Sample Preparation, Data Collection and Phase-ID using Powder XRD

Pamela WhitfieldNational Research Council, Ottawa9th 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 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 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°)

R

)θcos(π1802

θ2 deg

Note: convention is that –ve sample displacement = sample too highNot 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

Diffraction patterns from powdered sucrose as both deep and thin samples

2 (degrees CoK)

10 20 30 40 50Intensity

deep, top-loaded sample

thin, sieved sample

21.0 21.5 22.0 22.5 23.0 23.5 24.0

Peak positions – sources of error

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

Crystallite size range

15-20mm 5-50mm 5-15mm <5mm

Intensity reproducibility

18.2% 10.1% 2.1% 1.2%

Reproducibility of the intensity of the quartz (101) reflection with different crystallite sizes

Diameter 40mm 10mm 1mm

Crystallites / 20mm3 5.97 × 105 3.82 × 107 3.82 × 1010

No. of diffracting crystallites

12 760 38000

Comparison of the particle statistics for samples 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

problems• With a rotation stage phi is a set

angle instead of full rotation

Phi-scans across 5 fingers of quartz with different samples

<15 mm

2 (degrees CuK)

67.0 67.5 68.0 68.5 69.0 69.5Intensity (counts)

0

1000

2000

30000deg 45deg 90deg 135deg 180deg 225deg 270deg 315deg

-400 mesh sieve(<37 mm)

2 (degrees CuK)

67.0 67.5 68.0 68.5 69.0 69.5

Intensity (counts)

0

1000

2000

3000

40000deg 45deg 90deg 135deg 180deg 225deg 270deg 315deg

Micronized

2 (degrees CuK)

67.0 67.5 68.0 68.5 69.0 69.5

Intensity (counts)

0

500

1000

1500

2000 0deg 45deg 90deg 135deg 180deg 225deg 270deg 315deg

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

Overlay of 3 repeat patterns from un-micronized cement Overlay of 3 repeat patterns from micronized cement

periclase

2 (degrees CuK)

26.0 26.5 27.0 27.5 28.0 28.5

Intensity (counts)

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2000

4000

6000

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12000

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16000

2 (degrees CuK)

20 30 40 50

Intensity

Extreme examples…• Occasionally reflections are unexpectedly split

• Quartz is particularly prone….• Synchrotron data is no immune – in fact it can be worse due to

the extremely parallel beam

d-spacing (Å)2.4 2.5 2.9 3.0

Intensity

Flat-plate ( rocked ±2º)0.3mm capillary

Main 101 reflection of ~100 micron quartz with a fuller pattern inset showing spurious intensities

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:

m1001

max t m = linear absorption coefficient (LAC)

corundum magnetite zirconCuK LAC (cm-1) 125 1167 380

tmax (mm) 0.8 0.1 0.3

CoK 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 mabsorption – Brindley model estimates the effect

• Very coarse powders• mD > 1 severe m-absorption – forget it!

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…

• Breaking up the alignment of the plates by back-loading onto a rough surface such as sandpaper can help…

2 (degrees CuK)10 20 30 40 50 60 70 80

Intensity (counts)

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12000

14000

micronized micabackloaded onto smooth surface

2 (degrees CuK)10 20 30 40 50 60 70 80

Intensity (counts)

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6000

8000

10000

micronized micabackloaded onto sandpaper

• With plate-like samples if you have a capillary stage then use it!

• If not then spray-drying the sample can be an alternative…. 2 (degrees CuK)

10 20 30 40 50 60 70 80 90

Intensity (counts)

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5000

10000

15000

20000

25000

30000

micronized 40S mica - capillary

Background-subtracted data from micronized 40S mica in a 0.5mm capillary

Top-loaded, spray-dried 40S mica

SEM of spray-dried mica 2 (degrees CuK)

10 20 30 40 50 60 70 80 90

Intensity (counts)

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3000

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7000

Spray-dried 40S mica - top-loaded

200001

Going the extra mile…

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!

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 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• 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

Quartz zero background plate

Silicon 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…..

powder

glassslide

holder

plug

sample

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!

0.2 mm

1 mm

Capillaries – highly absorbing samples

2 (degrees CuK)

20 40 60 80 100 120 140

Arbitrary Intensity

0.3mm capillary

reflection geometry

2 (degrees CuK)

20 40 60 80 100 120 140

Intensity (counts)

0

10000

20000

30000

40000

50000

60000

70000

*

**

* *

* diamond

Rietveld refinement of ~10 vol% SnO2 in diamond powder

Capillary and reflection data from pure SnO2

• 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)

Foil transmission• Another approach for small samples• Not immune to preferential orientation – the plane is just rotated 90° so

the peak intensities change accordingly!• Sample can be very thin so highly absorbing samples possible• 1/cos() correction required for accurate relative intensities

2Th Degrees9080706050403020

Cou

nts

60,000

55,000

50,000

45,000

40,000

35,000

30,000

25,000

20,000

15,000

10,000

5,000

0

-5,000

Cassiterite 100.00 %

Quartz powder between Kapton

Rietveld refinement of SnO2 (1400cm-1)

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° 2 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

0.01º step, 1s countRwp = 15.2%

0.02º step, 2s countRwp = 12.0%

Lin

(Cps

)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

2-Theta - Scale

25.5 26 27 28

2 different datasets from quartz stone– both experiments took 25 seconds

Smaller Rwp corresponds to a better fit.

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

20 .0 0 40 .0 0 60 .0 0 80 .0 0 100 .0 0 120 .00 140 .0 0

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Which line would you choose?

Fluorescence• Fluorescence even adversely affects phase-ID detection limits

• a secondary monochromator on conventional system is an effective way to filter out fluorescence

CuK - Li1.15Mn1.85O3.9F0.1

Lin

(Cou

nts)

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100

200

300

400

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600

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1000

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1300

2-Theta - Scale15 20 30 40 50 60 70 80

No monochromator

Properly aligned monochromator/mirror

there is a real peak here!

Fluorescence – what to do about it?• With a PSD a conventional monochromator not possible – data

with CoK

Which dataset do you prefer?

CoK - LiMn1.5Ni0.5O4

Lin

(Cps

)

0

10

20

30

40

50

2-Theta - Scale

20 30 40 50 60

Fluorescence cont.• Can improve PSD data significantly by adjusting the detector’s

electronic discriminator window

Lin

(Cps

)

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1

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3

4

5

6

7

8

9

10

2-Theta - Scale

21.2 22 23 24

Rescaled to normalize background PHA

0.1 0.2 0.3 0.4 0.5 0.6

LL = 0.36WW = 0.06 P/B = 13.4

PHA0.1 0.2 0.3 0.4 0.5 0.6

LL = 0.28WW = 0.34

P/B = 4.5

PHA0.1 0.2 0.3 0.4 0.5 0.6

LL = 0.1WW = 0.5

P/B = 4.2

Sacrifice intensity to improve P/B ratio

P/B still along way off 50. Change radiation or instrument.

Problematic sample: quant analysis• FeS + Mg(OH)2 + SiO2

• CuK• Ground or unground?

• particle statistics• Microabsorption (FeS)

• ideally switch to CoK• Fluorescence (FeS)

• high background• monochromator, energy-discriminating detector, switch to CoK

• 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 CuK and CoK!• 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?• CrK 2.29Å (unusual, high air scatter/attenuation and limits lower d-

spacings attainable)• FeK 1.94Å (very unusual and low power tubes)• MoK 0.71Å (unusual and beta-filter artefacts visible)

LiMn1.4Ti0.1Ni0.5O4

2 (degrees - CoK)20 30 40 50 60

Inte

nsity

(cou

nts)

0

1000

2000

3000

4000 CoP/B = 4.5

2 (degrees - CuK)

20 30 40 50 60

Inte

nsity

(cou

nts)

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10000

20000

30000

40000

50000

60000

70000

80000 CuP/B = 9.4

2 (degrees - MoK)

10 20 30 40 50 60

Inte

nsity

(cou

nts)

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4000

6000

8000

10000

12000

14000Mo

A primary monochromator would get rid of this high angle tail

P/B = 84

2 (degrees - CrK)

30 40 50 60 70 80

Inte

nsity

(cou

nts)

0

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10000

15000

20000Cr

P/B = 87(P/B = 54 without air-scatter sink to reach angles >100)

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

Boehmite (Madsen, 1992)

Variable Counting TimeConstant Counting Time

I ~ LP * thermal vibration * f2

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)

Two theta (degrees)20 40 60 80 100 120 140

Inte

nsity

(cou

nts)

0

10000

20000

30000

400000.0284º/10s0.0214º/5s0.0142º/1.5s0.0072º/0.5s

overall Rwp = 4.3%overall RB = 1.4%

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 K2 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)

CrKCrKbWL

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)

LaCoO3, R-3ca = 5.449, c = 13.104Å

LaNiO3, R-3c a = 5.456, c = 13.143Å

LiMnO2 LiCrO2

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/powder-diffraction/projects

• links to all the round-robin information, guidelines and papers (freely available)

Questions?