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

1. 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

2. 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.

3. 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?

4. 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!

5. Where to start? • What errors affects peak positions? • What affects relative intensities? • Preparing the samples • Different types of sample holders

6. 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°) Note: convention is that –ve sample displacement = sample too high Not an issue for parallel beam systems

7. Peak positions – sources of error • 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

8. Relative intensities • Particle statistics (grain size) • Preferential orientation • Crystal structure • Microabsorption (multiphase samples)

9. 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)

10. 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 (101) reflection with different crystallite sizes

11. “Seeing” particle statistics Playing Russian roulette with a grainy sample Stacking the odds in your favour by micronizing….

12. 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 

13. 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

14. 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

15. 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 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

16. 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

17. 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!

18. Effect of particle size • Brindley proposed that a maximum acceptable particle size for QPA can be calculated by: m = linear absorption coefficient (LAC)

19. 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!

20. 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

21. 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…

22. Going the extra mile… • 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 • If not then spray-drying the sample can be an alternative…. 001 200 Top-loaded, spray-dried 40S mica SEM of spray-dried mica

23. 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!

24. The different preparation techniques Reflection • Top-loading • Flat plate • Back-loading • Side-loading Transmission • Capillary • Foil transmission

25. 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

26. 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)

27. Back-loading

28. Side-loading • I don’t have one of these! • but basic principle….. plug powder sample holder glass slide

29. 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

30. 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) Rietveld refinement of ~10 vol% SnO2 in diamond powder Capillary and reflection data from pure SnO2

31. 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(q) correction required for accurate relative intensities Quartz powder between Kapton Rietveld refinement of SnO2 (1400cm-1)

32. Data collection strategies • Rietveld analysis guidelines published by McCuskeret 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! • 1stdatapoint 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

33. 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.

34. 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….

35. 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 Which line would you choose?

36. 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 • a secondarymonochromator on conventional system is an effective way to filter out fluorescence there is a real peak here! No monochromator Properly aligned monochromator/mirror 50 60 70 80

37. Fluorescence – what to do about it? • With a PSD a conventional monochromator not possible – data with CoKa CoKa - LiMn1.5Ni0.5O4 Which dataset do you prefer?

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

39. 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

40. 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)

41. 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

42. 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 Variable Counting Time Constant Counting Time I ~ LP * thermal vibration * f2 Boehmite (Madsen, 1992)

43. 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

44. 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 structure with thermal ellipsoids

45. 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

46. 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

47. 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

48. 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

49. 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 LiCrO2 LaNiO3, R-3c a = 5.456, c = 13.143Å LaCoO3, R-3c a = 5.449, c = 13.104Å

50. 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!