sample prep CPDW 2012 extended

Report
Sample Preparation, Data
Collection and Phase-ID using
Powder XRD
Pamela Whitfield
National Research Council, Ottawa
9th Canadian Powder Diffraction Workshop, Saskatoon, 23-25 May 2012
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.
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 or foil transmission? (not an option for many people)
• smear mount?
• We’ll assume conventional reflection geometry unless stated
otherwise
• 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 errors affects peak positions?
• What affects relative intensities?
• Preparing the samples
• Different types of sample holders
Peak positions – sources of error
• Zero point error - is the system properly aligned?
• use a NIST standard periodically to check it
• Sample displacement - sample too high/low?
(0.1 mm  ~ 0.045°)
 2θ deg  

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 X-rays penetrate a long way
into the sample can get a
‘sample displacement’ even if
the height is perfect
• not an issue for parallel-beam
systems
• if necessary use a thin sample
to avoid transparency peak
shifts
• relative intensities will be
affected
Intensity
Peak positions – sources of error
21.0
21.5
22.0
22.5
23.0
23.5
24.0
thin, sieved sample
deep, top-loaded sample
10
20
30
40
2 (degrees CoK )
Diffraction patterns from powdered
sucrose as both deep and thin samples
50
Parallel versus para-focussing
• The systems don’t look that different but don’t behave the same..
• Parallel-beam immune to sample displacement & transparency
but has worse peak resolution – twin mirror system excepted
Divergent-beam without & with
secondary graphite monochromator
Parallel-beam setups with long slits and
secondary mirror
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
• Extinction
“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 (101) reflection with different crystallite sizes
“Seeing” particle
statistics
Playing Russian roulette with a
grainy sample
Stacking the odds in your favour by
micronizing….
How to improve particle statistics
• There are a number of potential ways to improve particle
statistics
– Increase the area illuminated by X-rays
• Divergence angle
– Rotate samples
– Use a PSD
– Reduce the particle size
(without damaging crystallites!) McCrone mill = good

Mortar and pestle = bad

I don’t have a 2D detector – now what?
• A series of phi-scans can show up
Intensity (counts)
problems
3000
• With a rotation stage phi is a set
angle instead of full rotation
<15 m
0deg
45deg
90deg
135deg
180deg
225deg
270deg
315deg
2000
1000
-400 mesh sieve
(<37 m)
0deg
45deg
90deg
135deg
180deg
225deg
270deg
315deg
3000
2000
0
67.0
2000
1000
0
67.0
67.5
68.0
68.5
2 (degrees CuK )
69.0
67.5
68.0
68.5
69.0
69.5
2 (degrees CuK )
Intensity (counts)
Intensity (counts)
4000
69.5
Micronized
0deg
45deg
90deg
135deg
180deg
225deg
270deg
315deg
1500
1000
500
0
67.0
67.5
68.0
68.5
2 (degrees CuK )
Phi-scans across 5 fingers of quartz with different samples
69.0
69.5
I don’t have a 2D detector – now what?
• Can also run repeats after reloading sample each time (get real
stats as a bonus)
• Unmicronized : 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
Extreme examples…
• Occasionally reflections are unexpectedly split
• Quartz is particularly prone….
• Synchrotron data are not immune – in fact it can be worse due to
the extremely parallel beam
16000
Flat-plate ( rocked ±2º)
0.3mm capillary
12000
10000
Intensity
Intensity
Intensity (counts)
14000
8000
6000
20
4000
30
40
50
2 (degrees CuK )
2000
0
26.0
26.5
27.0
27.5
28.0
28.5
2 (degrees CuK )
Main 101 reflection of ~100 micron quartz with a fuller
pattern inset showing spurious intensities
2.4
2.5
2.9
3.0
d-spacing (Å)
Capillary and rocked reflection data from LaB6 on a strip
heater taken with the Australian synchrotron
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?
• Need to know absorption of each phase
• Need to know particle (not crystallite!) size for each phase
• 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!
Radiation dependence of mD
CoKa (7 keV)
Size
mm
corundum (Al2O3) magnetite (Fe3O4)
mD
mD
zircon (ZrSiO4)
mD
0.1
0.002
0.002
0.006
0.2
0.004
0.005
0.011
0.5
0.010
0.012
0.029
1
0.019
0.024
0.057
2
0.039
0.048
0.115
5
0.097
0.120
0.287
10
0.195
0.240
0.574
20
0.389
0.480
1.148
fine
mD < 0.01
medium
0.01 < mD < 0.1
coarse
0.1 < mD < 1
very coarse
mD > 1
Radiation dependence of mD
CuKa (8 keV)
Size
mm
corundum (Al2O3) magnetite (Fe3O4)
mD
mD
zircon (ZrSiO4)
mD
0.1
0.001
0.012
0.004
0.2
0.003
0.023
0.008
0.5
0.006
0.058
0.019
1
0.013
0.117
0.038
2
0.025
0.233
0.076
5
0.063
0.584
0.190
10
0.125
1.167
0.380
20
0.251
2.344
0.759
fine
mD < 0.01
medium
0.01 < mD < 0.1
coarse
0.1 < mD < 1
very coarse
mD > 1
Radiation dependence of mD
MoKa (17 keV)
Size
mm
note new
rows!
{
corundum (Al2O3) magnetite (Fe3O4)
mD
mD
zircon (ZrSiO4)
mD
0.1
0.000
0.001
0.000
0.2
0.000
0.003
0.001
0.5
0.001
0.007
0.002
1
0.001
0.014
0.004
2
0.003
0.028
0.009
5
0.006
0.071
0.022
10
0.013
0.142
0.044
20
0.025
0.284
0.088
50
0.063
0.709
0.219
100
0.126
1.418
0.438
fine
mD < 0.01
medium
0.01 < mD < 0.1
coarse
0.1 < mD < 1
very coarse
mD > 1
QXRD Round Robin:
CPD #4 unmilled
• Unmilled grain sizes: Al2O3 28mm, Fe3O4 36mm, ZrSiO4 21mm
• Can’t fit intensities
• very poor particle statistics? (<200 diffracting crystallites per phase)
• Al2O3 mD = 0.345, Fe3O4 mD = 4.15!!, ZrSiO4 mD = 0.788
Counts
Counts
0.1 < mD < 1 large mabsorption – Brindley model estimates the effect
mD > 1
severe m-absorption – forget it!
3,600
3,600
3,400
3,400
3,200
3,200
3,000
3,000
2,800
2,800
2,600
2,600
2,400
2,400
2,200
2,200
2,000
2,000
1,800
1,800
1,600
1,600
1,400
1,400
1,200
1,200
1,000
1,000
800
800
600
600
400
400
200
200
00
-200
-200
-400
-400
-600
-600
-800
-800
-1,000
-1,000
-1,200
-1,200
-1,400
-1,400
-1,600
-1,600
Corundum 73.71
60.69%
%
Corundum
Magnetite 6.53
15.14
Magnetite
%%
Zircon
24.16%
%
Zircon
19.76
weighed amounts
Al2O3 50.5%
Fe3O4 19.6%
ZrSiO4 29.9%
without
Brindley
with Brindley
correction
correction
10
10
20
20
30
30
40
40
50
50
60
60
70
70
80
80
2ThDegrees
Degrees
2Th
90
90
100
100
110
110
120
120
CuKa with graphite
monochromator
130
130
140
140
Thanks to Ian Madsen for the data
CPD #4
confirming what we suspected
• Large grains can be confirmed using 2D detector as before or
using a series of scans with different phi angles (no rotation)
M ZC
ZM
Al2O3
9000
Fe3O4
ZrSiO4
8000
Lin (Counts)
7000
6000
5000
4000
3000
2000
1000
0
34.6
35
36
2-Theta - Scale
• N.B. Al2O3 may still have preferential orientation
Thanks to Arnt Kern for the data
Micronized CPD #4?
• Particle statistics no longer a problem
• Al2O3 and ZrSiO4 still have some orientation – corrected
• CoKa doesn’t help much as problem switches from Fe3O4 to ZrSiO4
• How about an SEM?
Counts
Counts
23,000
18,000
22,000
17,000
21,000
16,000
20,000
19,000
15,000
18,000
14,000
17,000
13,000
16,000
12,000
15,000
14,000
11,000
13,000
10,000
12,000
9,000
11,000
10,000
8,000
9,000
7,000
8,000
6,000
7,000
6,000
5,000
5,000
4,000
4,000
3,000
3,000
2,000
2,000
1,000
1,000
0
0
-1,000
-1,000
-2,000
-3,000
-2,000
-4,000
15
15
CuKa
CoKawith
with
graphite
graphitemonochromator
monochromator
20
20
25
25
30
30
35
35
40
40
45
45
50
50
55
60
65
70
75
80
85
2Th Degrees
Degrees
2Th
90
95
100
Corundum
Corundum 56.31
59.76 %
%
Magnetite
20.01
%
Magnetite 14.17 %
Zircon
23.68
Zircon
26.06 %
%
weighed amounts
Al2O3 50.5%
Fe3O4 19.6%
ZrSiO4 29.9%
105 110
110 115
115 120
120 125
125 130
130 135
135 140
140
105
Thanks to Ian Madsen for the data
Al2O3, Fe3O4, ZrSiO4 - Micronised
Global copyright Ian Madsen!
Corundum
Magnetite
Zircon
Pick a number, any number…
• What value of particle size do we choose for the Brindley
correction?
Wt%
Corundum Magnetite
Zircon
Weighed
50.46
19.46
29.90
No correction
Mean
Bias
56.52
6.06
17.06
-2.58
26.42
-3.48
55.76
5.30
17.81
-1.83
26.43
-3.47
21.18
1.54
26.33
-3.57
26.15
6.51
26.08
-3.82
Brindley model, Ø = 1mm
Mean
Bias
Brindley model, Ø = 5 mm
Mean
Bias
52.49
2.03
Brindley model, Ø = 10mm
Mean
Bias
47.76
-2.70
Thanks to Ian Madsen for the analysis
What does reality matter anyway?
• Can fudge the particle size numbers and packing so the Brindley
correction gives the right result for CoKa
• but mD for ZrSiO4 and Al2O3 well into coarse range
Counts
• CuKa not as good (and mDs are even worse)
23,000
18,000
22,000
17,000
21,000
16,000
20,000
19,000
15,000
18,000
14,000
17,000
13,000
16,000
12,000
15,000
14,000
11,000
13,000
10,000
12,000
9,000
11,000
Corundum 51.31
50.22 %
Magnetite 19.70
19.89 %
Zircon
29.89 %
28.99
CoKa
CuKa
Al22O33 17mm 40%PD
Fe33O44 5mm 40%PD
ZrSiO44 14mm 40%PD
weighed amounts
Al2O3 50.5%
Fe3O4 19.6%
ZrSiO4 29.9%
if these numbers
correct send your
McCrone mill and
SEM back!
10,000
8,000
9,000
7,000
8,000
6,000
7,000
6,000
5,000
5,000
4,000
4,000
3,000
3,000
2,000
2,000
1,000
1,000
0
0
-1,000
-1,000
-2,000
-3,000
-2,000
-4,000
15
15
20
20
25
25
30
35
40
45
50
55
60
65
70
75
80
85
2Th Degrees
90
95
100
105
110
115
120
125
130
135
140
Preferential orientation (texture…)
• 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
Orientation of plate-like samples
• There’s no getting away from it – they can be a real pain
• Top-loading is hopeless as you make it worse….
• Back-loading the usual approach but not always enough…
14000
10000
micronized mica
backloaded onto smooth surface
Intensity (counts)
Intensity (counts)
12000
10000
8000
6000
4000
micronized mica
backloaded onto sandpaper
8000
6000
4000
2000
2000
0
0
10
20
30
40
50
60
2 (degrees CuK )
70
80
10
20
30
40
50
60
70
80
2 (degrees CuK )
• Breaking up the alignment of the plates by back-loading onto a
rough surface such as sandpaper can help…
Going the extra mile…
30000
• With plate-like samples if
you have a capillary
stage then use it!
Background-subtracted data
from micronized 40S mica in a
0.5mm capillary
Intensity (counts)
25000
micronized 40S mica - capillary
20000
15000
10000
5000
0
10
20
30
40
50
60
70
80
90
2 (degrees CuK )
• If not then spray-drying the sample can be an alternative….
7000
Spray-dried 40S mica - top-loaded
Intensity (counts)
6000
5000
001 200
Top-loaded,
spray-dried 40S
mica
4000
3000
2000
1000
0
10
SEM of spray-dried mica
20
30
40
50
60
70
2 (degrees CuK )
80
90
Just to prove the data is usable….
• Micas not pleasant to deal with at the best of times and this has
some messy anisotropic broadening..
• However, the data from the top-loaded, spray-dried sample fits a
un-refined literature biotite structure very well with no orientation
correction
6,000
Structure
• That’s not to say
5,500
5,000
4,500
Rwp = 11.8%
GOF = 1.82
4,000
3,500
Counts
there were no
corrections
needed at all! 
100.00 %
3,000
2,500
2,000
1,500
Refinement of the toploaded, spray-dried 40S
mica using a literature
biotite structure without
orientation correction
1,000
500
0
-500
10
20
30
40
50
2Th Degrees
60
70
80
90
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
• No orientation direction required
• Number of variables increase with reducing cell symmetry
• Be very careful in quantitative analysis with severe peak overlap (e.g.
cements)
• Negative peaks are very common and very meaningless!
Extinction
• Reduction in the intensity of a Bragg reflection by re-diffraction by
the successive planes back in the direction of the incident beam
• Dependent on size/shape of the coherently-diffracting domains
• Primary  re-diffraction within a single crystallite
• Effect minimized by reducing grain size – ideally submicron
• Normally seen in large, ‘perfect’ crystallites such as silicon or quartz
• Secondary  mosaic crystals, not seen in powders
The different preparation techniques
Reflection
• Top-loading
• Flat plate
• Back-loading
• Side-loading
Transmission
• Capillary
• Foil transmission
Top-loading
• Simplest but most prone to inducing preferential orientation
• Special holders often in this category
Alternative holders such
as cavity zero
background silicon or
air-sensitive often toploading 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
• Vaseline, vacuum grease, hairspray (spray ~12” from holder)
• Slurry with ethanol or acetone – tricky to get right consistency
• N.B some quartz plates 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!
• but basic principle…..
plug
powder
sample
glass
slide
holder
Capillaries
• Probably best way to reduce orientation in platey materials
• Commercially either quartz, borosilicate or soda-glass
• range in diameter from 2mm to 0.1mm
• Or use thin-walled polymer tubing of Kapton, PET, etc
• Most useful where sample absorption is low, e.g. organics
• Can be extremely fiddly to fill!
Capillary instrument setup
• Capillary setups can be quite specialized
• Focussing optics specific to transmission geometry
• Even qq systems better run as q2q in capillary mode
• Transmission better for low angles
Capillary setup on qq system at
very high 2q angle using detector
scan with focussing primary optic,
PSD, radial Soller slits and primary
slit setup to reduce low angle scatter
reaching detector
Capillaries – highly absorbing samples
• Absorption reduces the peak intensities at low angles
• Corrections exist but they have limits
• Smaller capillaries and/or dilution with a ‘light’ phase will help
(e.g. diamond, amorphous boron, carbon black, etc)
70000
Intensity (counts)
Arbitrary Intensity
60000
0.3mm capillary
reflection geometry
*
50000
40000
30000
20000
*
*
10000
0
20
20
40
60
80
100
120
140
2 (degrees CuK )
Capillary and reflection data from
pure SnO2
* diamond
40
60
80
*
100
*
120
2 (degrees CuK )
Rietveld refinement of ~10 vol%
SnO2 in diamond powder
140
Foil transmission
• Another approach for small samples
• Powders can be mounted between films of Kapton, Mylar, etc
• Not immune to preferential orientation – the plane is just rotated
90° so the peak intensities change accordingly!
Quartz powder between Kapton
Twin-mirror system set up for foil transmission
Foil transmission
• Sample can be very thin so highly absorbing samples possible
without dilution
• 1/cos(q) correction required for accurate relative intensities
Cassiterite 100.00 %
60,000
55,000
Rietveld refinement of SnO2 (1400cm-1)
50,000
45,000
Counts
40,000
35,000
30,000
25,000
20,000
15,000
10,000
5,000
0
-5,000
20
30
40
50
60
2Th Degrees
70
80
90
Data collection strategies
• Rietveld analysis guidelines published by McCusker et al in 1999
• Choose beam divergence so the beam doesn’t overspill the
sample at low angle
• remember the under-scan when a PSD is used!
• 1st datapoint may be at 10° 2q but the scan may start at 8°!
(ENeqV1_0.xls very handy for working out correct divergence)
(http://ig.crystallography.org.uk/spreadsh/eneqv1_0.xls)
• Rule of thumb - step size of ~ FWHM/5 to FWHM/8
• 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
9000
0.01º step,
1s count
Rwp = 15.2%
8000
7000
0.02º step, 2s
count
Rwp = 12.0%
2 different datasets from quartz stone
– both experiments took 25 seconds
Smaller Rwp corresponds to a better fit.
Lin (Cps)
6000
5000
4000
3000
2000
1000
0
25.5
26
27
2-Theta - Scale
28
Peak-to-background
• A number of things affect the peak-to-background
• 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 extracted 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
22
100.00
120.00
140.00
Fluorescence
• Fluorescence even adversely affects phase-ID detection limits
• a secondary monochromator on conventional system is an effective
way to filter out fluorescence
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 conventional monochromator not possible – 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 PSD data significantly by adjusting the detector’s
electronic discriminator window
LL = 0.36
WW = 0.06
Rescaled to normalize
background
P/B = 13.4
10
0.1
9
0.2
0.3
0.4
0.5
0.6
PHA
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: 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 and CoKa !
• 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 = 9.4
P/B = 4.5
Intensity (counts)
Intensity (counts)
70000
Co
4000
60000
50000
40000
30000
20000
3000
2000
1000
10000
0
0
20
30
40
50
60
20
30
2q (degrees - CuKa)
14000
P/B = 84
P/B = 87
4000
Intensity (counts)
Intensity (counts)
A primary
monochromator
would get rid of this
high angle tail
6000
60
Cr
20000
12000
8000
50
2q (degrees - CoKa)
Mo
10000
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 counting time (VCT)
• The physics of XRD dictate that intensities drop with angle
• Most of the information (reflections) is at higher angles
• Can regain much of the information by counting for longer at
higher angles
Constant Counting Time
I ~ LP * thermal vibration * f2
Boehmite (Madsen, 1992)
Variable Counting Time
VCT data - quantitative analysis
• Also 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
• Extract more structural details if reflections still visible at high angles
• Using a PSD split pattern into sections
• can also increase step size with angle as well to save some time…
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 ICDD 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
• A free database called the Crystallographic Open Database
(COD) exists but there is no quality checking – user beware…
• The Powder Diffraction File uses XRD ‘fingerprints’ – if they
haven’t been deposited they won’t show up
• Database 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
Be sensible…
• 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 very
helpful in identifying minor
phases
Don’t be led astray…
• 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?
• your diffractometer alignment should be checked regularly with a standard
• 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
• 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
• then the fun really starts!
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 if possible!
• Preferential orientation can be corrected during analysis but the
others can’t……
• Assumptions of the Brindley correction never met in real life
• Poor application of Brindley correction worse than no correction
Yet more conclusions….
• There are times when the newest diffractometer (PSD, etc) isn’t
the best one for the job
• fluorescence can be your #1 enemy!
• secondary optics can be your friend 
• 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 CPD QPA round robin sample 4 which still give
people nightmares
• Mati Raudsepp (UBC) for spray drying the mica sample
and the SEM
References
• 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
• Commission on Powder
Diffraction webpage
• www.iucr.org/resources/
commissions/powderdiffraction/projects
• links to all the roundrobin information,
guidelines and papers
(freely available)
Questions?
OK so you found a new phase….
• Before getting to the refinement step you have to figure out a
rough idea of the structure
• There are some different steps in the process
• Peak fitting (most of the time)
• Indexing
• Space group determination
• Structure determination
Indexing….
• You need to know whether cubic, monoclinic, etc and what are
the lattice parameters
• The instrument should be as good as you can get it
• lab data more difficult than synchrotron
• In the lab it may require a special high resolution dataset over a
limited range (usually only use the first 15-25 lines)
• Accurate peak positions the main goal
• Software packages
• TOPAS (LSI and LP Search), Crysfire (Dicvol, Treor, etc), MacMaille,
etc
Space group determination….
• This is where you often need to know a little crystallography….
• Conventional way to do this is to study systematic absences
(International Tables necessary and Chekcell software useful)
• Maximum likelihood software Extsym can give a list of probable
extinction symbols from a Pawley refinement (not same as SG!)
• http://www.markvardsen.net/projects/ExtSym/main.html
• TOPAS makes a guess at extinction symbol and often correct
• Sample density very useful (buy a pycnometer!)
• Can then calculate ‘volume per formula unit’
• Allows easy exclusion of space groups where the multiplicities are too high
• With organics a rough guide of 18Å3 per non-H atom can help
Solution….
• A number of approaches possible
• Conventional direct methods (EXPO, SXTL software)
• Real space methods (DASH, TOPAS, FOX, etc)
• Charge flipping (Superflip, TOPAS, etc)
• Real space methods tend to be more reliable with poor resolution
data
• Powder diffraction data often regarded as ‘poor’ by default
• Charge-flipping a powerful method with higher resolution powder
data
• has been know to work with iffy data, but not mine…..!
Example…. aspirin
• Aspirin tablets usually very pure
• Done with capillary transmission but…
• Highly crystalline organic with reflections to high 2q angles
• Easily indexed to a monoclinic cell
• Charge flipping does need a space group to work
• Basic structure solves in a matter of minutes…
340
320
300
280
Sqrt(Counts)
260
240
220
200
180
160
140
120
100
80
60
10
20
30
40
50
60
70
2Th Degrees
80
90
100
110
Example... sucrose
• Even easier to get hold of than aspirin
• Also highly crystalline
• Simulated annealing with a z-matrix works with normal data,
charge flipping with VCT data
Example... wollastonite
• An inorganic reflection example…. but a difficult one
• Wollastonite needles show severe preferential orientation when
top-loaded
• Normally I would say ‘make a better sample’ but sometimes it will still works
• The basic simulated annealing approach still the same with some
tweaks
SEM of wollastonite
Wollastonite – SA strategy
• TOPAS input file setup
• Space group P-1
• SiO4 always tetrahedral – safe to use simple z-matrix
• Anti-bump for Ca-Ca, Ca-O and Si-Si
• Octahedrally coordinated Ca-O
• Too many oxygens with SiO4 z-matrices
• Need to merge oxygens to get correct unit cell contents
• Use “occ_merge O* occ_merge_radius 0.9”
Wollastonite – SA strategy
• TOPAS lets you to use a ‘trick’ to solve badly orientated
data
• for details read the paper!
• The structure from simulated annealing matches the literature
c/2
Literature structure
Raw structure from SA with 4th order SH
– blue atoms are merging oxygens
Wollastonite – SA strategy
• Just a couple of plots to prove the sample was orientated!
Fit to the data for the raw SA structure
with 4th order SH
Simulated powder pattern from the SA
structure without PO correction

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