Lesson 1 Good Diffraction Data

Transcription

1 Lesson 1 Good Diffraction Data Nicola Döbelin RMS Foundation, Bettlach, Switzerland

2 Digital Diffractometers Transmission Geometry Debye-Scherrer Geometry Reflective Geometry Bragg-Brentano Geometry Glass Capillary Flat powder sample Foil Fluid Cell Capillaries are ideal for: Light atoms (Polymers, Pharmaceuticals) Small amounts Hazardous materials Air-sensitive materials Reflective Geometry is ideal for: Absorbing materials (Ceramics, Metals) Thin films Texture analysis Use characteristic radiation with low absorption coefficient Use characteristic radiation with high absorption coefficient 2

3 Bragg-Brentano Parafocusing Diffractometer Typical Configuration (with Kβ filter) Focusing circle Receiving slit X-ray tube Divergence slit Soller slit Anti-Scatter slit Anti-Scatter slit Soller slit Detector Kβ Filter Beam mask Goniometer circle Sample 3

4 Bragg-Brentano Parafocusing Diffractometer Typical Configuration (with secondary monochromator) Secondary monochromator Soller slit Receiving slit Anti-Scatter slit Detector Modern instruments are modular. Sample Configuration can be changed easily. PANalytical: «PreFIX» Bruker: «SNAP-LOCK» 4

5 Instrument Configuration Many optical elements = many options to optimize data quality How to find the best configuration? Soller Slits Beam Mask Programmable Anti-Scatter Slit Ni-Filter Tube Programmable Divergence Slit Anti-Scatter Slit Sample Stage «Spinner» Soller Slits Detector 5

6 Optimum Data Quality Beam Geometry Radiation Spectrum Sample Preparation 6

7 Optimum Settings: Divergence Slit & Beam Mask Controlled by: Beam mask Beam divergence θ Beam width Controlled by: Divergence slit Irradiated length 7

8 Optimum Settings: Divergence Slit Fix divergence slit: 8 2θ - Constant intensity + No mechanical parts 2 2θ Variable / Automatic divergence slit: + Gain of intensity at high 2θ - Backlash in slit mechanics 8 2θ 8

9 Intensity [a.u.] Fixed vs. Variable Divergence Slit More than 2x higher intensity at 9 2θ with variable DS fix Diffraction Angle [ 2 ] variable 9

10 Intensity [counts] Intensity [%] Divergence Slit: Irradiated Length 3 15 mm 1 mm 5 mm 1 5mm 1mm 15mm Diffraction Angle [ 2 ] Diffraction Angle [ 2 ] Soller Slits:.2 rad, Beam Mask: 1mm 1

11 Intensity [counts] Intensity [%] Beam Mask 35 2 mm 1 mm 5 mm 1 5mm 1mm 2mm Diffraction Angle [ 2 ] Diffraction Angle [ 2 ] Soller Slits:.2 rad, Irradiated Length: 1mm 11

12 Optimum Settings: Divergence Slit & Beam Mask Correct! Wrong! Wrong! Sample holder Reduce «irradiated length» of divergence slit Use a smaller Beam Mask Primary beam Irradiated area Sample surface For phase quantifications: - Variable divergence slit - Adjust «irradiated length» and beam mask for maximum illumination - Avoid beam spill-over! For structure refinements: - Fix divergence slit - Adjust «slit size» and beam mask for maximum illumination at lowest 2θ - Avoid beam spill-over! 12

13 Optimum Settings: Divergence Slit & Beam Mask Using sample holders of various sizes? Match your Divergence Slit and Beam Mask! Or else: Waste of intensity or Beam spill-over 13

14 Intensity [counts] Intensity [%] Soller Slits / Collimators 7.4 rad.2 rad 1.2rad.4rad 6 5 Asymmetry more 8 pronounced at low 2θ, less at high 2θ Diffraction Angle [ 2 ] Diffraction Angle [ 2 ] In primary & secondary beam, Beam Mask: 1mm, Irradiated Length: 1mm 14

15 Intensity (Counts) Intensity (%) Receiving Slit / Detector Slit mm.225 mm.525 mm Diffraction Angle ( 2theta) Diffraction Angle ( 2theta) Soller Slits:.25 rad, Irradiated Length: 15mm, LynxEye XE in D mode 15

16 Anti-Scatter Slits Focusing circle Receiving slit X-ray tube Divergence slit Soller slit Anti-Scatter slit Anti-Scatter slit Soller slit Detector Kβ Filter Beam mask Goniometer circle Sample 16

17 Anti-Scatter Slits (& Beam Knifes) Removes radiation scattered at air molecules = Divergence slit Lower background signal Anti-Scatter slit In primary beam: Same settings as divergence slit Sample 17

18 Anti-Scatter Slits In secondary beam: Point detector (D): Same settings as divergence slit Linear / area detector (1D / 2D): Wide open or removed 18

19 Intensity (Counts) Intensity (%) Anti-Scatter Slits mm 4.5 mm 2.25 mm 1 9. mm 4.5 mm 2.25 mm Blocking detector channels Diffraction Angle ( 2theta) Diffraction Angle ( 2theta) Soller Slits:.25 rad, Irradiated Length: 15mm, LynxEye XE in 1D mode 19

20 Summary: Optical Elements Optical Element Effect (Too) Small (Too) Large Divergence Slit Beam Mask Adjusts beam length on the sample Adjusts beam width on the sample Loss of intensity Loss of intensity Soller Slit Reduces peak asymmetry Loss of intensity, Better resolution Beam spills over sample Beam spills over sample More asymmetry, Less resolution Anti-Scatter Slit Reduces background signal Loss of intensity High background Receiving Slit Adjusts peak width / resolution Loss of intensity Better resolution Loss of resolution Higher intensity Intensity Resolution 2

21 Optimum Data Quality Beam Geometry Radiation Spectrum Sample Preparation 21

22 Intensity (counts) Unfiltered Diffraction Pattern 5 4 (14) peak of Al 2 O 3 (corundum): d = Å λ = Å n λ = 2d sin (θ) 2θ = 2 asin ( λ 2d ) 2θ = Diffraction Angle ( 2theta) Al 2 O 3 pattern, Cu radiation, (Bruker D8 Advance, LynxEye XE) 22

23 Intensity (counts) Unfiltered Diffraction Pattern 5 Unfiltered Diffraction Angle ( 2theta) Al 2 O 3 pattern, Cu radiation, (Bruker D8 Advance, LynxEye XE) 23

24 Intensity Monochromatic X-Radiation Kα 1 Kα 2 Kβ Bremsstrahlung Cu Wavelength (nm) Ideally: Isolate Kα 1 Reality*: Suppress Kβ and Bremsstrahlung * Exception: Multi-bounce monochromators (mostly used with Synchrotron or rotating anode radiation) 24

25 Intensity (counts) Unfiltered Diffraction Pattern 5 Unfiltered n λ = 2d sin (θ) CuKα 1 4 2θ = 2 asin ( λ 2d ) 3 λ Kα1 λ Kα2 λ Kβ CuKα 2 2 CuKβ Diffraction Angle ( 2theta) Al 2 O 3 pattern, Cu radiation, (Bruker D8 Advance, LynxEye XE) 25

26 Intensity Monochromatic X-Radiation Kα 1 Graphite Monochromator CuKα 1/2 Radiation Kα 2 Kβ Bremsstrahlung Characteristic Radiation filtered Radiation Wavelength (nm) Cu Kβ filter Ideally: Isolate Kα 1 Reality*: Suppress Kβ and Bremsstrahlung Cu Radiation Digital filtering * Exception: Multi-bounce monochromators (mostly used with Synchrotron or rotating anode radiation) 26

27 Intensity (counts) Unfiltered Diffraction Pattern 5 Unfiltered CuKα CuKα 2 2 CuKβ Diffraction Angle ( 2theta) Al 2 O 3 pattern, Cu radiation, (Bruker D8 Advance, LynxEye XE) 27

28 Intensity Graphite Monochromator 1 9 CuKα CuKα Diffraction Angle ( 2 ) LaB 6 pattern, Cu radiation, Graphite monochromator, secondary beam (PANalytical CubiX 3 ) 28

29 Kβ-filtered Diffraction Pattern 5 Unfiltered 5 Kbeta Filter.125 mm 2 18 Kbeta Filter.125 mm Diffraction Angle ( 2theta) Diffraction Angle ( 2theta) Diffraction Angle ( 2theta) Al 2 O 3 pattern, Cu radiation, (Bruker D8 Advance, LynxEye XE) 29

30 Kβ-filtered Diffraction Pattern 5 Unfiltered 5 Kbeta Filter.2 mm 2 18 Kbeta Filter.2 mm Diffraction Angle ( 2theta) Diffraction Angle ( 2theta) Diffraction Angle ( 2theta) Al 2 O 3 pattern, Cu radiation, (Bruker D8 Advance, LynxEye XE) 3

31 Kβ-filtered Diffraction Pattern 5 Unfiltered 5 Digitally filtered 2 18 Digitally filtered Diffraction Angle ( 2theta) Diffraction Angle ( 2theta) Diffraction Angle ( 2theta) Al 2 O 3 pattern, Cu radiation, (Bruker D8 Advance, LynxEye XE) 31

32 Kβ- and digitally filtered Diffraction Pattern 5 Unfiltered 5 2 Digitally filtered + Kbeta Filter.125 mm 18 Digitally filtered + Kbeta Filter.125 mm Diffraction Angle ( 2theta) Diffraction Angle ( 2theta) Diffraction Angle ( 2theta) Al 2 O 3 pattern, Cu radiation, (Bruker D8 Advance, LynxEye XE) 32

33 Intensity (Counts) Calcite + metallic Fe 14 Unfiltered Fe radiation Diffraction Angle ( 2theta) Fe rich sample Cu radiation, (Bruker D8 Advance, LynxEye XE) 33

34 Intensity (Counts) Fluorescent radiation Unfiltered Ni.125 mm Diffraction Angle ( 2theta) 34

35 Intensity (Counts) Fluorescent radiation Unfiltered Ni.125 mm Ni.2 mm Diffraction Angle ( 2theta) 35

36 Intensity (Counts) Fluorescent radiation Unfiltered digital Filter Diffraction Angle ( 2theta) 36

37 Monochromators Kβ Filter: Selectively suppresses Kβ and parts of Bremsstrahlung «Notch filter» Monochromator crystal and energy dispersive detector: Suppress everything BUT Kα «Bandpass filter» Important difference for fluorescent samples 37

38 Summary: Monochromators Optical Element Effect on Spectrum Effect on Intensity Kβ Filter Reduces Kβ peaks Moderate loss Graphite Monochromator Eliminates Kβ peaks Eliminates Fluorescence Multi-bounce Monochromator Eliminates Kβ and Kα 2 Eliminates Fluorescence Energy dispersive Detector Reduces Kβ peaks Eliminates Fluorescence Strong loss Massive loss (mostly used on Synchrotrons) Little loss Monochromator crystals don t work with modern linear (1D) detectors Only in D mode no speed advantage anymore 38

39 Optimum Data Quality Beam Geometry Radiation Spectrum Sample Preparation 39

40 Sample Preparation Sample preparation is ABSOLUTELY CRUCIAL for a good diffraction pattern! Some problems encountered during Rietveld refinement are inherent to the sample. These are hard enough to deal with! Some are related to sample preparation errors. So let s try to minimize these! 4

41 Problems - Graininess - Micro-absorption - Texture - Crystallite size - Sample height displacement - Surface roughness - Sample transparency 41

42 Graininess Single crystals generate spotty diffracted rays. Fine powders generate smooth diffraction rings. 42

43 Graininess Spotty diffraction rings Very strong peak The same sample, at the same 2θ position, but different intensities! Grainy samples: - non-reproducible intensities - «phantom» peaks - «missing» peaks Very weak peak 43

44 Graininess: Rocks in Dust «Rocks in Dust»: A few large crystals in a fine matrix Usually invisible, but if scanned: Strong peaks out of nowhere! 44

45 Graininess Reducing graininess: - Grinding / milling - Adjust divergence slit and beam mask for largest possible irradiated area (= more particles contribute to diffraction pattern) - Use spinning sample stage (= better randomisation) - Counting time per step 1 revolution of samples stage spinner Few diffracting crystallites Many diffracting crystallites 45

46 Micro-absorption X-rays Phase 1: High absorption coefficient for X-radiation X-rays Phase 2: Low absorption coefficient for X-radiation 46

47 Micro-absorption Strong attenuation by phase 1 Large particles absorb significant part of the radiation. Small volume of interaction Weak attenuation by phase 2 Large volume of interaction Small particles absorb insignificant part of the radiation. Volumes of interaction with phases 1 & 2 are representative for phase composition 47

48 Micro-absorption Micro-absorption occurs in samples with - large particles (not crystallites!) - phases with large contrast in absorption coefficients Reducing micro-absorption: - Grinding / milling to reduce particle size 48

49 Texture, Preferred Orientation Platelets, Needles, Fibers, Whiskers Random orientation Preferred orientation SEM Images: L. Galea, RMS Foundation 49

50 Texture, Preferred Orientation Try to avoid orientation at the surface of the sample: - Press powder without «rubbing» the surface - Use back-loading sample holder - Disorder surface with textured stamp - Various creative solutions can be found on the internet (involving Vaseline, hair spray, ) PO can be corrected mathematically, but phase quantification will be biased. (more on this in the lesson on «Rietveld refinement») 5

51 Sample Height Displacement X-ray tube Focusing circle Results: - 2θ shift of peaks - Diffuse / broad peaks - Loss of intensity - «Funny» peak profiles (beam partially blocked) - Focus of diffracted beam is displaced - Receiving slit / 1D / 2D detector out of focus - Diffracted beam blocked by optical elements Detector Sample surface is too low / high Goniometer circle Sample 51

52 Sample Height Displacement Focusing circle Similar effects with rough sample surface and sample transparency Detector X-ray tube Sample surface is too low / high Goniometer circle Sample 52

53 Summary: The Perfect Sample The perfect sample for Bragg-Brentano diffractometers: - Crystallites and particles of 1-5 µm size - Perfectly random orientation - Perfectly flat surface - Surface precisely centered in the goniometer - Not transparent (absorb all primary radiation) 53