Lesson 2 Diffractometers Nicola Döbelin RMS Foundation, Bettlach, Switzerland January 14 16, 2015, Bern, Switzerland
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
Repetition: Generation of X-rays Kα 1 Intensity Kβ Kα 2 Cu 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Wavelength (nm) Kβ absorption filter Digital filtering Monochromator Crystal 3
Repetition: Powder Diffraction λ n λ = 2 d sin(θ) (120) θ θ 2θ d (100) (010) Powder sample 4
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
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 http://adias-uae.com 6
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
Bragg-Brentano Parafocusing Diffractometer Tube and Detector move symmetrically End Scan Start Sample Detector X-ray Tube 8
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
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
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
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
Beam Divergence Divergence Slit Soller Slit Beam Masks 13
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
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
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
Fixed vs. Variable Divergence Slit More than 2x higher intensity at 90 2θ with variable DS Intensity [a.u.] fixed 20 30 40 50 60 70 80 90 Diffraction Angle [ 2θ] variable 17
Divergence Slit: Irradiated Length 3000 15 mm 10 mm 5 mm 100 2500 80 2000 Intensity [counts] 1500 1000 Intensity [%] 60 40 500 20 0 30.2 30.3 30.4 30.5 3 Diffraction Angle [ 2θ] 0 30.2 30.3 30.4 30.5 Soller Slits: 0.02 rad, Beam Mask: 10mm 18
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
Beam Mask 3500 20 mm 10 mm 5 mm 100 3000 80 2500 Intensity [counts] 2000 1500 1000 Intensity [%] 60 40 500 20 0 30.2 30.3 30.4 30.5 Diffraction Angle [ 2θ] 0 30.2 30.3 30.4 30.5 Soller Slits: 0.02 rad, Irradiated Length: 10mm 20
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
Soller Slits / Collimators 7000 0.04 rad 0.02 rad 100 6000 80 5000 Intensity [counts] 4000 3000 2000 Intensity [%] 60 40 20 1000 0 30.2 30.3 30.4 30.5 Diffraction Angle [ 2θ] 0 30.2 30.3 30.4 30.5 In primary & secondary beam, Beam Mask: 10mm, Irradiated Length: 10mm 22
Receiving Slit / Detector Slit 7000 6000 0.075 mm 0.225 mm 0.375 mm 100 80 0.075 mm 0.225 mm 0.375 mm 5000 Intensity [counts] 4000 3000 2000 Intensity [%] 60 40 20 1000 0 34.9 35.0 35.1 35.2 35.3 35.4 Diffraction Angle [ 2θ] 0 34.9 35.0 35.1 35.2 35.3 35.4 Diffraction Angle [ 2θ] Al 2 O 3, 15 mm irradiated length, 2.5 soller slit 23
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 = 0.3352 nm) CuKα 1/2 Radiation Cu Radiation Digital filtering Energy dispersive Detector 24
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 - - 25
Bragg-Brentano Parafocusing Diffractometer Detector X-ray tube Sample 26
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
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
Measurement parameters Angular Range Step Size Counting Time 29
Angular Range 14000 Start before first peak 12000 Intensity [Counts] 10000 8000 6000 Typical Ranges: 5-60 2θ End at 60 (higher = better) 4000 2000 0 0 10 20 30 40 50 60 Diffraction Angle [ 2theta] 30
Angular Range Avoid the primary beam! 20000 Intensity [Counts] 15000 10000 5000 0 0 10 20 30 40 50 60 Diffraction Angle [ 2theta] 31
Angular Range 30000 25000 Intensity [Counts] 20000 15000 10000 No need to measure empty background 5000 0 0 10 20 30 40 50 60 Diffraction Angle [ 2theta] 32
Step Size 25000 At least 5 data points per peak 20000 Intensity [Counts] 15000 10000 Typically 0.02 2θ 5000 Here: 0.0122 0 35.0 35.1 35.2 35.3 Diffraction Angle [ 2theta] 33
Time per Step 1D Energy dispersive Detector 0D Detector 7000 180 6000 160 140 Intensity [counts] 5000 4000 3000 2000 Intensity [counts] 120 100 80 60 40 1000 20 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Diffraction Angle [ 2theta] Diffraction Angle [ 2theta] 12.5 min 12.5 min 34
Examples Jagged peak shape Intensity [counts] 7000 6000 5000 4000 3000 2000 1000 180 160 100 Check your S/N ratio and peak shape! 140 No recommendation! Intensity [counts] 120 80 60 40 20 0 30 31 32 33 34 35 Diffraction Angle [ 2theta] 0 30 31 32 33 34 35 Diffraction Angle [ 2theta] Noise or peak? 35
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