improve P/B ratio
Lin (Cps)
7
6
0.1
0.2
0.3
0.4
0.5
0.6
PHA
5
4
LL = 0.1
WW = 0.5
P/B = 4.2
3
P/B still along way off
50. Change radiation
or instrument.
2
0.1
0.2
0.3
0.4
0.5
0.6
PHA
1
0
21.2
22
23
2-Theta - Scale
24
Problematic sample:
Phase-ID
• Aspirin
– organic sample
– large transparency effects in reflection (peak shifts & poor
resolution)
• use smear mount
Comparison of data from
aspirin using lab toploading and capillary
compared to synchrotron
data.
Problematic sample:
Quant Analysis
• FeS + Mg(OH)2 + SiO2
– CuKa
• Ground or unground?
– particle statistics
• Microabsorption (FeS)
– ideally switch to CoKa
• Fluorescence (FeS)
– high background
– monochromator, energy-discriminating detector, switch to CoKa
• Preferential orientation (Mg(OH)2)
• Extinction? (SiO2)
– Micronize!!!!
• All of these problems are reduced by micronizing to sub-micron
particle/crystallite size
Problematic sample:
Rietveld analysis
• LiMn1.4Ti0.1Ni0.5O4 (lithium battery cathode material)
– Mn fluoresces with both CuKa (1.54Å) and CoKa(1.79Å)!
– Worse with CoKa in this case
– Use a monochromator or energy discriminating detector
• Good peak-to-background, but...
• Fluorescence is still there even if you can’t see it
– Very high absorption impacts particle statistics (X-rays only penetrate
a few 10s of microns)
– Solution by changing tube?
• CrKa 2.29Å (unusual, high air scatter/attenuation and limits lower
d-spacings attainable)
• FeKa 1.94Å (very unusual and low power tubes)
• MoKa 0.71Å (unusual and beta-filter artefacts visible)
LiMn1.4Ti0.1Ni0.5O4
Cu
80000
P/B = 4.5
P/B = 9.4
Intensity (counts)
70000
Intensity (counts)
Co
4000
60000
50000
40000
30000
20000
3000
2000
1000
10000
0
0
20
30
40
50
20
60
30
Mo
P/B = 84
P/B = 87
Intensity (counts)
Intensity (counts)
A primary
monochromator
would get rid of this
high angle tail
8000
6000
4000
60
Cr
20000
12000
10000
50
2q (degrees - CoKa)
2q (degrees - CuKa)
14000
40
15000
(P/B = 54 without air-scatter sink
to reach angles >100)
10000
5000
2000
0
0
10
20
30
40
2q (degrees - MoKa)
50
60
30
40
50
60
2q (degrees - CrKa)
70
80
Variable Count Time
• One problem with XRD is the drop in intensity with increasing 2q
• Most of the ‘information’ is at the higher angles but least-squares
practically ignores it
1.4e+6
6.2
Data from the mineral
stichtite
6.0
5.8
1.0e+6
Log intensity
Intensity (counts)
1.2e+6
8.0e+5
6.0e+5
5.6
5.4
5.2
5.0
4.8
4.6
4.0e+5
20
40
60
80
100
120
140
2q (degrees - CuKa)
2.0e+5
0.0
20
40
60
80
100
2q (degrees - CuKa)
120
140
VCT continued
• Error in intensity = intensity (Poisson statistics)
– can reduce error (and increase weighting) by counting for longer….
– In practice split into ranges and double count time for each range (can
increase step size to partially compensate for increased time)
1200000
Raw VCT capillary data for
stichtite
1100000
1000000
900000
Data reformatted into ASCII
format xye file
Lin (Counts)
800000
700000
600000
500000
400000
Remember if subtracting
background (e.g. capillary
blank) that the error is 
original intensity!
300000
200000
100000
0
10
20
30
40
50
60
70
80
2-Theta - Scale
90
100
110
120
130
140
VCT data
Quantitative analysis
• Possible to improve detection limits in quant analysis by
counting for longer where minor phases expected
Fixed count time
Variable count time (normalized)
Example from presentation by Lachlan Cranswick
VCT data
Structure refinement
• You can extract more structural details if reflections still resolvable up
to high angles
Jadarite (variable count/step)
40000
Intensity (counts)
0.0072º/0.5s 0.0142º/1.5s
0.0284º/10s
0.0214º/5s
overall Rwp = 4.3%
overall RB = 1.4%
30000
20000
10000
0
20
40
60
80
100
Two theta (degrees)
120
140
Jadarite structure with thermal
ellipsoids
Phase-ID
• Phase-ID usually undertaken using vendor-supplied software
with the Powder Diffraction Database (PDF2 or PDF4)
• The database is not free so budget accordingly
– PDF4 requires yearly renewal but has more features
– PDF2 good enough for search-match and OK for 10 years
• The PDF2 uses XRD ‘fingerprints’ – if they haven’t been
deposited they won’t show up
• PDF2 entries are allocated a ‘quality mark’ but occasionally the
newer ones are actually worse!
– Experimental quality marks ‘*’ > ‘I’ > ‘A’ > ‘N’ > ‘D’
– Calculated from ICSD, etc ‘C’
• Background subtraction recommended before search-match if it
is high but don’t bother with Ka2 stripping, etc
Phase-ID
• Improve your odds in the search-match
– make a sensible guess as to the likely elements
• does your sample really have plutonium in it?!
– if you have elemental analysis results then use them
• but consider possibility of amorphous phases!
Search-match in
EVA on a sample
of zircon
• Use common/chemical sense
– don’t believe results just because the computer tells you
– even oxygen has entries in the PDF2!
• Where software supports it
‘residue’ searches can be
helpful in identifying minor
phases
• Minor peaks - make sure they aren’t Kb or tungsten lines!
– vendor software can often identify these (e.g. EVA below)
WLa
CrKa
CrKb
No luck – what next?
• Do you have a large systematic error in the data?
– check your diffractometer alignment if not sure
– modern search-match software can cope with a reasonable error but it has
limits
• Look for possible analogues which may appear in the PDF2
– LaCoO3 similar to LaNiO3 with slightly different lattice parameters
– analogues may have significantly different relative intensities
– however: LiMnO2 (Pmmn) completely different from LiCrO2 (R-3m)
LiMnO2
LaCoO3, R-3c
a = 5.449, c = 13.104Å
LaNiOLiCrO
3, R-3c2
a = 5.456, c = 13.143Å
Getting desperate yet?
• Put the sample under optical microscope with polarizers
– does it seem to have the number of phases you expect?
• If it contains Fe or Co try a magnet!
• Possible contamination
– mortar and pestle not clean
– material from micronizer grinding elements (newer corundum
elements not as good as the older ones – use agate)
• Last possibility to consider….
– maybe you have found a new phase
Conclusions…
• Use the appropriate sample mounting technique for the sample
and the data requirements
• Graininess, microabsorption and preferential orientation are all
related to particle and crystallite size
• Do yourself a big favour by micronizing your sample!
• Preferential orientation can be corrected during analysis but the
others can’t
– The assumptions required by the Brindley correction are never
met in real life
• There are times when the newest diffractometer (PSD, etc) isn’t
the best one for the job!
• No such thing as the perfect configuration for everyone
• VCT data can help in a number of ways
– improve the detection limit for minor phases
– significantly improve the quality of a structure refinement
• If you don’t remember anything else remember this!
– think about your samples!
– a one size fits all approach doesn’t work!
Acknowledgements
• Ian Madsen (CSIRO)
– I couldn’t improve on his explanation of microabsorption so I
used it!
– Responsible for the QPA XRD round robin samples which still
give people nightmares
• Lyndon Mitchell (NRC-IRC)
– cement samples
References
•
•
•
•
L.B. McCusker et al, “Rietveld refinement guidelines”, J. Appl. Cryst., 32
(1999), 36-50
R.J. Hill and L.M.D. Cranswick, “IUCr Commission for Powder Diffraction
Rietveld refinement round robin II. Analysis of monoclinic ZrO2”, J. Appl.
Cryst., 27 (1994), 802-844
G.W. Brindley, “The effect of grain and particle size on X-ray reflections from
mixed powders and alloys….”, Philosophical Magazine, 3 (1945), 347-369
Quantitative Phase Analysis Round Robin
– Link to papers and background information on the Commission for Powder
Diffraction webpage
– www.iucr.org/resources/commissions/powder-diffraction/projects
Questions?