Rietveld Refinement
Why do Reitveld Refinement?
What are the limitations of Rietveld?
• Phase detection limit of 1-5%• Impurity atom determination• New/unknown crystal structures• Determination of disorder parameter• Multiple parameters can have same effect diffraction
patterns• Equipment (powder vs. single crystal, x-rays vs.
neutrons)• Garbage in, garbage out! (noisy data, poor quality
sample, poor sample prep)
Basic refinement procedure
Experimental diffraction pattern
Starting crystal structure (.cif, ICSD)
Instrument file (.inst, LaB6 standard)
Refinement program: GSAS, FullProf
Refine:• Background• Lattice parameters• Peak intensities• Peak shapes• Peak positions• Phase fractions
Assess:• Goodness of fit/R factors• Impurity phases• Peak/background shapes• Difference pattern
Some common challenges/problems
• False minimas• Refinement diverges (“blows
up”)• Which goodness of fit to
choose? R vs. Chi sq?• Preferred orientation• Large amorphous background• Incorrect starting crystal
structure or lattice parameter• Poor quality data!
Resources and links
• GSAS/EXPGUI: http://www.ncnr.nist.gov/programs/crystallography/software/gsas.html
• GSAS tutorials: http://www.aps.anl.gov/Xray_Science_Division/Powder_Diffraction_Crystallography/
• FullProf: https://www.ill.eu/sites/fullprof/• PowDLL (file conversion):
http://users.uoi.gr/nkourkou/powdll/• Lots of info about fitting parameters:
http://www.ing.unitn.it/~luttero/laboratoriomateriali/RietveldRefinements.pdf
• People: Lauryn Baranowski ([email protected]), Lakshmi Krishna ([email protected])