Difference between revisions of "GSAS Profile Terms"
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For the synchrotron powder XRD data from 11-BM, the instrumental resolution is well described by Gaussian terms. Sample effects, such as size and microstrain broadening (i.e. local variations in the lattice parameters) are usually fit best by Lorentzian terms. | For the synchrotron powder XRD data from 11-BM, the instrumental resolution is well described by Gaussian terms. Sample effects, such as size and microstrain broadening (i.e. local variations in the lattice parameters) are usually fit best by Lorentzian terms. | ||
Please consult other references (such as the GSAS maunal) for details on Rietveld profile functions. | |||
== Fitting 11-BM Data == | |||
Representative LaB6 data for 11-BM (high resolution powder XRD) can be [downloaded from the 11-BM webpage http://11bm.xor.aps.anl.gov/standards_data.html] here (pick your format): | |||
more details about this specific LaB6 dataset follow: | more details about this specific LaB6 dataset follow: | ||
Revision as of 22:06, 4 January 2012
Peaks Profile terms for Rietveld Analysis
Most Rietveld refinement programs use a pseudo-Voigt term combining Gaussian and Lorentzian peak shapes (plus other correction terms). In general, the X-ray source can be described by a Gaussian function, while sample effects are described by Lorentzian terms.
For the synchrotron powder XRD data from 11-BM, the instrumental resolution is well described by Gaussian terms. Sample effects, such as size and microstrain broadening (i.e. local variations in the lattice parameters) are usually fit best by Lorentzian terms.
Please consult other references (such as the GSAS maunal) for details on Rietveld profile functions.
Fitting 11-BM Data
Representative LaB6 data for 11-BM (high resolution powder XRD) can be [downloaded from the 11-BM webpage http://11bm.xor.aps.anl.gov/standards_data.html] here (pick your format):
more details about this specific LaB6 dataset follow:
precise wavelength = 0.412235 data was collected on a spinning 0.8 mm diameter capillary of LaB6 660a [NIST SRM certificate lattice value = 4.15691(1) A] . Estimated muR is ~ 1.0 collection Temp: 295 K 2theta range: 0.5 deg - 50.0 deg step size: 0.001 deg
My GSAS/EXPGUI refinement gives a good fit (image attached) with the following parameters:
SG: Pm-3m a = 4.156917(1) zero shift: -0.00029 deg 2theta La @ 0, 0, 0 (Ui/Ue*100 = 0.62) B @ 0.1984(1), 1/2, 1/2 (Ui/Ue*100 = 0.29)
GSAS Profile type 4: (non-listed terms = 0.0) Coeff. : GU GV GW LX S/L H/L Value : 2.552E+00 -5.439E-01 5.990E-02 2.790E-01 1.2E-03 1.2E-03
4-term Shifted Chebyschev (type #1) background
wRp = 0.0645, Rp = 0.0486 CHI**2 = 3.349 for 14 variables
Let me know if you need any additional information.
Conversion of pseudo-Voigt function terms
GSAS <-> Fullprof Gaussian Parameters GSAS Term <=> Fullprof Term : (description) GU = 1803.4 * U : (instrumental term, ~ tan^2 of theta) GV = 1803.4 * V : (instrumental term, ~ tan of theta) GW = 1803.4 * W : (instrumental term, ~ constant with theta) GP = 1803.4 * IG : (size broadening)
note: 1083.4 => centidegrees squared divided by 8*ln(2)
Lorentzian Parameters
GSAS Term <=> Fullprof Term : (description)
LX = 100 * Y : (size broadening)
LY = 100 * X : (microstrain)
note: 100 => degrees to centidegrees
Finger-Cox-Jephcoat asymmetry parameters
GSAS S/L = Fullprof S_L
GSAS H/L = Fullprof D_L
Note: terms are equivalent
S / L = the sample “half height”/diffractometer radius H / L = the slit half height/diffractometer radius
"Typical values of Rietveld instrument profile coefficients" Kaduk J, Reid J. Powder Diffraction (2011) vol. 26 pp. 88
size and microstrain broadening (i.e. local variations in the lattice parameters) are Lorentzian for 11-BM data
Only rarely is Gaussian size broadening observed; this re- quires a very tight monodisperse distribution, which is rarely encountered in powder specimens but is sometimes seen in polycrystalline solid specimens such as pieces of metal.
