Sample preparation data collection and phase id using powder xrd
1 / 52

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

  • Uploaded on

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

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'Sample Preparation, Data Collection and Phase-ID using Powder XRD' - otylia

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
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… Powder XRD

  • 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 Powder XRD

  • 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? Powder XRD

  • 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? Powder XRD

  • What affects peak positions?

  • What affects relative intensities?

  • Preparing the samples

  • Different types of sample holders

Peak positions l.jpg
Peak positions Powder XRD

  • 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 Powder XRD

  • Particle statistics (grain size)

  • Preferential orientation

  • Crystal structure

  • Microabsorption (multiphase samples)

Sample related problems l.jpg
Sample-related problems Powder XRD

  • 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 Powder XRD

size range









Crystallites / 20mm3

5.97 × 105

3.82 × 107

3.82 × 1010

Intensity reproducibility





No. of diffracting crystallites




“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

How to improve particle statistics l.jpg
How to improve particle statistics Powder XRD

  • 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? Powder XRD

  • 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


Overlay of 3 repeat patterns from un-micronized cement

Overlay of 3 repeat patterns from micronized cement

Microabsorption l.jpg
Microabsorption Powder XRD

  • 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? Powder XRD

  • 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 Powder XRD

  • 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! Powder XRD

  • 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 Powder XRD

  • 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 Powder XRD

  • Top-loading

  • Flat-plate

  • Back-loading

  • Side-loading

  • Capillary

Top loading l.jpg
Top-loading Powder XRD

  • 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 plate Powder XRDaka: 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 (

Back loading l.jpg
Back-loading Powder XRD

Side loading l.jpg
Side-loading Powder XRD

  • I don’t have one of these!

  • Basic principle…..







Capillaries l.jpg
Capillaries Powder XRD

  • 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 Powder XRD

  • 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 Powder XRD



  • 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 Powder XRD

  • 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 Powder XRD

  • 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)


  • 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 Powder XRD

  • ‘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 Powder XRD

  • 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? Powder XRD

  • 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 Powder XRD

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






Lin (Counts)













2-Theta - Scale


  • 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






Fluorescence what to do about it l.jpg
Fluorescence – what to do about it? Powder XRD

  • 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. Powder XRD

  • 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: Powder XRDPhase-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 Powder XRD

  • 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 Powder XRD

  • 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
LiMn Powder XRD1.4Ti0.1Ni0.5O4



P/B = 4.5

P/B = 9.4



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 Powder XRD

  • 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 Powder XRD

  • 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 data Powder XRDQuantitative 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 data Powder XRDStructure 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 Powder XRD

  • 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 Powder XRD

  • 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 Powder XRD

    • 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




No luck what next l.jpg
No luck – what next? Powder XRD

  • 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)



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? Powder XRD

  • 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… Powder XRD

  • 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 isn’t the best one for the job!

  • 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 isn’t the best one for the job!

  • 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


Slide52 l.jpg

Questions? isn’t the best one for the job!