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Optical Components of Basic Powder Diffractometers

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Presentation on theme: "Optical Components of Basic Powder Diffractometers"— Presentation transcript:

1 Optical Components of Basic Powder Diffractometers
X-ray Powder Diffractometry Optical Components of Basic Powder Diffractometers

2 Classical Powder Diffractometer
goniometer Detector X-ray tube Monochromator Soller slit Soller slit Receiving slit Divergence slit Anti-scatter slit Mask Sample stage

3 Classical Powder Diffractometer
tube focus soller slit divergence slit anti-scatter slit receiving slit diffracted beam monochromator detector Optical components in X-ray beam path

4 Classical Powder Diffractometer
Sample surface bisects incident and scattered beams Scattered beams focus at the same distance as the tube focus in receiving slit Optimal resolution

5 Polycrystalline sample
Classical diffractometer: the parafocusing effect Scan direction Scan direction Receiving slit + detector Divergence slit Tube focus Polycrystalline sample

6 The Bragg-Brentano diffractometer
2 Receiving slit X-ray tube Goniometer axis Sample : angle between incident beam and sample 2: angle between incident and diffracted beam

7 Classical Powder Diffractometer
Origin and characteristics of X-rays The X-ray beam path Sample stages Detection of X-rays

8 The X-ray source What are X-rays? Generation of X-rays
Characteristic radiation Continuous radiation X-ray operation conditions Tube efficiency Anode material: choice of wavelength Power rating: mA & kV Focus type and orientation

9 Generation of X-rays Electrons are emitted by a hot filament
High voltage accelerates electrons Electrons bombard anode material with high speed Kinetic energy of electrons largely transferred into heat and X-ray radiation Current (mA) Voltage (kV)

10 Generation of X-rays Mo-anode Continuous radiation
Characteristic radiation Spectrum of X-rays depending on applied voltage and anode material. Characteristic radiation used for experiments.

11 X-ray Operation Conditions
Anode material = choice of wavelength

12 X-ray Operation Conditions
Operating conditions: anode material

13 X-ray Operation Conditions
Focus orientation: line focus and point focus Line general phase analysis stress Point microdiffraction texture stress point focus line focus

14 Optical components for the powder diffractometer
Intermezzo: optical path in various powder diffractometer setups

15 The Bragg-Brentano diffractometer
2 Receiving slit X-ray tube Goniometer axis Sample : angle between incident beam and sample 2: angle between incident and diffracted beam

16 What really happens Each d-spacing in the polycrystalline material forms “rings” centered around the incident beam

17 The Debye-Scherrer camera
Each d-spacing in the polycrystalline material forms “rings” centered around the incident beam

18 The area detector The area detector catches a part of several diffraction rings at once

19 Enlarging the irradiated area
Large irradiated width, but impossible to derive good peak shapes: Area detector needs a small irradiated width

20 Use of the line focus - traditionally
Receiving slit Soller slits Line focus: more signal due to larger irradiated volume Soller slits are necessary

21 Use of the line focus – parafocusing geometry
Receiving slit Soller slits 2 R2 R1 Para-focusing geometry (Bragg-Brentano): large irradiated area, focusing on receiving slit Prerequisites:  = 2/2 and R1=R2

22 Point detector vs. area detector
Large irradiated area Good statistical description of the sample Excellent resolution, determined by receiving slit Slow Small irradiated area Possibility for micro-diffraction Moderate resolution, determined by irradiated area Fast

23 The linear detector Large irradiated area Excellent resolution Fast 2
Soller slits 2 R2 R1

24 The X-ray Beam Path Optical components in X-ray beam path tube focus
soller slit divergence slit anti-scatter slit receiving slit diffracted beam monochromator detector Optical components in X-ray beam path

25 The X-ray Beam Path Incident beam: Diffracted beam: Soller slit
X-ray tube focus Soller slit Divergence slit Width mask Beta-filter Monochromator Diffracted beam: Receiving slit Soller slit Anti-scatter slit Width mask Beta-filter Monochromator

26 Divergence Slit & Anti-scatter Slit
determining illuminated and observed length divergence slit anti-scatter slit

27 Divergence Slit & Anti-scatter Slit
Divergence slit determines the irradiated length on the sample major effect on intensities minor effect on resolution Anti-scatter slit determines observed length on the sample major effect on the intensity used to reduce background ideally the illuminated area equals the observed area

28 Divergence Slit & Anti-scatter Slit
fixed mode (4, 2, 1, 1/2, 1/4, 1/8, 1/16, 1/32º) automatic mode (length in mm) Anti-scatter slit fixed mode & automatic mode following receiving slit (reflectivity)

29 Fixed slits: always choose them correctly!
Make sure that you never over-irradiate the sample at low 2Theta angles… What determines the resolution in the X’Celerator geometry? In the previous course you have learned that the resolution in the traditional diffractometer is determined by the receiving slit. In this case, however, there is no physical receiving slit. In many cases, the resolution in the diffractogram is determined by the sample. A crystallite size which is too small and/or crystallites with a lot of defects can cause a broadening of the peaks beyond that caused by the instrument. Of course it does not make sense to increase instrument resolution beyond that of the sample. The resolution that one can obtain with the X’Celerator is comparable with a receiving slit of about 0.1 mm. Other ways to influence the resolution are (in order of importance): reduction of divergence slit size reduction of Soller slit size - less peak asymmetry at low angles reduction of active length - effect is generally too small to be detected. … it will give a very high background!

30 Divergence Slit Size & Peak Intensities
factor 3.75 Hint: you can observe the irradiated length on the status bar of the X’Pert DataCollector! factor 1.47! 20 mm length

31 Soller Slits Soller slits: limiting axial divergence Soller slit

32 Soller Slits Soller slits consist of large numbers of parallel plates in plane of diffraction. Soller slits limit the spread of the incident and diffracted X-ray beam out of the plane of diffraction: 0.01, 0.02, 0.04 rad and 0.08 rad. large effect on intensities moderate effect on resolution (low/high 2q) It is good practice to place similar Soller slits in the incident and diffracted beam.

33 Extending the angular resolution: 0.01 rad Soller slits
2 x 0.04 rad, 15 minutes 2 x 0.02 rad, 45 minutes 2 x 0.01 rad, 110 minutes

34 Soller slits also work in transmission geometry

35 -filter & Monochromator
-filter and/or monochromator: removing unwanted radiation Possible places -filter diffracted beam monochromator

36 b-filter & Monochromator
b-filter and diffracted beam monochromator remove unwanted wavelengths like Kb-line and continuous radiation b -filters selectively absorb radiation Monochromators select radiation by means of diffraction and take out sample fluorescence

37 Intermezzo What is sample fluorescence, why do people worry about it, and how to deal with it?

38 Sample fluorescence, what is it?
8 keV 7 keV “halo” 8 keV Sealed X-ray tube intensity 2 angle Diffraction peak from 8 keV photon Powder sample containing Co Constant background from sample fluorescence

39 Sample fluorescence, an example
Cu K1 radiation 1/4º divergence slit Higher background due to the presence of Co in the sample Low intensity in peaks as a consequence of low penetration depth in the sample

40 Sample Fluorescence Problematic elements for Cu radiation: H He Li Be
Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V V Cr Cr Mn Mn Fe Fe Co Co Ni Ni Cu Cu Zn Zn Ga Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba L Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra A L La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu A Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

41 Who worries about sample fluorescence?
People measuring samples with these elements, especially in research battery materials metals/mining magnetic materials

42 How to deal with this effect?
Incident beam monochromator or filter Change the PHD levels of the detector Different anode materials Optimize the kV/mA settings Sample with high % of transition metals Diffracted beam monochromator or filter

43 The incident beam side: limit the tube spectrum
The spectrum from an X-ray tube is NOT monochromatic! Cu K As diffractionists, we only want to see K! In other words: Use tricks to limit the spectrum: kV/mA settings, filters, monochromators,… Cu K Continuous or white radiation (influenced by kV setting) 10 20 30 40 kV

44 Experimental procedure
Samples: powders from pure metals or their oxides Instrument: X’Pert PRO MPD, X’Celerator detector Scan range: 10 to 100 º2 Scan time: between 10 minutes and 5 hours (depending on configuration) Data used for evaluation: intensity of strongest peak background near 90 º2 Peak and background are plotted as a function of the sample TiO2 V2O5 Cr Mn Fe Co3O4 Ni Cu ZnO Ga2O3

45 An example of the resulting graph
The traditional powder diffraction geometry: curved diffracted beam monochromator takes out almost (!) everything Net intensity of strongest peak Background near 90º 2 Strong Cu fluorescence from excitation by Cu K!

46 How can I see that my detector picks up fluorescence?
Remove incident beam anti-scatter slit and beam knife Measure a scan with constant irradiated and observed length from 5 –100 deg 2Theta If sample fluorescence is picked up by the detector, you will see a background that increases with 2Theta After divergence slit conversion to fixed slits values, the background is flat again

47 Example: fluorescent sample, constant irr. length
TbVO4 at 20 K

48 What is the competition doing?
How well can the Vantec deal with fluorescent samples? Bruker claims: no diffracted beam monochromator necessary Instead, electronic settings of the detector are changed (Pulse Height Discrimination, PHD) We can do this too, with better results

49 Pulse Height Discrimination
Selecting the correct detector pulses

50 The influence of Pulse Height Discrimination (PHD)
Ni filter in diffracted beam Also LL = 58 and LL = 68 datasets were recorded Improvement in peak/bg especially with Mn and Fe

51 Changing the lower level, does it improve?
Peak-to-background improvement relative to default setting So: for a pure Fe sample, you can improve p/bg with a factor of almost 15 at the cost of intensity of more than 8 Let us compare this with the diffracted beam monochromator K intensity loss relative to default setting

52 Changing the lower level, does it improve?
Peak-to-background improvement relative to default setting X’Celerator with diffracted beam monochromator K intensity loss relative to default setting

53 Example 1: Comparison Alpha-1 with Vario-1
Sample: Na3Nd9O3(BO3) Alpha-1 data was recorded in the beginning of 2004 At that time, we were not aware of the possibilities of X’Celerator energy discrimination Vario-1 data was recorded in Karlsruhe in 2005; I suspect that Bruker specialist had our data Nd has L lines with an energy of about 5.3 keV, more or less comparable in energy with Cr K lines

54 Example 1: Comparison Alpha-1 with Vario-1
Peaks scaled to the same height Bruker scan shows better peak/bg! Comment in Bruker scan says: “vantec-discrimination) Customer bought Bruker based on these results…

55 Example 1: Comparison Alpha-1 with Vario-1
Scans on Cr show effect of PHD changes See table in Martijn’s manuscript for p/bg improvements and attenuation factor of K-Alpha!

56 Example 2: Direct energy resolution comparison
Sample: Haematite (Fe2O3) Cu radiation Set of Bruker .raw files in which their lower level setting is adjusted Scan time, step size and scan range in PANalytical scans were kept similar to Bruker scans Our scans show better intensities, similar default p/bg and quicker improvement upon raising the lower level!

57 Example 2: Direct energy resolution comparison
6000 Intensity (counts) 5000 4000 3000 2000 1000 31.5 32.5 33.5 34.5 2Theta (°) More intensity and better peak resolution in the same time (upper graph = us) Quicker improvement in peak-to-background ratio upon changing the PHD levels, indicating better energy resolution

58 Conclusions Energy discrimination of a linear detector can help to overcome issues with sample fluorescence in certain cases Improved peak/bg ratio may look attractive to customer Correct implementation of PHD settings will come in next Data Collector version Note that later X’Celerators have LOWER default Lower Level! Take a look at the background when measuring with the X’Celerator… Although Bruker uses this as a lockout of their detector, our X’Celerator actually performs better!

59 Sample Stages For circular samples, no spinning

60 Sample Stages Sample Stages Multi Purpose Sample Stage (MPSS)
For flat samples and odd shaped samples up to 1 kg.

61 Sample Stages Sample Stages Sample spinner to eliminate inhomogeneous
sample statistics

62 Sample Stages Sample Stages Displacement error:
Sample stage and/or sample above or below diffraction plane: peaks are displaced from original position by R s

63 Summary These slides were meant to give you an overview of the optical components of the powder diffractometer Remember the remarks about sample displacement! Tomorrow we’ll discuss alternative geometries: parallel beam, transmission, non-ambient, …

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