Lesson 2 Diffractometers

Transcription

1 Lesson 2 Diffractometers Nicola Döbelin RMS Foundation, Bettlach, Switzerland January 14 16, 2015, Bern, Switzerland

2 Repetition: Generation of X-rays / Diffraction SEM: BSE detector, BSED / SAED detector SEM: SE detector XRD (WAXS) SAXS, XRR Target Characteristic X-radiation Absorption: XAS, EXAFS, XANES Sample SEM: EDX detector EMPA: EDX / WDX XPS XRF Anode 2

3 Repetition: Generation of X-rays Kα 1 Intensity Kβ Kα 2 Cu Wavelength (nm) Kβ absorption filter Digital filtering Monochromator Crystal 3

4 Repetition: Powder Diffraction λ n λ = 2 d sin(θ) (120) θ θ 2θ d (100) (010) Powder sample 4

5 Repetition: Powder Diffractometer Diffraction Cones «Secondary Beams» X-ray tube Primary Beam Powder Sample X-ray Detector scanning X-ray intensity vs. 2θ angle 5

6 Analogue Cameras Debye-Scherrer Camera: Powder in Glass Capillary Diffraction pattern recorded on photographic film Various alternative setups: Gandolfi Guinier Straumanis Bradley Seemann-Bohlin Camera 2θ angle 6

7 Digital Diffractometers Transmission Geometry Reflective 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 7

8 Bragg-Brentano Parafocusing Diffractometer Tube and Detector move symmetrically End Scan Start Sample Detector X-ray Tube 8

9 Instruments Lab Instrument Monochromator Configuration RMS Foundation Bruker D8 Energy dispersive Detector Bragg-Brentano (Reflection) Debye-Scherrer (Capillary) Uni Bern PanalyticalX Pert Ni-Filter Bragg-Brentano (Reflection) Uni Bern PanalyticalCubiX Graphite Monochromator Bragg-Brentano (Reflection) Bruker D8 Panalytical X Pert Panalytical CubiX 9

10 Bragg-Brentano Diffractometer Detector Kβ filter X-ray tube Flat powder sample Primary beam Sample holder More optical elements are required to control the beam pattern. Irradiated area Sample surface 10

11 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 11

12 Bragg-Brentano Parafocusing Diffractometer Typical Configuration (with secondary monochromator) Secondary monochromator Soller slit Receiving slit Anti-Scatter slit Detector Modern instruments are modular. Configuration can be changed easily. Sample PANalytical: «PreFIX» Bruker: «SNAP-LOCK» 12

13 Beam Divergence Divergence Slit Soller Slit Beam Masks 13

14 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 14

15 Optimum Settings: Divergence Slit 20 2θ Beam overflow! - Wrong peak intensities - Artifact signal from sample holder Reduced beam divergence angle 20 2θ 80 2θ 80 2θ 15

16 Optimum Settings: Divergence Slit Fixed divergence slit: 20 2θ 80 2θ Low incident angle: - Low penetration depth - Large illuminated area High incident angle: - Deep penetration depth - Small illuminated area Irradiated Volume is constant Constant intensity of diffraction pattern Variable divergence slit: 20 2θ Low incident angle: - Narrow divergence slit - Low penetration depth 80 2θ High incident angle: - Wide divergence slit - Deep penetration depth Irradiated Area is constant Higher diffracted intensity at high 2θ angle 16

17 Fixed vs. Variable Divergence Slit More than 2x higher intensity at 90 2θ with variable DS Intensity [a.u.] fixed Diffraction Angle [ 2θ] variable 17

18 Divergence Slit: Irradiated Length mm 10 mm 5 mm Intensity [counts] Intensity [%] Diffraction Angle [ 2θ] Soller Slits: 0.02 rad, Beam Mask: 10mm 18

19 Optimum Settings: Divergence Slit Correct! Wrong! Wrong! Sample holder Reduce «irradiated length» of divergence slit Use a smaller Beam Mask Primary beam Irradiated area Sample surface Beam Mask 19

20 Beam Mask mm 10 mm 5 mm Intensity [counts] Intensity [%] Diffraction Angle [ 2θ] Soller Slits: 0.02 rad, Irradiated Length: 10mm 20

21 Optimum Settings: Divergence Slit Using sample holders of various sizes? Match your Divergence Slit and Beam Mask! Or else: Waste of intensity or Beam spill-over 21

22 Soller Slits / Collimators rad 0.02 rad Intensity [counts] Intensity [%] Diffraction Angle [ 2θ] In primary & secondary beam, Beam Mask: 10mm, Irradiated Length: 10mm 22

23 Receiving Slit / Detector Slit mm mm mm mm mm mm 5000 Intensity [counts] Intensity [%] Diffraction Angle [ 2θ] Diffraction Angle [ 2θ] Al 2 O 3, 15 mm irradiated length, 2.5 soller slit 23

24 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-bounceMonochromator EliminatesKβandKα 2 Eliminates Fluorescence Energy dispersive Detector Reduces Kβ peaks Eliminates Fluorescence Strong loss Massive loss (mostly used on Synchrotrons) No loss Cu Radiation Kβ absorption filter filtered Radiation Cu Radiation Monochromator Crystal (Graphite, d = nm) CuKα 1/2 Radiation Cu Radiation Digital filtering Energy dispersive Detector 24

25 Summary: Optical Elements Optical Element Effect Too Small Too Large Divergence Slit Soller Slit Anti-Scatter Slit Beam Mask ReceivingSlit Adjusts beam length on the sample Reduces peak asymmetry Reduces background signal Adjusts beam width on the sample Adjustspeakwidth/ resolution Loss of intensity Loss of intensity, Better resolution Loss of intensity Loss of intensity Loss of intensity Better resolution Beam spills over sample More asymmetry, Less resolution High background Beam spills over sample Loss of resolution Higher intensity Kβ Filter Reduces Kβ peaks - - Graphite Monochromator Eliminates Kβ peaks

26 Bragg-Brentano Parafocusing Diffractometer Detector X-ray tube Sample 26

27 Detectors Detector Type Point Detector (0D) Linear Detector (1D) Area Detector (2D) Detector s window Receiving slit Position-sensitive «bins» Receiving slit determines active height Linear array of solid state detectors 2D array of solid state detectors Example Scintillation counter (various) SOL-XE (Bruker) XFlash (Bruker) X Celerator (PANalytical) PIXcel 1D (PANalytical) LynxEye (Bruker) LynxEye XE (Bruker) Våntec-1 (Bruker) D/teX Ultra (Rigaku) PIXcel 3D (PANalytical) Våntec-500 (Bruker) Key Features SOL-XE: Energy dispersive XFlash: Combines XRD + XRF Fast LynxEye XE: Energy dispersive Fast 2D image of Debye rings 27

28 Instruments Lab Instrument Monochr. Detector RMS Foundation Bruker D8 Energy dispersive Detector 1D LynxEye XE Uni Bern Panalytical X Pert Ni-Filter 1D X Celerator Uni Bern Panalytical CubiX Graphite 0D Scintillation Counter Bruker D8 Panalytical X Pert Panalytical CubiX 28

29 Measurement parameters Angular Range Step Size Counting Time 29

30 Angular Range Start before first peak Intensity [Counts] Typical Ranges: θ End at 60 (higher = better) Diffraction Angle [ 2theta] 30

31 Angular Range Avoid the primary beam! Intensity [Counts] Diffraction Angle [ 2theta] 31

32 Angular Range Intensity [Counts] No need to measure empty background Diffraction Angle [ 2theta] 32

33 Step Size At least 5 data points per peak Intensity [Counts] Typically θ 5000 Here: Diffraction Angle [ 2theta] 33

34 Time per Step 1D Energy dispersive Detector 0D Detector Intensity [counts] Intensity [counts] Diffraction Angle [ 2theta] Diffraction Angle [ 2theta] 12.5 min 12.5 min 34

35 Examples Jagged peak shape Intensity [counts] Check your S/N ratio and peak shape! 140 No recommendation! Intensity [counts] Diffraction Angle [ 2theta] Diffraction Angle [ 2theta] Noise or peak? 35

36 Data Quality Checklist For linear detector with Kβ filter Incident beam path Optical Element Divergence Slit Soller Slit Mask Anti-scatter slit Ideal setup Automatic Max irr. length w/o beam overflow Installed Small opening Installed Max irr. width w/o beam overflow Identical to divergence slit Diffracted beam path Sample Anti-scatter slit Soller slit Additional slits Kβ filter Spinning Wide open Installed Small opening Wide open Installed 36