Instrument Configuration for Powder Diffraction

Transcription

1 Instrument Configuration for Powder Diffraction Advanced X-ray Workshop S.N. Bose National Centre for Basic Sciences, 14-15/12/2011 Innovation with Integrity

2 Overview What is the application? What are the requirements of your application? Early Decisions Sample Instrument Data Collection Early decisions: What are the sample properties? What data quality is necessary? What instrument and measurement parameters to use? Advanced X-ray Workshop 2

3 Overview "Although the physical nature of a specimen from which the X-ray powder-diffraction data are collected is seemingly simple, sample preparation and data collection are generally the source of most of the serious problems with accurate X-ray diffraction analysis" Too small number of scattering particles / coarse grains (spotiness) Preferred orientation Inappropriate instrument geometry / sample presentation Unsuited step size / measurement time Advanced X-ray Workshop 3

4 The Sample Advanced X-ray Workshop 4

5 The Sample What is the form of the sample? "Powder Diffraction" is more aptly named "Polycrystalline Diffraction" Samples can be powder, sintered pellets, coatings on substrates, engine blocks, What is in your sample? Inorganics often better collected in reflection, organics often better collected in transmission Organics are poor scatterers at high angles Fluorescence can cause problems in data quality How much sample is there? Small quantities suggest capillary geometry (but absorption needs to be considered) Advanced X-ray Workshop 5

6 The Sample Minimize systematic sample related effects! This is as important as the optimization minimization of the instrument configuration! Avoid persisting with poor data (if possible) Find a better sample Re-prepare or remake the sample Change instrument or instrument setup Improve data collection parameters Don t rely on any software corrections! Variable slits conversion Preferred orientation corrections Microabsorption corrections (worst!)... Advanced X-ray Workshop 6

7 The Sample The grains in a powder should be randomly oriented: Front loading Sample prone to preferred orientation Back loading Better, but not effective on preferred orientation in all cases; consider using sandpaper to create a rough sample surface Use of capillary techniques Most effective but potential absorption issues No automation (sample preparation) Sample motion Motion should be 90 to the diffraction vector No effect on preferred orientation in reflection geometry! May slightly improve particle statistics; no improvements if large grains are present Advanced X-ray Workshop 7

8 The Sample Rotation parallel to the scattering vector does not reduce preferred orientation effects! Bragg-Brentano Reflection Debye-Scherrer Capillary Advanced X-ray Workshop 8

9 The Sample Debye cone of diffracted beam Ideally some crystallites in the beam Ideally completely random orientation Incident beam Adapted from S. Misture, 2002 Advanced X-ray Workshop 9

10 The Sample Phase Grain size ( crystallite size!) (C)orundum 28.0µm (M)agnetite 36.2µm (Z)ircon 21.1µm M Z C ZM Advanced X-ray Workshop 10

11 The Sample Spotiness effects cannot be corrected! Advanced X-ray Workshop 11

12 The Sample Need to get sample to a fine powder (preferably in the micron size range) without destroying or detrimentally affecting the sample The higher energy the grinding, the more likely the sample could undergo a phase transition Need to check different methods / grinding times to see which gets the job done effectively with the minimum of effort Peak broadening, apparent increase of background? Overgrinding? Crystallite size already too small? Samples gets amorphous? Variable relative intensities? New or disappearing peaks? Spotiness effect? Preferred orientation? Phase transformation? Try to grind materials with soft phases in liquid nitrogen Advanced X-ray Workshop 12

13 The Instrument Advanced X-ray Workshop 13

14 The Instrument The choice of the optimum instrument configuration must consider the aim of the experiment as well as specific sample properties Advanced X-ray Workshop 14

15 The Instrument Bragg-Brentano geometry with secondary monochromator (K α1+2 ) Advanced X-ray Workshop 15

16 The Instrument Bragg-Brentano geometry with incident beam monochromator (pure K α1 ), reflection geometry Focusing monochromator Asymmetric Ge(111) for λ Cu, asymmetric Ge(220) for λ Mo Advanced X-ray Workshop 16

17 The Instrument Bragg-Brentano geometry with incident beam monochromator, transmission geometry Foil transmission - capillary transmission Advanced X-ray Workshop 17

18 The Instrument ~ 1990: Introduction of parabolic graded multilayers Basic idea: Convert a divergent beam into a parallel beam Nowadays also eliptical graded multilayers for focussing beam Göbel Mirror X-ray Tube Advanced X-ray Workshop 18

19 The Instrument Parallel Beam Geometry Reflection Foil Transmission Capillary Transmission Advanced X-ray Workshop 19

20 The Instrument Bragg-Brentano Geometry - Properties Excellent resolution / line profile shapes ++ Up to 90% intensity loss with monochromators Large footprint of the beam (up to several cm) Good crystallite statistics Possibility of beam overflow at low angles 2θ Flat specimen error Sample displacement error Sample transparency error Advanced X-ray Workshop 20

21 Bragg-Brentano Geometry Flat Specimen Error 2Θ Sample is tangent to the variable focussing circle leading to peak shifts and asymmetric broadening Small divergence slits help at the expense of intensity Sample Advanced X-ray Workshop 21

22 Bragg-Brentano Geometry Sample Displacement Error 2Θ The sample must be tangent to the focussing circle Any deviations lead to peak shifts and asymmetric broadening Note: The sample displacement error is typically the largest error found in Bragg-Brentano geometry Sample Advanced X-ray Workshop 22

23 Bragg-Brentano Geometry Sample Transparency Error 2Θ In low absorbing samples the average diffracting surface lies below the physical sample surface leading to peak shifts and asymmetric broadening Note: The sample transparency error is equivalent to the sample displacement error Use transmission geometry Sample Advanced X-ray Workshop 23

24 The Instrument Parallel beam geometry - Properties Offers new possibilites for lab X-ray powder diffraction by overcoming significant limitations of the Bragg-Brentano geometry Ill-shaped samples may be used Minimized sample displacement, sample transparency, and beam overflow errors Easy change between reflection and transmission geometry - with foil transmission even without touching the instrument Intensity gain of a factor of up to 10??? Advanced X-ray Workshop 24

25 Parallel Beam Geometry Intensity Gain? An intensity gain of a factor of up to 10 may be obtained, if collimators / pinholes / slits are replaced by a Göbel mirror Thin film analysis (high resolution, grazing incidence, reflectometry) SAXS... This is NOT the case for standard powder diffraction applications, intensity may be even less! Advanced X-ray Workshop 25

26 The Instrument Parallel beam geometry - Properties Poor to medium resolution Not enough scattering particles ("Spotiness" effect) Parallel beam conditions Small footprint of the beam (few mm at maximum) Can deal with sample surface roughness, but... Advanced X-ray Workshop 26

27 Parallel Beam Geometry "Spotiness" effect Bragg-Brentano geometry Horizontal beam divergence significantly increases the number of diffracting crystallites Parallel beam geometry Significant reduction of the number of diffracting particles due to low beam divergence Advanced X-ray Workshop 27

28 Parallel Beam Geometry Surface Roughness With decreasing 2q diffracted intensity may not reach the detector due to "self-absorption" leading to severe intensity errors Note: Surface roughness will normally prevent reliable quantitative analysis and structure analysis Advanced X-ray Workshop 28

29 The Instrument The Bragg-Brentano is the preferred geometry for most powder diffraction applications, including non-ambient measurements The parallel beam geometry is required for ill-shaped samples and micro-diffraction. Applications are normally limited to phase ID. Advanced X-ray Workshop 29

30 Data Collection Advanced X-ray Workshop 30

31 Data Collection Need of accurate profile parameters: Phase ID Quantitative Analysis 2θ Intensity Resolution Indexing Structure Analysis Advanced X-ray Workshop 31

32 Data Collection Phase ID has the least stringent requirements on both sample prep and data collection Highly tolerant to large peak position and intensity errors Highly tolerant to sample preparation and presentation, unless there are issues related to complex samples (peak overlap) and lower limits of detection Size-strain analysis, quantitative phase analysis, and structure analysis require careful sample preparation and presentation! Advanced X-ray Workshop 32

33 Data Collection Phase ID Make sure to get enough peaks for confident phase identification. Using Cu X-rays: 2 to 90 degrees 2θ; most PDF entries dont go beyond Count time depends on whether only interested in major phases, minor or trace phases Consider VCT All search / match results given can only be a list of suggestions. It is up to the user to pick the correct suggestions or to recognise that there are no reasonable hits The best match does not necessarily give you the correct / best crystal structure! Advanced X-ray Workshop 33

34 Data Collection Indexing Data collection range to get 20 to 40 peaks For low absorbing samples (organics), floating onto a low background plate can be desirable (reflection geometry) Accurate peak positions are important with indexing, not intensities Use the Fundamental Parameters Approach to obtain the best peak positions possible XRD optimized for maximum resolution Monochromatic radiation desirable! Small peaks shouldn't be obscured by Kα 2 peaks! Advanced X-ray Workshop 34

35 Data Collection Size-Strain Analysis Measure the maximum accessible 2θ-range to be able to distinguish between crystallite size and strain XRD optimized for line profile shape Bragg-Brentano geometry gives best defined line profile shapes Fundamental Parameters Approach XRD optimized for line profile shape if crystallite size is large and / or strain is small Advanced X-ray Workshop 35

36 Data Collection Structure Analysis Optimize sample preparation and presentation to minimize any sample related effects Preferred orientation, spotiness, absorption Inorganics: Measure in reflection or transmission; for transmission Mo radiation is strongly recommended Organics: Measure in transmission Consider Mo + capillary rather than Vario-1 with Cu, specifically if there are preferred orientation and absorption issues! XRD rather optimized for high intensity rather than maximum resolution High intensity using Kα 1,2 may be more important than getting rid of Kα 2 High angle data are particularily important, VCT is crucial Measure the maximum accessible 2θ-range Advanced X-ray Workshop 36

37 Data Collection Quantitative Analysis Similar to structure analysis Optimize sample preparation and presentation to minimize any sample related effects Preferred orientation, spotiness, absorption XRD rather optimized for high intensity rather than maximum resolution High intensity using Kα 1,2 may be more important than getting rid of Kα 2 High angle data are particularily important, VCT is crucial Measure the maximum accessible 2θ-range For well characterized mixtures a smaller 2θ-range maybe sufficient Advanced X-ray Workshop 37

38 Innovation with Integrity Copyright 2011 Bruker Bruker Corporation. Corporation. All rights reserved. All rights reserved.