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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

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

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Sample preparation data collection and phase id using powder xrd l.jpg

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

Pamela Whitfield

Canadian Powder Diffraction Workshop


Horses for courses l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

Where to start?

  • What affects peak positions?

  • What affects relative intensities?

  • Preparing the samples

  • Different types of sample holders


Peak positions l.jpg

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°)

  • 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

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

Not an issue for parallel beam systems


Relative intensities l.jpg

Relative intensities

  • Particle statistics (grain size)

  • Preferential orientation

  • Crystal structure

  • Microabsorption (multiphase samples)


Sample related problems l.jpg

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 l.jpg

Crystallite

size range

Diameter

15-20mm

5-50mm

40mm

10mm

5-15mm

1mm

<5mm

Crystallites / 20mm3

5.97 × 105

3.82 × 107

3.82 × 1010

Intensity reproducibility

18.2%

10.1%

2.1%

1.2%

No. of diffracting crystallites

12

760

38000

“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

Reproducibility of the intensity of the quartz (113) reflection with different crystallite sizes


Seeing particle statistics l.jpg

“Seeing” Particle Statistics


How to improve particle statistics l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

Effect of particle size

  • Brindley proposed that a maximum acceptable particle size for QPA can be calculated by:

m = linear absorption coefficient (LAC)


The scale of escalating despair l.jpg

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 l.jpg

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 l.jpg

Different preparation techniques

  • Top-loading

  • Flat-plate

  • Back-loading

  • Side-loading

  • Capillary


Top loading l.jpg

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 l.jpg

Flat plateaka: 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 l.jpg

Back-loading


Side loading l.jpg

Side-loading

  • I don’t have one of these!

  • Basic principle…..

plug

powder

sample

holder

glass

slide


Capillaries l.jpg

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!

0.2 mm

1 mm


Example hydrated cement l.jpg

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 top loaded and capillary l.jpg

15 day cement – top-loaded and capillary

Capillary

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 l.jpg

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 l.jpg

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 l.jpg

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%

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.


Peak to background l.jpg

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 l.jpg

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

Which line would you choose?


Fluorescence l.jpg

1300

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

1200

1100

1000

900

800

Lin (Counts)

700

600

500

400

300

200

100

0

15

20

30

40

2-Theta - Scale

Fluorescence

  • Fluorescence even adversely affects phase-ID detection limits

    • secondary monochromator on conventional system is an effective filter

there is a real

peak here!

No monochromator

Properly aligned

monochromator/mirror

50

60

70

80


Fluorescence what to do about it l.jpg

Fluorescence – what to do about it?

  • With a PSD a monochromator not possible – Vantec data with CoKa

CoKa - LiMn1.5Ni0.5O4

Which dataset do you prefer?


Fluorescence cont l.jpg

Fluorescence cont.

  • Can improve data significantly by adjusting the detector discriminator window

P/B = 13.4

Rescaled to normalize background

P/B = 4.5

Sacrifice intensity to improve P/B ratio

P/B = 4.2

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


Problematic sample phase id l.jpg

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 top-loading and capillary compared to synchrotron data.


Problematic sample quant analysis l.jpg

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 l.jpg

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)


Limn 1 4 ti 0 1 ni 0 5 o 4 l.jpg

LiMn1.4Ti0.1Ni0.5O4

Co

Cu

P/B = 4.5

P/B = 9.4

Mo

Cr

P/B = 84

P/B = 87

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

A primary monochromator would get rid of this high angle tail


Variable count time l.jpg

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

Data from the mineral stichtite


Vct continued l.jpg

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)

Raw VCT capillary data for stichtite

Data reformatted into ASCII format xye file

Remember if subtracting background (e.g. capillary blank) that the error is  original intensity!


Vct data quantitative analysis l.jpg

VCT dataQuantitative 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 l.jpg

VCT dataStructure refinement

  • You can extract more structural details if reflections still resolvable up to high angles

Jadarite structure with thermal ellipsoids


Phase id l.jpg

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 id43 l.jpg

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


Slide44 l.jpg

  • 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


Slide45 l.jpg

  • 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 l.jpg

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

LiCrO2

LaNiO3, R-3c

a = 5.456, c = 13.143Å

LaCoO3, R-3c

a = 5.449, c = 13.104Å


Getting desperate yet l.jpg

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 l.jpg

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


Slide49 l.jpg

  • 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 l.jpg

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.jpg

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


Slide52 l.jpg

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