1
Specimen type and preparation Radiation source Wavelength Instrument geometry Detector type Instrument setup Scan parameters 2
Specimen type and preparation Slide mount Front loading cavity Back loading cavity Side drifting cavity Low backgrd plate Several spherical particle techniques 3
Specimen type and preparation Slide mount Front loading cavity Back loading cavity Side drifting cavity Low backgrd plate Several spherical particle techniques Preferred orientation is worst prep problem 4
Preferred orientation 5
Preferred orientation 6
Specimen type and preparation Slide mount Front loading cavity Back loading cavity Side drifting cavity Low backgrd plate Several spherical particle techniques Low angle problem - fixed divergence slit: X specimen 7
Specimen type and preparation To get good particle statistics, generally want size < 10 µ Poorly ground sample: 8
Specimen type and preparation Slide mount Front loading cavity Back loading cavity Side drifting cavity Low backgrd plate Several spherical particle techniques Neutron diffraction requires larger specimens 9
Radiation sources Lab x-rays Rotating anode x-rays Synchrotron x-rays Constant wavelength neutrons TOF neutrons 10
X-rays vs neutrons X-rays - atomic scatt power (ƒ) decreases w/ 2Θ Neutrons - atom scatt cross sections constant w/ 2Θ 11
X-rays vs neutrons X-rays - low atomic no. ƒs very small Neutrons - little variation of atom scatt cross sections w/ atomic no. 12
X-rays vs neutrons X-rays - low atomic no. ƒs very small Neutrons - little variation of atom scatt cross sections w/ atomic no. magnetic spin use for magnetic structure detn 13
X-rays vs neutrons X-rays - usually α 1 -α 2 doublet used (not w/ synchrotron x-rays) 14
X-rays vs neutrons 15
Radiation sources Lab x-rays relatively low intensity Rotating anode x-rays much higher intensity 16
Radiation sources Lab x-rays relatively low intensity Rotating anode x-rays much higher intensity Synchrotron x-rays extremely high intensity monochromatic continuously variable wavelength very tiny beam 17
Radiation sources Lab x-rays relatively low intensity Rotating anode x-rays much higher intensity Synchrotron x-rays extremely high intensity monochromatic very high resolution continuously variable wavelength very tiny beam 18
Radiation sources Reactor neutrons continuous wavelength distribution monochromator req'd 19
Radiation sources Reactor neutrons continuous wavelength distribution monochromator req'd generally low flux, low resolution 20
Radiation sources Spallation source (pulsed) time-of-flight (TOF) energy (wavelength) analysis used 21
Radiation sources Spallation source (pulsed) time-of-flight (TOF) energy (wavelength) analysis used very high flux, high resolution 22
Radiation sources Spallation source (pulsed) time-of-flight (TOF) energy (wavelength) analysis used very high flux, high resolution 23
Wavelength Shorter wavelengths more Bragg peaks more peak overlap 24
Wavelength Shorter wavelengths more Bragg peaks more peak overlap (keep in mind peak broadening due to sample and/or no. phases present) 25
Wavelength Shorter wavelengths more Bragg peaks more peak overlap (keep in mind peak broadening due to sample and/or no. phases present) X-rays most atom types have very strong absorption of characteristic wavelengths 26
Instrument geometry Choices: a. conventional Bragg-Brentano diffractometer (includes Θ-Θ) b. Guinier camera or diffractometer c. diffractometer w/ curved PSD d. TOF neutron instrument e. 4-circle diffractometer 27
Instrument geometry Choices: a. conventional Bragg-Brentano diffractometer (includes Θ-Θ) b. Guinier camera or diffractometer c. diffractometer w/ curved PSD d. TOF neutron instrument e. 4-circle diffractometer Generally want good resolution & high intensity can be obtained w/ all but (c) above, & (a) w/reactor neutrons (CW) 28
Instrument geometry Choices: a. conventional Bragg-Brentano diffractometer (includes Θ-Θ) b. Guinier camera or diffractometer c. diffractometer w/ curved PSD d. TOF neutron instrument e. 4-circle diffractometer Generally want good resolution & high intensity can be obtained w/ all but (c) above, & (a) w/reactor neutrons (CW) Instrument geometry affects instrument file 29
Detector type Conventional scintillation or proportional counter energy resolution not high usually need monochromator 30
Detector type Conventional scintillation or proportional counter energy resolution not high usually need monochromator Also common solid state detector very high energy resolution monochromator not needed 31
Detector type Conventional scintillation or proportional counter energy resolution not high usually need monochromator Also common solid state detector very high energy resolution monochromator not needed Neutrons He counter 32
Detector type Conventional scintillation or proportional counter energy resolution not high usually need monochromator Also common solid state detector very high energy resolution monochromator not needed Neutrons He counter What about image plates? poor resolution, hi bkgrd 33
Instrument setup Divergence and receiving slit sizes 34
Instrument setup Divergence and receiving slit sizes Theta-compensating divergence slit keeps irradiated area constant, But changes intensity distribution vs 2Θ 35
Instrument setup Divergence and receiving slit sizes 36
Instrument setup Divergence and receiving slit sizes 37
Instrument setup Divergence and receiving slit sizes Use of monochromator changes polarization correction in LP factor Integrated intensities of Bragg reflections: I hkl = scale factor x mult factor hkl x LP Θ x absorb factor Θ x pref orient factor hkl x extinction factor hkl x F hkl 2 38
Scan setup Scan range no. of reflections want >5 x no. parameters refined wavelength dependent low angle reflections may not be useful due to specimen configuration larger inherent instrumental errors extinction effects 39
Scan setup Step size sample dependent - peak widths need 5 observations across top of peak usually 0.01-0.05 2Θ 40
Scan setup Step size sample dependent - peak widths need 5 observations across top of peak usually 0.01-0.05 2Θ Count time longer times > higher intensities > greater precision at some point, little improvement in refinement process for longer count times 41
Specimen type and preparation Radiation source Wavelength Instrument geometry Detector type Instrument setup Scan parameters Choose according to objective(s) of experiment 42