Dinnebier & Billinge, TA+PXRD course - Part 1, The Equipment

Transcription

1 Powder X-ray Diffraction (PXRD) in short MATR362 - Workshop on X-ray diffraction and thermoanalytical methods (5 cr) Prof. Markku Leskelä / Mikko Heikkilä Dinnebier & Billinge, 1 2 Aim of these lectures Aim of the course the aim of these lectures is to introduce you to The equipment (Part 1, Tuesday 7.11.) Powder diffractogram (Part 2, Tuesday 7.11.) How to perform measurements (Part 3, Wednesday 8.11.) How to interpret the results (Part 4, Wednesday 8.11.) High temperature XRD (and XRR) (Part 5, Tuesday ) Something more if there s time Laboratory exercises are done in pairs or small groups PANalytical X Pert Pro MPD will be used together with Anton-Paar HTK1200N high temperature oven What will be included: Introduction to the equipment Measurement and phase analysis of the test sample Determination of the suitable measurement parameters for HTXRD measurement Start the overnight measurement Analysis next morning or suitable date later 3 4

2 References Lecture material was made using the following sources: TA+PXRD course Pecharsky and Zavalij, Fundamentals of Powder Diffraction and Structural Characterization of Materials, 2009, Springer Part 1, The equipment Dinnebier and Billinge (Ed.): Powder diffraction Theory and Practice, 2008, RSC Clearfield, Reibenspies and Bhuvanesh (Ed.), Principles and Applications of Powder Diffraction, 2008, Wiley Snyder and Jenkins, Introduction fo X-ray Powder Diffractometry, 1996, Wiley 5 6 X-ray sources Main components X-ray tubes (right) X-ray source synchrotron (e.g. ESRF, bottom left) goniometer pulsars (rotating neutron stars) Optical elements Crab nebula on bottom right, often used as calibration standard for xrays X-ray detector Main function is to measure the scattered intensity as a function of the scattering vector proportional to sin Since wavelength is constant, the angle is adjusted 7 8

3 goniometer (para)focusing Bragg-Brentano geometry is shown below The angle between the x-ray source and the sample is always equal to the angle between the sample and the detector X-ray source F and the detector D move along the goniometer circle F and D are constantly on the same focusing circle goniometer Either sample is stationary and both F and D rotate the same angle q, or the source is stationary while the sample rotates angle q and detector rotates 2q In capillary geometry the tube can remain stationary while only the detector moves 9 10 goniometer goniometer Horizontal (two leftmost) or vertical (two rightmost) arrangements Horizontal is more convenient with respect to the sample, but more demanding on balancing the motors (heavy counterweight needed) In transmission geometry the sample is either a thin film or a capillary Capillary is either filled with powder or the sample is sprinkled on the its surface 11 12

4 Optical elements are based on absorption slits, Soller-slits, collimators filters, attenuators/absorbers reflection mirrors Typical components of Bragg-Brentano diffractometer are shown below F focus of the source; SoS Soller slit; DS divergence slit; Fi beta filter; ScS (anti)scatter slit; RS receiving slit; M monochromator; D detector diffraction monochromators Divergence slits Size of the slit can be either constant Easiest way to collimate a beam is to place a slit between the source and the sample Sometimes a second divergence slit is added N.B. slit always means a loss in intensity Most often used sizes: ¼, ½, 1, 2 Smaller sizes used for special purposes With small samples to avoid beam overspill 1/16 or even 1/32 for X-ray reflectivity (although one should always use parallel beam instead) 15 16

5 or variable Automatic slits can be used to keep the irradiated length constant Divergence slits Divergence of the beam can be calculated as shown in the image Diffracted volume changes, except with thin layers fluorescence problems increase with the volume Intensities need to be corrected with software to fixed slit values Take-off angle Increases the signal/noise ratio at higher angles, but simultaneously makes resolution worse as RS size increases Beam overspill could be an issue with fixed slits as the beam might hit the sample holder at small angles Receiving slit Diffracted beam converges (self-focuses) at the receiving slit Placed at the same distance from the sample as the focal point of the source Largely affects the resolution of the experiment, but decreasing the size decreases Snyder & Jenkins the intensity as well 19 20

6 Antiscatter slit(s) decreases background by reducing unwanted scatter Receiving slits are not needed with 1D detectors Detector surface acts as one e.g. from air or sample holder if fixed receiving slit is used, antiscatter slit size should be as small as possible without decreasing the observed intensity Soller slits Soller slits angular divergence in the direction parallel to the goniometer axis (=axial divergence) is controlled by Soller slits set of parallel, equally spaced thin metal plates Slit sizes usually 2 5 ( rad) Very helpful in reducing the asymmetry of the small angle reflections Axial divergence can be defined as Snyder & Jenkins 23 Snyder & Jenkins 24

7 Parallel plate collimator Much longer plates and parallel to the divergence slit Used with point detectors to select a direction in parallel beam setups Sizes available to our PANalytical tool are 0.09, 0.18 and 0.27 Determine the instrumental resolution smaller size leads to smaller intensity, careful balancing Monochromatization of the incident beam More than one wavelength due to the characteristic radiation Unwanted wavelengths can be removed by filtering diffracting monochromators mirrors to some extent with detector settings especially with energy dispersive detectors filters filters K b -radiation can be removed by using a suitable filter material filter material is selected so that its K-absorption edge for x-rays is betveen K a and K b energies Most often used filters for different anode materials are tabulated below For a Cu tube a suitably thick Ni filter halves the K a radiation intensity while decreasing K b component to two percent Snyder & Jenkins 27 28

8 monochromators Called as primary for incident beam and/or secondary for diffracted beam monochromator Several different arrangements as shown on the right monochromators Primary monochromators single crystals either cut to certain direction or bent (Si, Ge, quartz), based on diffraction from the crystals Usually q-2q geometry, (tube side is stationary) Tube is usually far away from the sample as in the image monochromators monochromators Primary monochromators Secondary monochromators pro K a1 and K a2 can be separated Possible to work with 1D and 2D detectors con Serious drop of intensity Not helping with fluorescence Since monochromator is moving, the crystal is not stable enough to separate K a1 and K a2 smaller quality demands for the crystal Removes only K b radiation but also fluorescence Intensity drops much less than with primary monochromators To about one third, but on the other hand no need for intensity halving filters 31 32

9 monochromators Example measurement shown in the figure Primary monochromator measurement in the topmost diffractogram, secondary in the one below Higher background of the upper due to fluorescence X-ray mirrors (Göbel mirrors) Multilayer structures often W-B 4 C, W-Si, Ni-Si or a combination Surface shaped as parabola, focusing point at the radiation source Parallel radiation, high intensity removes K b radiation and bremsstrahlung secondary mirror removes also fluorescence difficult to align X-ray mirrors Capillary optics for point focus Analyzer crystal or slits define the resolution Often much worse than with (para)focusing slit geometry Due to parallel beam, sample height has no effect on peak position Suitable for measuring irregularly shaped objects No need for symmetric geometry grazing incidence XRD (GIXRD) Bilderback, X-ray Spectrom., 32 (2003) 195 Based on external total reflection of the x-rays Can be focused or parallel One or more reflections with monocapillaries 50 nm - 10 µm rays, capillary must be next to the sample More intensity with polycapillaries (on the right)

10 Detectors detectors films bottom left shows the areas covered by different detectors Full image with area detector, rectangle with line detector and point detectors points with point detector scintillator, proportional counter, semiconductor Rightmost image shows how point detector might give distorted line detectors intensities position sensitive detector (PSD), Real time multiple strip (RTMS) area detectors image plate (IP), two dimensional proportional counters (MWPC), CCD, CMOS Point detectors proportional counter Point detectors scintillation counter X-rays hit a scintillating substance (most often NaI:Tl) that Gas filled space with a metal wire anode in the middle produces visible light Xe or Ar, and small amount of quenching gas (e.g. CH4 or CO2) Photomultiplier enhances the signal X-rays coming through the window ionize the gas Good efficiency and linearity, but worse Formed electron travels towards the anode and ionizes some more gas on the way electrical current at the anode energy resolution than proportional counter Quite good resolution Kβ can t be separated It is possible to discriminate the Kβ-radiation by analyzing current pulse heights 39 Wikipedia 40

11 Point detectors semiconductors PIN-diode, intrinsic part is made by diffusing lithium to p-type silicon or germanium Si(Li) or Ge(Li), nowadays also pure Si or Ge A bit like ionization chamber, in this case electron hole pairs are formed that travel to the electrodes and generate the current Point detectors semiconductors e.g. Bruker Sol-X Excellent energy resolution, but usually requires cooling LN2 or Peltier Linearity is lost at relatively low intensities Summary of point detectors below Clearfield et al Line detectors position sensitive detector (PSD) Basic principle equal to proportional counter Either anode wire or knife (latter patented by INEL) Signal is measured from both ends of the wire and angular position is calculated from the time difference Line detectors position sensitive detector (PSD) Can be either straight (covers 5-10 angle) or bent (even 120 ) Resolution and linearity not that good (for big bent detectors FWHM and I max ~200kcps) but measuring speed and signal/noise ratio obviously very good Well suited for e.g. non-ambient measurements where fast measurements are required 43 44

12 Line detectors real time multiple strip (RTMS) Same principle as with semiconductor point detectors p-type strips made with photolithography to n-type Si, each strip works as a separate detector Since there is no cooling, background noise is much larger than with point detector Neverthless, faster measurement allows longer time/step improving the s/n Line detectors Bruker Mikrogap Miniaturized proportional counter Resistive anode allows longer drift time for ions Better resolution and linearity Våntec-1, e.g. PANalytic X Celerator, Bruker LynxEye Area detectors image plate (IP) Based on phosphors in the detector E.g. BaFBr:Eu 2+ X-ray photon Eu 2+ excites to Eu 3+ After the data has been collected, pixels are scanned with lasers and excited state is released as blue emission which is collected with photomultiplier Area detectors two dimensional proportional counters multi wire proportional counter (MWCP) Basic principle is a proportional counter Two wires perpendicular to each other and a third parallel to the first Afterwards the plate is reactivated with another laser Large area, good dynamic range and linearity, but slow readout time (10-30s)

13 Area detectors two dimensional proportional counters Mikrogap by Bruker is otherwise similar except that lowest row is straight on circuit board and resistive anode closer to that Resolution depends on wire distances that depends on electronics Limited dynamic range, Mikrogap a bit better Area detectors charged coupled cell (CCD) Also in CCD cells x-ray photons are converted to visible light using a phosphor (most commonly Gd 2 O 2 S:Tb) Often phosphor area is bigger than the actual cell, and light is led to the cell via fiber optic taper Resolution depends on the thickness of the phosphor thinner layer gives better resolution but also less emitted light Apex-II by Needs cooling due to large background noise Data readout takes seconds Våntec-2000, 2D Mikrogap by Area detectors CMOS Development of CCD cells is approaching its end, CMOS cells are developing faster and displacing CCD s sensor is directly on top of CMOS Sensitivity already on par with best CCDs Much smaller noise and higher dynamic range Some detector properties are compared in the table CMOS detectors are missing, though e.g. Bruker Photon 100, PANalytical PIXcel, Dectris Mythen and Pilatus Photon-100 by 51 Clearfield et al. 52

14 Sample holders Sample stages Spinners for powders, either in reflection or transmission mode Eulerian cradles for determining texture and macro stresses Improves particle statistics and decreases possible orientation effects 53 November 2017, Mikko Heikkilä PANalytical 54 PANalytical Sample stages Sample stages So called in-plane arm in Rigaku Smartlab 5-axis goniometer Ovens for measurements conducted in non ambient In plane diffraction, phi scans, pole figures, Temperatures from liquid helium up to 2000 C Variable atmospheres (inert, oxidizing, reducing), moisture adjustment Manufacturers e.g. Anton-Paar, MRI 55 56

15 Sample stages Sample stages ScatterX78 attachment to PANalytical s Empyrean allows small Diamond anvil cells (DAC) for measurements done in high pressure (example below) footprint SAXS/WAXS measurements in standard laboratory E.g. colloidal dispersions, polymers, protein solutions, surfactants, Sample is measured in transmission geometry straight through nanopowders and liquid crystals diamonds Sample stage is just in front of the mirror and enclosure is in vacuum to reduce air scattering Focusing geometry incident beam side Automatic sample changers X-RAY TUBE Goniometer center is often hollow and that space can be used ANTISCATTER SLIT (see the middle image) Bruker also sells a sample changer with six 15 sample cassettes MASK and sample changer robot Often combined with robotic XRF PDS can be used in fixed or automatic mode in automatic mode the irradiated length can be set to constant value 59 60

16 Parallel beam geometry incident beam side Focusing geometry diffracted beam side DIVERGENCE SLIT FILTER/ ATTENUATOR/ MASK Ni FILTER ANTISCATTER SLIT SOLLER SLIT PASS can also be used in fixed or automatic mode in automatic mode, the detector can be set to measure constant irradiated length BEAM KNIFE when measuring XRR, a beam knife has to be set above the sample Parallel beam geometry diffracted beam side End of part 1 Questions, anyone? SOLLER SLIT RECEIVING SLIT receiving slit should be used when measuring XRR diffracted Soller is not always necessary 63 64