Instrument Configuration for Powder Diffraction

1 Advanced X ray Bose National Centre for Basic Sciences, 14 15/12/2011 Instrument Configuration for PowderDiffractionInnovation with Integrity What is the application? What are the requirements of your application? Early decisions: What are the sample properties? What data quality is necessary? What Instrument and measurement parameters to use?Advanced X ray Workshop2 OverviewSampleInstrumentDataCollectionEa rly Decisions14 15/12/ 2011"Although the physical nature of a specimen from which the X ray Powder Diffraction data are collected is seemingly simple, sample preparation and data collection are generally the source of most of the serious problems with accurate X ray Diffraction analysis" Too small number of scattering particles / coarse grains (spotiness)

2 Preferred orientation Inappropriate Instrument geometry / sample presentation Unsuited step size / measurement timeAdvanced X ray Workshop3 Overview14 15/12/ 2011 The SampleAdvanced X ray Workshop414 15/12/ 2011 What is the form of the sample? " Powder Diffraction " is more aptly named "Polycrystalline Diffraction " Samples can be Powder , sintered pellets, coatings on substrates, engine blocks, .. What is in your sample? Inorganics often better collected in reflection, organics often better collected in transmission Organics are poor scatterers at high angles Fluorescence can cause problems in data quality How much sample is there?

3 Small quantities suggest capillary geometry (but absorption needs to be considered)Advanced X ray Workshop5 The Sample14 15/12/ 2011 Minimize systematic sample related effects! This is as important as the optimization minimization of the Instrument Configuration ! Avoid persisting with poor data (if possible) Find a better sample Re prepare or remake the sample Change Instrument or Instrument setup Improve data collection parameters Don t rely on any software corrections! Variable slits conversion Preferred orientation corrections Microabsorption corrections (worst!).

4 Advanced X ray Workshop6 The Sample14 15/12/ 2011 The grains in a Powder should be randomly oriented: Front loading Sample prone to preferred orientation Back loading Better, but not effective on preferred orientation in all cases;consider using sandpaper to create a rough sample surface Use of capillary techniques Most effective but potential absorption issues No automation (sample preparation) Sample motion Motion should be 90 to the Diffraction vector No effect on preferred orientation in reflection geometry! May slightly improve particle statistics; no improvements if large grains are present Advanced X ray Workshop7 The Sample14 15/12/ 2011 Rotation parallel to the scattering vector does not reduce preferred orientation effects!

5 Advanced X ray Workshop8 The SampleDebye-Scherrer CapillaryBragg-Brentano Reflection14 15/12/ 2011 The Sample Ideally some 108 1010crystallites in the beam Ideally completely random orientationIncident beamDeb ye cone ofdiffracted beamIncident beamDeb ye cone ofdiffracted beamAdapted from S. Misture, 200214 15/12/ 2011 Advanced X ray Workshop9 PhaseGrain size ( crystallite size!) (C)orundum m (M)agnetite m (Z) mThe SampleCM ZZM14 15/12/ 2011 Advanced X ray Workshop10 The SampleSpotiness effects cannot be corrected!14 15/12/ 2011 Advanced X ray Workshop11 Need to get sample to a fine Powder (preferably in the micron size range) without destroying or detrimentally affecting the sample The higher energy the grinding, the more likely the sample could undergo a phase transition Need to check different methods / grinding times to see which gets the job done effectively with the minimum of effort Peak broadening, apparent increase of background?

6 Overgrinding? Crystallite size already too small? Samples gets amorphous? Variable relative intensities? New or disappearing peaks? Spotiness effect? Preferred orientation? Phase transformation? Try to grind materials with soft phases in liquid nitrogenAdvanced X ray Workshop12 The Sample14 15/12/ 2011 The InstrumentAdvanced X ray Workshop1314 15/12/ 2011 The choice of the optimum Instrument Configuration must consider the aim of the experiment as well as specific sample propertiesAdvanced X ray Workshop14 The Instrument14 15/12/ 2011 Bragg Brentano geometry with secondary monochromator(K 1+2)The Instrument14 15/12/ 2011 Advanced X ray Workshop15 Bragg Brentano geometry with incident beam monochromator (pure K 1)

7 , reflection geometry Focusing monochromator Asymmetric Ge(111) for Cu, asymmetric Ge(220) for MoThe Instrument14 15/12/ 2011 Advanced X ray Workshop16 Bragg Brentano geometry with incident beam monochromator, transmission geometry Foil transmission capillary transmissionThe Instrument14 15/12/ 2011 Advanced X ray Workshop17 ~ 1990: Introduction of parabolic graded multilayers Basic idea: Convert a divergent beam into a parallel beam Nowadays also eliptical graded multilayers for focussingbeamThe InstrumentG bel MirrorX-ray Tube14 15/12/ 2011 Advanced X ray Workshop18 Parallel Beam GeometryAdvanced X ray Workshop19 The InstrumentReflectionFoilTransmissionCapi llaryTransmission14 15/12/ 2011 Bragg Brentano Geometry Properties Excellent resolution / line profile shapes++ Up to 90% intensity loss with monochromators Large footprint of the beam (up to several cm)

8 Good crystallite statistics Possibility of beam overflow at low angles 2 Flat specimen error Sample displacement error Sample transparency errorThe Instrument14 15/12/ 2011 Advanced X ray Workshop20 Bragg Brentano Geometry Flat Specimen Error Sample is tangent to the variable focussing circle leading to peak shifts and asymmetric broadening Small divergence slits help at the expense of intensitySa mple 2 14 15/12/ 2011 Advanced X ray Workshop21 Bragg Brentano Geometry Sample Displacement Error The sample must be tangent to the focussing circle Any deviations lead to peak shifts and asymmetric broadeningNote: The sample displacement error is typically the largest error found in Bragg Brentano geometrySa mple 2 14 15/12/ 2011 Advanced X ray Workshop22 Bragg Brentano Geometry Sample Transparency Error In low absorbing samples the average diffracting surface lies below the physical sample surface leading to peak shifts and asymmetric broadeningNote.

9 The sample transparency error is equivalent to the sample displacement error Use transmission geometrySa mple 2 14 15/12/ 2011 Advanced X ray Workshop23 Parallel beam geometry Properties Offers new possibilites for lab X ray Powder Diffraction by overcoming significant limitations of the Bragg Brentano geometry Ill shaped samples may be used Minimized sample displacement, sample transparency, and beam overflow errors Easy change between reflection and transmission geometry with foil transmission even without touching the Instrument Intensity gain of a factor of up to 10 ?

10 ??The Instrument14 15/12/ 2011 Advanced X ray Workshop24 Parallel Beam GeometryIntensity Gain ? An intensity gain of a factor of up to 10 may be obtained, if collimators / pinholes / slits are replaced by a G bel mirror Thin film analysis (high resolution, grazing incidence, reflectometry) SAXS .. This is NOT the case for standard Powder Diffraction applications, intensity may be even less!14 15/12/ 2011 Advanced X ray Workshop25 Parallel beam geometry Properties Poor to medium resolution Not enough scattering particles ("Spotiness" effect) Parallel beam conditions Small footprint of the beam (few mm at maximum) Can deal with sample surface roughness, but.